Digestion and absorption of protein | My Assignment Tutor

CHAPTER2180The chemist’s view of proteins• Amino acids• ProteinsDigestion and absorption of protein• Protein digestion• Protein absorptionProteins in the body• Protein synthesis• Roles of proteins• A preview of protein metabolismProtein in foods• Protein quality• Protein regulations for food labelsHealth effects and recommended intakesof protein• Protein–energy malnutrition• Health effects of protein• Recommended intakes of protein• Protein and … Continue reading “Digestion and absorption of protein | My Assignment Tutor”

CHAPTER2180The chemist’s view of proteins• Amino acids• ProteinsDigestion and absorption of protein• Protein digestion• Protein absorptionProteins in the body• Protein synthesis• Roles of proteins• A preview of protein metabolismProtein in foods• Protein quality• Protein regulations for food labelsHealth effects and recommended intakesof protein• Protein–energy malnutrition• Health effects of protein• Recommended intakes of protein• Protein and amino acid supplementsHighlight 6: Nutritional genomicsCHAPTER OUTLINEPUTTING COMMONSENSE TO THE TESTCircle your answer T FMeat is the most important source of protein in the diet.T FWhen proteins are denatured they cease being proteins.T FAmino acids are the building blocks of proteins.T FProteins have many roles in the body, including that of energy provision throughglucose production.Foods derived from animals are considered high-quality proteins.T F PROTEIN: AMINO ACIDSNutrition in your lifeTheir versatility in the body is impressive. They help muscles to contract, blood to clotand eyes to see. They keep you alive and well by facilitating chemical reactions anddefending against infections. Without them, your bones, skin and hair would have nostructure. No wonder they were named proteins, meaning ‘of prime importance’. Doesthat mean proteins deserve top billing in your diet as well? Are the best sources ofprotein beef, beans or broccoli? This chapter will help you learn which foods will supplyyou with enough, but not too much, high-quality protein.6Throughout thischapter, theCourseMate logoindicates anopportunity foronline self-study,linking you toactivities, videosand other onlineresources.• Figure 6.6:Animated!Protein digestionin the GI tract• Figure 6.7:Animated!Protein synthesis• Figure 6.10:Animated! Anexample ofprotein transport• How to: Practiceproblems• Nutrition portfoliojournal• Nutritioncalculations:Practice problemsEleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.Misconceptions surround the roles of protein in the body and the importance of protein in thediet. For example, people who associate meat with protein and protein with strength may eatsteak to build muscles. Their thinking is only partly correct, however. Protein is a vital structuraland working substance in all cells – not just muscle cells. To build strength, muscle cells needphysical activity and all the nutrients – not just protein. Furthermore, protein is found in milk,eggs, legumes and many grains and vegetables – not just meat. By overvaluing protein andoveremphasising meat in the diet, a person may mistakenly crowd out other, equally importantnutrients and foods. As this chapter describes the various roles of protein in the body and foodsources in the diet, keep in mind that protein is one of many nutrients needed to maintaingood health.THE CHEMIST’S VIEW OF PROTEINSLEARN ITRecognise the chemical structures of amino acids and proteins.Chemically, proteins contain the same atoms as carbohydrates and lipids – carbon (C), hydrogen(H) and oxygen (O) – but proteins also contain nitrogen (N) atoms. These nitrogen atoms givethe name amino (nitrogen containing) to the amino acids – the links in the chains of proteins.AMINO ACIDSAll amino acids have the same basic structure – a central carbon (C) atom with a hydrogenatom (H), an amino group (NH2) and an acid group (COOH) attached to it. However,carbon atoms need to form four bonds, so a fourth attachment is necessary. This fourthsite distinguishes each amino acid from the others. Attached to the carbon atom at thefourth bond is a distinct atom, or group of atoms, known as the side group or side chain(see Figure 6.1).Unique side groupsThe side groups on amino acids vary from one amino acid to the next, making proteins morecomplex than both carbohydrates and lipids. A polysaccharide (starch, for example) may beseveral thousand units long, but each unit is a glucose molecule just like all the others. A protein,on the other hand, is made up of about 20 different amino acids, each with a different sidegroup. Table 6.1 lists the amino acids most common in proteins.*The simplest amino acid, glycine, has a hydrogen atom as its side group. A slightly morecomplex amino acid, alanine, has an extra carbon with three hydrogen atoms. Other aminoacids have more complex side groups (see Figure 6.2 for examples). Thus, althoughall amino acids share a common structure, they differ in size, shape, electricalcharge and other characteristics because of differences in these side groups.Reminder:• H forms 1 bond• O forms 2 bonds• N forms 3 bonds• C forms 4 bondsPUTTINGCOMMONSENSE TOTHE TESTMeat is the mostimportant source ofprotein in the diet.FALSEH NH HC C O HOAminogroup AcidgroupSide groupvariesFIGURE 6.1 Amino acid structure*Besides the 20 common amino acids, which can all be components of proteins, others do not occur inproteins but can be found individually (for example, taurine and ornithine). Some amino acids occur inrelated forms (for example, proline can acquire an OH group to become hydroxyproline).CHAPTER 6 PROTEIN: AMINO ACIDS 181Eleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.Non-essential amino acidsMore than half of the amino acids are non-essential, meaning that the body can synthesise themfor itself. Proteins in foods usually deliver these amino acids, but it is not essential that they doso. The body can make all non-essential amino acids, given nitrogen to form the amino groupand fragments from carbohydrate or fat to form the rest of the structure.Essential amino acidsThere are nine amino acids that the human body either cannot make at all or cannotmake in sufficient quantity to meet its needs. These nine amino acids must be suppliedby the diet; they are essential. The first column in Table 6.1 presents the essentialamino acids.Conditionally essential amino acidsSometimes a non-essential amino acid becomes essential under special circumstances. Forexample, the body normally uses the essential amino acid phenylalanine to make tyrosineSome researchersrefer to essential aminoacids as indispensableand to non-essentialamino acids asdispensable.TABLE 6.1 Amino acidsProteins are made up of about 20 common amino acids. The first column lists the essentialamino acids for human beings (those the body cannot make – that must be provided in thediet). The second column lists the non-essential amino acids. In special cases, some nonessential amino acids may become conditionally essential (see the text). In a newborn, forexample, only five amino acids are truly non-essential; the other non-essential amino acidsare conditionally essential until the metabolic pathways are developed enough to make thoseamino acids in adequate amounts. ESSENTIAL AMINO ACIDSNON-ESSENTIAL AMINO ACIDSHistidineIsoleucineLeucineLysineMethioninePhenylalanineThreonineTryptophanValineAlanineArginineAsparagineAspartic acidCysteineGlutamic acidGlutamineGlycineProlineSerineTyrosine Glycine Alanine Aspartic acid PhenylalanineH NH HC O HOH CH NHC O HOC C HH HHH NHC O HOC C HH HH NHC O HO C C HH HC O HOFIGURE 6.2 Examples of amino acidsNote that all amino acids have a common chemical structure but that each has a different side group.Appendix C presents the chemical structures of the 20 amino acids most common in proteins.182 UNDERSTANDING NUTRITIONEleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.(a non-essential amino acid). But if the diet fails to supply enough phenylalanine, or ifthe body cannot make the conversion for some reason (as happens in the inherited diseasephenylketonuria), then tyrosine becomes a conditionally essential amino acid.PROTEINSCells link amino acids end to end in a variety of sequences to form thousands of differentproteins. A peptide bond unites each amino acid to the next.Amino acid chainsCondensation reactions connect amino acids, just as they combine monosaccharides to formdisaccharides and fatty acids with glycerol to form triglycerides. Two amino acids bondedtogether form a dipeptide (see Figure 6.3). By another such reaction, a third amino acid can beadded to the chain to form a tripeptide. As additional amino acids join the chain, a polypeptideis formed. Most proteins are a few dozen to several hundred amino acids long; Figure 6.4 providesan example – insulin.Cys Leu His GlnAsnValPheLeu Ala Glu Val Leu His Ser GlyTyrLeuValPhe Phe Gly Arg Glu Gly CysTyrThrPro Lys AlaCys Tyr Asn Glu Leu Gln Tyr Leu Ser CysValSAsnGly Ile Val Glu Gln Cys CysAlaSerSSSSSFIGURE 6.4 Amino acid sequence of human insulinHuman insulin is a relatively small protein that consists of 51 amino acids in two short polypeptidechains. (For amino acid abbreviations, see Appendix C.) Two bridges link the two chains. A thirdbridge spans a section within the short chain. Known as disulphide bridges, these links always involvethe amino acid cysteine (Cys), whose side group contains sulphur (S). Cysteines connect to each otherwhen bonds form between these side groups.Amino acidAn OH group from the acid end of one aminoacid and an H atom from the amino group ofanother join to form a molecule of water.A peptide bond (highlighted inred) forms between the two aminoacids, creating a dipeptide.+ amino acid DipeptideWaterH NHC O HOC C HH HHN HC O HOC NHC O HOCH HH C H H C HHHOHH NHO CC C HH HHFIGURE 6.3 Condensation of two amino acids to form a dipeptideCHAPTER 6 PROTEIN: AMINO ACIDS 183Eleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.Amino acid sequences – primary structureThe primary structure of a protein is determined by the sequence of amino acids. If a personcould walk along a carbohydrate molecule like starch, the first stepping stone would be a glucose.The next stepping stone would also be a glucose, and it would be followed by another glucose,and yet another glucose. But if a person were to walk along a polypeptide chain, each steppingstone would be one of 20 different amino acids. The first stepping stone might be the aminoacid methionine. The second might be an alanine. The third might be a glycine, and the fourtha tryptophan, and so on. Walking along another polypeptide path, a person might step on aphenylalanine, then a valine, and then a glutamine. In other words, amino acid sequenceswithin proteins vary.The amino acids can act somewhat like the letters in an alphabet. If you had onlythe letter G, all you could write would be a string of Gs: G-G-G-G-G-G-G. But with26 different letters available, you can create poems, songs and novels. Similarly, the20 amino acids can be linked together in a variety of sequences – even more than are possiblefor letters in a word or words in a sentence. Thus, the variety of possible sequences forpolypeptide chains is tremendous.Polypeptide shapes – secondary structureThe secondary structure of proteins is determined not by chemical bonds as between the aminoacids but by weak electrical attractions within the polypeptide chain. As positively chargedhydrogen atoms attract nearby negatively charged oxygen atoms, sections of the polypeptidechain twist into a helix or fold into a pleated sheet, for example. These shapes give proteinsstrength and rigidity.Polypeptide tangles – tertiary structureThe tertiary structure of proteins occurs as long polypeptide chains twist and fold into a varietyof complex, tangled shapes. The unique side group of each amino acid gives it characteristicsthat attract it to, or repel it from, the surrounding fluids and other amino acids. Some aminoacid side groups are attracted to water molecules; they are hydrophilic. Other side groupsare repelled by water; they are hydrophobic. As amino acids are strung together to make apolypeptide, the chain folds so that its hydrophilic side groupsare on the outer surface near water; the hydrophobic groups tuckthemselves inside, away from water. Similarly, the disulphidebridges in insulin (see Figure 6.4) determine its tertiary structure.The extraordinary and unique shapes of proteins enable themto perform their various tasks in the body. Some form globularor spherical structures that can carry and store materialswithin them, and some, such as those of tendons, form linearstructures that are more than ten times as long as they are wide.The intricate shape a protein finally assumes gives it maximumstability.Multiple polypeptide interactions – quaternarystructuresSome polypeptides are functioning proteins just as they are; othersneed to associate with other polypeptides to form larger workingcomplexes. The quaternary structure of proteins involves theinteractions between two or more polypeptides. One molecule ofhaemoglobin – the large, globular protein molecule that, by thebillions, packs the red blood cells and carries oxygen – is made offour associated polypeptide chains, each holding the mineral iron(see Figure 6.5).IronFour highly folded polypeptide chainsform the globular haemoglobin protein.Haem, thenon-proteinportion ofhaemoglobin,holds iron.The amino acid sequencedetermines the shapeof the polypeptide chain.FIGURE 6.5 The structure of haemoglobin184 UNDERSTANDING NUTRITIONEleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.LEARN ITSummarise protein digestion and absorption.DIGESTION AND ABSORPTION OF PROTEINChemically speaking, proteins are more complex than carbohydrates or lipids,being made of some 20 different amino acids, nine of which the body cannotmake. Each amino acid contains an amino group, an acid group, a hydrogen atomand a distinctive side group, all attached to a central carbon atom. Cells linkamino acids together in a series of condensation reactions to create proteins. Thedistinctive sequence of amino acids in each protein determines its unique shapeand function.REVIEW ITCooking an egg denatures its proteins.Matthew FarruggioProteins in foods do not become body proteins directly. Instead, they supply the aminoacids from which the body makes its own proteins. When a person eats foods containingprotein, enzymes break the long polypeptide strands into shorter strands, the shortstrands into tripeptides and dipeptides, and, finally, the tripeptides and dipeptides intoamino acids.PROTEIN DIGESTIONFigure 6.6 illustrates the digestion of protein through the gastrointestinal (GI) tract. Proteins arecrushed and moistened in the mouth, but the real action begins in the stomach.In the stomachThe major event in the stomach is the partial breakdown (hydrolysis) of proteins. Hydrochloricacid uncoils (denatures) each protein’s tangled strands so that digestive enzymes can attack thepeptide bonds. The hydrochloric acid also converts the inactive form of the enzyme pepsinogento its active form, pepsin. Pepsin cleaves proteins – large polypeptides – into smaller polypeptidesand some amino acids.In the small intestineWhen polypeptides enter the small intestine, several pancreatic and intestinal proteaseshydrolyse them further into short peptide chains, tripeptides, dipeptides and amino acids. Thenpeptidase enzymes on the membrane surfaces of the intestinal cells split most of the dipeptidesand tripeptides into single amino acids. Only a few peptides escape digestion and enter the bloodintact. Figure 6.6 includes names of the digestive enzymes for protein and describes their actions.The inactive form ofan enzyme is called aproenzyme or a zymogen.A string of four tonine amino acids is anoligopeptide.• oligo 5 fewProtein denaturationWhen proteins are subjected to heat, acid or other conditions that disturbtheir stability, they undergo denaturation – that is, they uncoil and lose theirshapes and, consequently, also lose their ability to function. Past a certain point,denaturation is irreversible. Familiar examples of denaturation include thehardening of an egg when it is cooked, the curdling of milk when acid is added,and the stiffening of egg whites when they are whipped.PUTTINGCOMMONSENSE TOTHE TESTWhen proteinsare denaturedthey cease beingproteins.FALSECHAPTER 6 PROTEIN: AMINO ACIDS 185Eleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.PROTEINHYDROCHLORIC ACIDAND THEDIGESTIVE ENZYMESMouth and salivary glandsStomachChewing and crushing moistenprotein-rich foods and mix them withsaliva to be swallowedHydrochloric acid (HCl) uncoilsprotein strands and activatesstomach enzymes:Small intestine and pancreasPancreatic and small intestinalenzymes split polypeptides further:Then enzymes on the surface of thesmall intestinal cells hydrolyse thesepeptides and the cells absorb them:MouthSalivaryglands(Oesophagus)(Liver)(Gall bladder)StomachPancreaticductPancreasSmallintestineProteinpepsin,HCl smallerpolypeptidesPeptidesintestinaltripeptidasesanddipeptidases amino acids(absorbed)Polypeptidespancreaticandintestinalproteasestripeptides,dipeptides,amino acidsIn the stomach:Hydrochloric acid (HCl)Denatures protein structureActivates pepsinogen to pepsinIn the small intestine:EnteropeptidaseaConverts pancreatic trypsinogento trypsinPepsinCleaves proteins to smallerpolypeptides and some freeamino acidsInhibits pepsinogen synthesisaEnteropeptidase was formerly knownas enterokinase.Intestinal aminopeptidasesCleave amino acids from theamino ends of small polypeptides(oligopeptides)Intestinal dipeptidasesCleave dipeptides to amino acidsIntestinal tripeptidasesCleave tripeptides to dipeptidesand amino acidsElastase and collagenaseCleave polypeptides into smallerpolypeptides and tripeptidesCarboxypeptidasesCleave amino acids from the acid(carboxyl) ends of polypeptidesChymotrypsinCleaves peptide bonds next tothe amino acids phenylalanine,tyrosine, tryptophan, methionine,asparagine and histidineTrypsinInhibits trypsinogen synthesisCleaves peptide bonds next tothe amino acids lysine andarginineConverts pancreaticprocarboxypeptidases tocarboxypeptidasesConverts pancreaticchymotrypsinogen tochymotrypsinWatch an animationon this topic inCourseMate.FIGURE 6.6 Animated! Protein digestion in the GI tractPROTEIN ABSORPTIONA number of specific carriers transport amino acids (and some dipeptides and tripeptides)into the intestinal cells. Once inside the intestinal cells, amino acids may be used for energyor to synthesise needed compounds. Amino acids that are not used by the intestinal cells aretransported across the cell membrane into the surrounding fluid where they enter the capillarieson their way to the liver.186 UNDERSTANDING NUTRITIONEleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.Consumers lacking nutrition knowledge may fail to realise that most proteins are brokendown to amino acids before absorption. They may be misled by advertisements urging themto ‘Eat enzyme A. It will help you digest your food’. Or ‘Don’t eat food B. It contains enzymeC, which will digest cells in your body’. In reality, though, enzymes in foods are digested, justas all proteins are. Even the digestive enzymes – which function optimally at their specificpH – are denatured and digested when the pH of their environment changes. (For example,the enzyme pepsin, which works best in the low pH of the stomach, becomes inactive anddigested when it enters the higher pH of the small intestine.)Another misconception is that eating pre-digested proteins (amino acid supplements) savesthe body from having to digest proteins and keeps the digestive system from ‘overworking’. Sucha belief grossly underestimates the body’s abilities. Actually, the digestive system handles wholeproteins better than pre-digested ones because it dismantles and absorbs the amino acids atrates that are optimal for the body’s use. (The last section of this chapter discusses amino acidsupplements further.)LEARN ITDescribe how the body makes proteins and uses them to perform various roles.Digestion is facilitated mostly by the stomach’s acid and enzymes, which first denature dietaryproteins, then cleave them into smaller polypeptides and some amino acids. Pancreatic and intestinalenzymes split these polypeptides further, to oligo-, tri- and dipeptides, and then split most of these tosingle amino acids. Then carriers in the membranes of intestinal cells transport the amino acids intothe cells, where they are released into the bloodstream.REVIEW ITPROTEINS IN THE BODYThe human body contains an estimated 30000 different kinds of proteins. Of these, about 3000have been studied, although with the recent surge in knowledge gained from sequencing thehuman genome, this number is growing rapidly. Only about 10 are described in this chapter –but these should be enough to illustrate the versatility, uniqueness and importance of proteins.As you will see, each protein has a specific function, and that function is determined duringprotein synthesis.PROTEIN SYNTHESISEach human being is unique because of small differences in the body’s proteins. These differencesare determined by the amino acid sequences of proteins, which, in turn, are determined bygenes. The following paragraphs describe in words the ways cells synthesise proteins; Figure 6.7provides a pictorial description.The instructions for making every protein in a person’s body are transmitted by way of thegenetic information received at conception. This body of knowledge, which is filed in the DNA(deoxyribonucleic acid) within the nucleus of every cell, never leaves the nucleus.Delivering the instructionsTransforming the information in DNA into the appropriate sequence of amino acids neededto make a specific protein requires two major steps. In the first step, a stretch of DNA isused as a template to make a strand of RNA (ribonucleic acid) known as messenger RNA.Messenger RNA then carries the code across the nuclear membrane into the body of the cell.The study of thebody’s proteins isproteomics.The human genomeis the full set ofchromosomes, includingall of the genes andassociated DNA.This process ofmessenger RNA beingmade from a templateof DNA is known astranscription.CHAPTER 6 PROTEIN: AMINO ACIDS 187Eleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.124 563mRNADNAThe DNA serves as a template to make strandsof messenger RNA (mRNA). Each mRNAstrand copies exactly the instructions formaking some protein the cell needs.The mRNA attaches itself to the proteinmaking machinery of the cell, theribosomes.Another form of RNA, transfer RNA (tRNA), collectsamino acids from the cell fluid. Each tRNA carriesits amino acids to the mRNA, which dictates thesequence in which the amino acids will beattached to form the protein strands. Thus themRNA ensures the amino acids are linedup in the correct sequence.The mRNA leavesthe nucleus through thenuclear membrane. DNAremains inside the nucleus.RibosomemRNAmRNAAmino acidtRNAAs the amino acids are lined up in the rightsequence, and the ribosome moves alongthe mRNA, an enzyme bonds one aminoacid after another to the growing proteinstrand. The tRNA are freed to return formore amino acids. When all the aminoacids have been attached, thecompleted protein is released.Finally, the mRNA and ribosome separate. It takesmany words to describe these events, but in the cell,40 to 100 amino acids can be added to a growingprotein strand in only a second. Furthermore severalribosomes can simultaneously work on the samemRNA to make many copies of the protein.mRNAProtein strandRibosomes(protein-makingmachinery)DNA NucleusCellFIGURE 6.7 Animated! Protein synthesisWatch an animation on thistopic in CourseMate.188 UNDERSTANDING NUTRITIONEleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.There it seeks out and attaches itself to one of the ribosomes (a protein-making machine,which is itself composed of RNA and protein), where the second step takes place. Situatedon a ribosome, messenger RNA specifies the sequence in which the amino acids line up forthe synthesis of a protein.Lining up the amino acidsOther forms of RNA, called transfer RNA, collect amino acids from the cell fluid andbring them to the messenger. Each of the 20 amino acids has a specific transfer RNA.Thousands of transfer RNA, each carrying its amino acid, cluster around the ribosomes,awaiting their turn to unload. When the messenger’s list calls for a specific amino acid,the transfer RNA carrying that amino acid moves into position. Then the next loadedtransfer RNA moves into place and then the next and the next. In this way, the aminoacids line up in the sequence that is called for, and enzymes bind them together. Finally,the completed protein strand is released, and the transfer RNA are freed to return forother loads of amino acids.Sequencing errorsThe sequence of amino acids in each protein determines its shape, which supports a specificfunction. If a genetic error alters the amino acid sequence of a protein, or if a mistake ismade in copying the sequence, an altered protein will result, sometimes with dramaticconsequences. The protein haemoglobin offers one example of such a genetic variation. Ina person with sickle-cell anaemia, two of haemoglobin’sfour polypeptide chains (refer to Figure 6.5 on page 184)have the normal sequence of amino acids, but the othertwo chains do not – they have the amino acid valine in aposition that is normally occupied by glutamic acid (seeFigure 6.8). This single alteration in the amino acid sequencechanges the characteristics and shape of haemoglobin somuch that it loses its ability to carry oxygen effectively.The red blood cells filled with this abnormal haemoglobinstiffen into elongated sickle, or crescent, shapes insteadof maintaining their normal pliable disc shape – hencethe name, sickle-cell anaemia. Sickle-cell anaemia raisesenergy needs, causes many medical problems and can befatal.1 Caring for children with sickle-cell anaemia includesdiligent attention to their water needs, as dehydration cantrigger a crisis.Nutrients and gene expressionWhen a cell makes a protein as described earlier, scientistssay that the gene for that protein has been ‘expressed’. Cellscan regulate gene expression to make the type of protein,in the amounts and at the rate, they need. Nearly all of thebody’s cells possess the genes for making all human proteins,but each type of cell makes only the proteins it needs. Forexample, cells of the pancreas express the gene for insulin; inother cells, that gene is idle. Similarly, the cells of the pancreasdo not make the protein haemoglobin, which is needed onlyby the red blood cells.Recent research has unveiled some of the fascinating waysnutrients regulate gene expression and protein synthesis (seeAmino acid sequence of sickle-cell haemoglobin:Val His Leu Thr Pro GluAmino acid sequence of normal haemoglobin:Val His Leu Thr Pro Glu GluValSickle-shaped blood cell Normal red blood cellFIGURE 6.8 Sickle cell compared with normal redblood cellNormally, red blood cells are disc-shaped, but in theinherited disorder sickle-cell anaemia, red blood cells aresickle- or crescent-shaped. This alteration in shape occursbecause valine replaces glutamic acid in the amino acidsequence of two of haemoglobin’s polypeptide chains.As a result of this one alteration, the haemoglobin has adiminished capacity to carry oxygen.Science Photo Library/Dr. Stanley Flegler/Visuals UnlimitedThis process ofmessenger RNA directingthe sequence of aminoacids and synthesis ofproteins is known astranslation.Anaemia is not adisease but a symptomof various diseases. Inthe case of sickle cellanaemia, a defect in thehaemoglobin moleculechanges the shape ofthe red blood cells. Laterchapters describe theanaemias of vitamin andmineral deficiencies. Inall cases, the abnormalblood cells are unable tomeet the body’s oxygendemands.CHAPTER 6 PROTEIN: AMINO ACIDS 189Eleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.Highlight 6). Because diet plays an ongoing role in our lives from conception to death, it hasa major influence on gene expression and disease development. The benefits of polyunsaturatedfatty acids in defending against heart disease, for example, are partially explained by their rolein influencing gene expression for lipid enzymes. Later chapters provide additional examples ofrelationships among nutrients, genes and disease development.ROLES OF PROTEINSWhenever the body is growing, repairing or replacing tissue, proteins are involved. Sometimestheir role is to facilitate or to regulate; other times it is to become part of a structure. Versatilityis a key feature of proteins.Building materials for growth and maintenanceFrom the moment of conception, proteins form the building blocks of muscles, blood and skin –in fact, of most body structures. For example, to build a bone or a tooth, cells first lay down amatrix of the protein collagen and then fill it with crystals of calcium, phosphorus, magnesium,fluoride and other minerals.Collagen also provides the material of ligaments and tendons and the strengthening gluebetween the cells of the artery walls that enables the arteries to withstand the pressure of theblood surging through them with each heartbeat. Also made of collagen are scars that knit theseparated parts of torn tissues together.Proteins are also needed for replacing dead or damaged cells. The life span of a skin cellis only about 30 days. As old skin cells are shed, new cells made largely of protein grow fromunderneath to replace them. Cells in the deeper skin layers synthesise new proteins to go intohair and fingernails. Muscle cells make new proteins to grow larger and stronger in responseto exercise. Cells of the GI tract are replaced every few days. Both inside and outside, then, thebody continuously deposits protein into the new cells that replace those that have been lost.EnzymesSome proteins act as enzymes. Digestive enzymes have appeared in every chapter since Chapter 3,but digestion is only one of the many processes facilitated by enzymes. Enzymes not only breakdown substances, but they also build substances (such as bone) and transform one substanceinto another (amino acids into glucose, for example). Figure 6.9 displays a synthesis reaction.An analogy may help to clarify the role of enzymes. Enzymes are comparable to the clergyand judges who make and dissolve marriages. When a minister marries two people, they becomeNutrients can playkey roles in activatingor silencing genes.Switching genes on andoff, without changing thegenetic sequence itself,is known as epigenetics.• epi 5 amongPUTTINGCOMMONSENSE TOTHE TESTAmino acids arethe building blocksof proteins.TRUEBreaking-downreactions are catabolic;building-up reactionsare anabolic. (Chapter 7provides more details.)The separate compounds,A and B, are attracted to theenzyme’s active site, making areaction likely.The enzyme forms a complexwith A and B.The enzyme is unchanged, butA and B have formed a newcompound, AB.NewcompoundEnzyme EnzymeEnzyme A B A BABFIGURE 6.9 Enzyme actionEach enzyme facilitates a specific chemical reaction. In this diagram, an enzyme enables twocompounds to make a more complex structure, but the enzyme itself remains unchanged.190 UNDERSTANDING NUTRITIONEleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.a couple, with a new bond between them. They are joined together – but the minister remainsunchanged. The minister represents enzymes that synthesise large compounds from smallerones. One minister can perform thousands of marriage ceremonies, just as one enzyme canperform billions of synthetic reactions.Similarly, a judge who lets married couples separate may decree many divorces before retiring.The judge represents enzymes that hydrolyse larger compounds to smaller ones; for example, thedigestive enzymes. The point is that, like the minister and the judge, enzymes themselves are notaltered by the reactions they facilitate. They are catalysts, permitting reactions to occur morequickly and efficiently than if substances depended on chance encounters alone.HormonesThe body’s many hormones are messenger molecules, and some hormones are proteins.Various endocrine glands in the body release hormones in response to changes that challengethe body. The blood carries the hormones from these glands to their target tissues, where theyelicit the appropriate responses to restore and maintain normal conditions.The hormone insulin provides a familiar example. When blood glucose rises, the pancreasreleases its insulin. Insulin stimulates the transport proteins of the muscles and adipose tissueto pump glucose into the cells faster than it can leak out. (After acting on the message, thecells destroy the insulin.) Then, as blood glucose falls, the pancreas slows its release of insulin.Many other proteins act as hormones, regulating a variety of actions in the body (see Table 6.2for examples).Recall from Chapter 5that some hormones,such as oestrogen andtestosterone, derive fromcholesterol.TABLE 6.2 Examples of hormones and their actions HORMONESACTIONSPromotes growthGrowth hormoneInsulin and glucagonRegulate blood glucose (see Chapter 4)ThyroxineRegulates the body’s metabolic rate (seeChapter 8)Calcitonin and parathyroid hormoneRegulate blood calcium (see Chapter 12)Antidiuretic hormoneRegulates fluid and electrolyte balance (seeChapter 12) NOTE: Hormones are chemical messengers that are secreted by endocrine glands in response to alteredconditions in the body. Each travels to one or more specific target tissues or organs, where it elicits aspecific response. For descriptions of many hormones important in nutrition, see Appendix A.Regulators of fluid balanceProteins help to maintain the body’s fluid balance. Figure 12.1 (see page 414) illustrates acell and its associated fluids. As the figure explains, the body’s fluids are contained insidethe cells (intracellular) or outside the cells (extracellular). Extracellular fluids, in turn, canbe found either in the spaces between the cells (interstitial) or within the blood vessels(intravascular). The fluid within the intravascular spaces is called plasma (essentially bloodwithout its red blood cells). Fluids can flow freely between these compartments, but beinglarge, proteins cannot. Proteins are trapped primarily within the cells and to a lesser extentin the plasma.The exchange of materials between the blood and the cells takes place across the capillarywalls, which allow the passage of fluids and a variety of materials – but usually not plasmaproteins. Still, some plasma proteins leak out of the capillaries into the interstitial fluid betweenthe cells. These proteins cannot be reabsorbed back into the plasma; they normally re-enterCHAPTER 6 PROTEIN: AMINO ACIDS 191Eleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.circulation via the lymph system. If plasma proteins enter the interstitial spaces faster than theycan be cleared, fluid accumulates (because plasma proteins attract water) and causes swelling.Swelling due to an excess of interstitial fluid is known as oedema. The protein-related causes ofoedema include:• excessive protein losses caused by kidney disease or large wounds (such as extensive burns)• inadequate protein synthesis caused by liver disease• inadequate dietary intake of protein.Whatever the cause of oedema, the result is the same – a diminished capacity to delivernutrients and oxygen to the cells and to remove wastes from them. As a consequence, cells failto function adequately.Acid–base regulatorsProteins also help to maintain the balance between acids and bases within the body fluids.Normal body processes continually produce acids and bases, which the blood carries to thekidneys and lungs for excretion. The challenge is to do this without upsetting the blood’s acid–base balance.In an acid solution, hydrogen ions (H1) abound; the more hydrogen ions, the moreconcentrated the acid. Proteins, which have negative charges on their surfaces, attract hydrogenions, which have positive charges. By accepting and releasing hydrogen ions, proteins maintainthe acid–base balance of the blood and body fluids.The blood’s acid–base balance is tightly controlled. The extremes of acidosis and alkalosislead to coma and death, largely because they denature working proteins. Disturbing a protein’sshape renders it useless. To give just one example, denatured haemoglobin loses its capacity tocarry oxygen.TransportersSome proteins move about in the body fluids, carrying nutrients and other molecules. Theprotein haemoglobin carries oxygen from the lungs to the cells. The lipoproteins transport lipidsaround the body. Special transport proteins carry vitamins and minerals.The transport of the mineral iron provides an especially good illustration of these proteins’specificity and precision. When iron enters an intestinal cell after a meal has been digested andabsorbed, it is captured by a protein. Before leaving the intestinal cell, iron is attached to anotherprotein that carries it through the bloodstream to the cells. Once iron enters a cell, it is attached toa storage protein that will hold the iron until it is needed. When it is needed, iron is incorporatedinto proteins in the red blood cells and muscles that assist in oxygen transport and use. (Chapter13 provides more details on how these protein carriers transport and store iron.)Some transport proteins reside in cell membranes and act as ‘pumps’, picking upcompounds on one side of the membrane and releasing them on the other as needed.Each transport protein is specific for a certain compound or group of related compounds.Figure 6.10 illustrates how a membrane-bound transport protein helps to maintain thesodium and potassium concentrations in the fluids inside and outside cells. The balance ofthese two minerals is critical to nerve transmissions and muscle contractions; imbalancescan cause irregular heartbeats, muscular weakness, kidney failure and even death.AntibodiesProteins also defend the body against disease. A virus – whether it is one that causes flu, smallpox,measles or the common cold – enters the cells and multiplies there. One virus may produce 100replicas of itself within an hour or so. Each replica can then burst out and invade 100 differentcells, soon yielding 10000 virus particles, which invade 10000 cells. Left free to do their worst,they will soon overwhelm the body with disease.Fortunately, when the body detects these invading antigens, it manufactures antibodies,giant protein molecules designed specifically to combat them. The antibodies work so swiftlyand efficiently that in a normal, healthy individual, most diseases never have a chance to getCompounds that helpkeep a solution’s acidityor alkalinity constant arecalled buffers.192 UNDERSTANDING NUTRITIONEleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.started. Without sufficient protein, though, the body cannot maintain its army of antibodies toresist infectious diseases.Each antibody is designed to destroy a specific antigen. Once the body has manufacturedantibodies against a particular antigen (such as the measles virus), it ‘remembers’ how to makethem. Consequently, the next time the body encounters that same antigen, it produces antibodieseven more quickly. In other words, the body develops a molecular memory, known as immunity.(Chapter 16 describes food allergies – the immune system’s response to food antigens.)Source of energy and glucoseWithout energy, cells die; without glucose, the brain and nervous system falter. Even thoughproteins are needed to do the work that only they can perform, they will be sacrificed to provideenergy and glucose during times of starvation or insufficient carbohydrate intake. Thebody will break down its tissue proteins to make amino acids available for energy or glucoseproduction. In this way, protein can maintain blood glucose levels, but at the expense of losinglean body tissue. Chapter 7 provides many more details on energy metabolism.Other rolesAs mentioned earlier, proteins form integral parts of most body structures such as skin, musclesand bones. They also participate in some of the body’s most amazing activities such as bloodclotting and vision. When a tissue is injured, a rapid chain of events leads to the production offibrin, a stringy, insoluble mass of protein fibres that forms a solid clot from liquid blood. Later,more slowly, the protein collagen forms a scar to replace the clot and permanently heal thewound. The light-sensitive pigments in the cells of the eye’s retina are molecules of the proteinopsin. Opsin responds to light by changing its shape, thus initiating the nerve impulses thatconvey the sense of sight to the brain.A PREVIEW OF PROTEIN METABOLISMThis section previews protein metabolism; Chapter 7 provides a full description. Cells haveseveral metabolic options, depending on their protein and energy needs.PUTTINGCOMMONSENSE TOTHE TESTProteins havemany rolesin the body,including that ofenergy provisionthrough glucoseproduction.TRUEThe protein changes shape andreleases potassium inside thecell.The transport protein picks uppotassium from outside the cell.The transport protein picks upsodium from inside the cell.OutsidecellInsidecellCellmembraneTransportproteinKey:PotassiumSodiumThe protein changes shape andreleases sodium outside the cell.FIGURE 6.10 Animated! An example of protein transportThis transport protein resides within a cell membrane and acts as a two-door passageway.Molecules enter on one side of the membrane and exit on the other, but the protein doesn’t leavethe membrane. This example shows how the transport protein moves sodium and potassium inopposite directions across the membrane to maintain a high concentration of potassium and a lowconcentration of sodium within the cell. This active transport system requires energy.Watch an animationon this topic inCourseMate.Reminder: Proteinprovides 17 kJ/g. Returnto page 9 for a refresheron how to calculate theprotein kilojoules fromfoods.Reminder: The makingof glucose from noncarbohydrate sourcessuch as amino acids isgluconeogenesis.CHAPTER 6 PROTEIN: AMINO ACIDS 193Eleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.Protein turnover and the amino acid poolWithin each cell, proteins are continually being made and broken down, a processknown as protein turnover. When proteins break down, they free amino acids. Theseamino acids mix with amino acids from dietary protein to form an ‘amino acid pool’within the cells and circulating blood. The rate of protein degradation and the amountof protein intake may vary, but the pattern of amino acids within the pool remainsfairly constant. Regardless of their source, any of these amino acids can be used to makebody proteins or other nitrogen-containing compounds, or they can be stripped of theirnitrogen and used for energy (either immediately or stored as fat for later use).Nitrogen balanceProtein turnover and nitrogen balance go hand in hand. In healthy adults, proteinsynthesis balances with degradation, and protein intake from food balances withnitrogen excretion in the urine, faeces and sweat. When nitrogen intake equals nitrogenoutput, the person is in nitrogen equilibrium, or zero nitrogen balance. Researchers usenitrogen balance studies to estimate protein requirements.If the body synthesises more than it degrades and adds protein, nitrogen statusbecomes positive. Nitrogen status is positive in growing infants, children, adolescents,pregnant women and people recovering from protein deficiency or illness; theirnitrogen intake exceeds their nitrogen output. They are retaining protein in newtissues as they add blood, bone, skin and muscle cells to their bodies.If the body degrades more than it synthesises and loses protein, nitrogen status becomesnegative. Nitrogen status is negative in people who are starving or suffering other severe stressessuch as burns, injuries, infections and fever; their nitrogen output exceeds their nitrogen intake.During these times, the body loses nitrogen as it breaks down muscle and other body proteinsfor energy.Amino acids (orproteins) that derivefrom within the body areendogenous. In contrast,those that derive fromfoods are exogenous.• endo 5 within• gen 5 arising• exo 5 outside (the body)Nitrogen balance:• nitrogen equilibrium(zero nitrogenbalance):N in 5 N out• positive nitrogen:N in . N out• negative nitrogen:N in , N outCells synthesise proteins according to the genetic information provided by the DNA in the nucleus ofeach cell. This information dictates the order in which amino acids must be linked together to form agiven protein. Sequencing errors occasionally occur, sometimes with significant consequences.The protein functions discussed here are summarised in the table below. They are only a few ofthe many roles proteins play but they convey some sense of the immense variety of proteins and theirimportance in the body. Growth and maintenanceProteins form integral parts of most body structures such as skin,tendons, membranes, muscles, organs and bones. As such, they supportthe growth and repair of body tissues.EnzymesProteins facilitate chemical reactions.HormonesProteins regulate body processes. (Some, but not all, hormones are proteins.)Fluid balanceProteins help to maintain the volume and composition of body fluids.Acid–base balanceProteins help maintain the acid–base balance of body fluids by acting asbuffers.TransportationProteins transport substances, such as lipids, vitamins, minerals and oxygen,around the body.AntibodiesProteins inactivate foreign invaders, thus protecting the body against diseases.Energy and glucoseProteins provide some fuel, and glucose if needed, for the body’s energyneeds. REVIEW ITGrowing children end each day with morebone, blood, muscle and skin cells thanthey had at the beginning of the day.amanaimages/Fabio Cardoso194 UNDERSTANDING NUTRITIONEleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.Using amino acids to make proteins or non-essential amino acidsAs mentioned, cells can assemble amino acids into the proteins they need to do their work. If aparticular non-essential amino acid is not readily available, cells can make it from another amino acid.If an essential amino acid is missing, the body may break down some of its own proteins to obtain it.Using amino acids to make other compoundsCells can also use amino acids to make other compounds. For example, the amino acid tyrosineis used to make the neurotransmitters noradrenaline and adrenaline, which relay nervoussystem messages throughout the body. Tyrosine can also be made into the pigment melanin,which is responsible for brown hair, eye and skin colour, or into the hormone thyroxine, whichhelps to regulate the metabolic rate. For another example, the amino acid tryptophan servesas a precursor for the vitamin niacin and for serotonin, a neurotransmitter important in sleepregulation, appetite control and sensory perception.Using amino acids for energy and glucoseAs mentioned earlier, when glucose or fatty acids are limited, cells are forced to use amino acidsfor energy and glucose. The body does not make a specialised storage form of protein as it does forcarbohydrate and fat. Glucose is stored as glycogen in the liver and fat as triglycerides in adiposetissue, but protein in the body is available only from the working and structural componentsof the tissues. When the need arises, the body breaks down its tissue proteins and uses theiramino acids for energy or glucose. Thus, over time, energy deprivation (starvation) always causeswasting of lean body tissue as well as fat loss. An adequate supply of carbohydrates and fats sparesamino acids from being used for energy and allows them to perform their unique roles.Using amino acids to make fatAmino acids may be used to make fat when energy and protein intakes exceed needsand carbohydrate intake is adequate. When protein is abundant, energy metabolismshifts to use more protein instead of fat. Excess amino acids can also be converted to fatand stored for later use. Consequently protein-rich foods can contribute to weight gain.Deaminating amino acidsWhen amino acids are broken down (as occurs when they are used for energy),they are first deaminated – stripped of their nitrogen-containing amino groups.Two products result from deamination: one is ammonia (NH3); the other productis the carbon structure without its amino group – often a keto acid. Keto acidsmay enter a number of metabolic pathways; for example, they may be used forenergy or for the production of glucose, ketones, cholesterol, or fat.* They mayalso be used to make non-essential amino acids.Using amino acids to make proteins and non-essentialamino acidsAs mentioned, cells can assemble amino acids into the proteins they need todo their work. If an essential amino acid is missing, the body may break downsome of its own proteins to obtain it. If a particular non-essential amino acid isnot readily available, cells can make it from a keto acid – if a nitrogen source isavailable. Ammonia provides some of the nitrogen needed for the synthesis ofnon-essential amino acids from keto acids (see Figure 6.11). Cells can also make a*Chemists sometimes classify amino acids according to the destinations of their carbon fragments after deamination. If thefragment leads to the production of glucose, the amino acid is called glucogenic; if it leads to the formation of ketone bodies,fats and sterols, the amino acid is called ketogenic. There is no sharp distinction between glucogenic and ketogenic aminoacids, however. A few are both, most are considered glucogenic, only one (leucine) is clearly ketogenic.H C NH2COOHCCOOHNH3OAmino acid Keto acidH C NH2COOHCCOOHNH3OKeto acid Amino acidSynthesisGiven a source of NH3, the body can makenon-essential amino acids from keto acids.DeaminationThe deamination of an amino acidproduces ammonia (NH3) and a keto acid.SidegroupSidegroupSidegroupSidegroupFIGURE 6.11 Deamination andsynthesis of a non-essential amino acidCHAPTER 6 PROTEIN: AMINO ACIDS 195Eleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.H C NH2COOHCCOOHO H C NH2COOHCCOOHOKeto acid A + Amino acid B Amino acid A Keto acid B++ +SidegroupSidegroupSidegroupSidegroupThe body can transfer amino groups (NH2) from an amino acid to a keto acid,forming a new non-essential amino acid and a new keto acid. Transaminationreactions require the vitamin B6 coenzyme.FIGURE 6.12 Transamination and synthesis of a non-essential amino acidH NHO CH N+ +Ammonia AmmoniaCarbondioxideHOHH O HO CH N N HH HUreaWaterHFIGURE 6.13 Urea synthesisWhen amino acids are deaminated,ammonia is produced. The liver detoxifiesammonia before releasing it into thebloodstream by combining it with anotherwaste product, carbon dioxide, toproduce urea. See Appendix C for details.BloodstreamLiver(NH3)+CO2 Urea BloodstreamKidney UreaTo bladder andout of body Urea AmmoniaAmino acidsFIGURE 6.14 Urea excretionThe liver and kidneys both play a rolein disposing of excess nitrogen. Canyou see why the person with liverdisease has high blood ammonia,whereas the person with kidney diseasehas high blood urea? (Figure 12.2 onpage 418 provides details of how thekidneys work.)non-essential amino acid by transferringan amino group from one amino acidto its corresponding keto acid, as shownin Figure 6.12. Through many suchtransamination reactions, involving manydifferent keto acids, the liver cells cansynthesise the non-essential amino acids.Converting ammonia to ureaAmmonia is a toxic compound chemically identical to the strong-smellingammonia in bottled cleaning solutions. Because ammonia is a base, the blood’scritical acid–base balance becomes upset if the cells produce larger quantities thanthe liver can handle.To prevent such a crisis, the liver combines ammonia with carbon dioxide tomake urea, a much less toxic compound. Figure 6.13 provides a greatly oversimplifieddiagram of urea synthesis; details are shown in Appendix C. The production of ureaincreases as dietary protein increases, until production hits its maximum rate atintakes approaching 250 grams per day.Excreting ureaLiver cells release urea into the blood, where it circulates until it passes through thekidneys (see Figure 6.14). The kidneys then filter urea out of the blood for excretionin the urine. Normally, the liver efficiently captures all the ammonia, makes ureafrom it and releases the urea into the blood; then the kidneys clear all the urea fromthe blood. This division of labour allows easy diagnosis of diseases of both organs.In liver disease, blood ammonia will be high; in kidney disease, blood urea will behigh.Urea is the body’s principal vehicle for excreting unused nitrogen, and theamount of urea produced increases with protein intake. To keep urea in solution,the body needs water. For this reason, a person who regularly consumes a highprotein diet (for example, 100 grams a day or more) must drink plenty of waterto dilute and excrete urea from the body. Without extra water, a person on ahigh-protein diet risks dehydration because the body uses its water to rid itselfof urea. This explains some of the water loss that accompanies high-proteindiets. Such losses may make high-protein diets appear to be effective, but waterloss, of course, is of no value to the person who wants to lose body fat (asHighlight 8 explains).196 UNDERSTANDING NUTRITIONEleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.PROTEIN IN FOODSLEARN ITExplain the differences between high-quality and low-quality proteins, including notable foodsources of each.In Australia and New Zealand, where nutritious foods are abundant, most people eat protein insuch large quantities that they receive all the amino acids they need. In countries where food isscarce and the people eat only marginal amounts of protein-rich foods, however, the quality ofthe protein becomes crucial.PROTEIN QUALITYThe protein quality of the diet determines, in large part, how well children grow and howwell adults maintain their health. Put simply, high-quality proteins provide enough of all theessential amino acids needed to support the body’s work, and low-quality proteins don’t. Twofactors influence protein quality – the protein’s digestibility and its amino acid composition.DigestibilityAs explained earlier, proteins must be digested before they can provide amino acids. Proteindigestibility depends on such factors as the protein’s source and the other foods eaten withit. The digestibility of most animal proteins is high (90 to 99 per cent); plant proteins are lessdigestible (70 to 90 per cent for most, but over 90 per cent for soy and legumes).Amino acid compositionTo make proteins, a cell must have all the needed amino acids available simultaneously. Theliver can produce any non-essential amino acid that may be in short supply so that the cells cancontinue linking amino acids into protein strands. If an essential amino acid is missing, though,a cell must dismantle its own proteins to obtain it. Therefore, to prevent protein breakdown,dietary protein must supply at least the nine essential amino acids plus enough nitrogencontaining amino groups and energy for the synthesis of the others. If the diet supplies too littleof any essential amino acid, protein synthesis will be limited. The body makes whole proteinsonly; if one amino acid is missing, the others cannot form a ‘partial’ protein. An essential aminoacid supplied in less than the amount needed to support protein synthesis is called a limitingamino acid.Reference proteinThe quality of a food protein is determined by comparing its amino acid composition withthe essential amino acid requirements of preschool-age children. Such a standard is called areference protein. The rationale behind using the requirements of this age group is that if aprotein will effectively support a young child’s growth and development, then it will meet orexceed the requirements of older children and adults.In the past, eggprotein was commonlyused as the referenceprotein. Table D.1 inAppendix D presentsthe amino acid profileof egg. As the referenceprotein, egg wasassigned the value of100; Table D.3 includesseveral other foodproteins for comparison.Proteins are constantly being synthesised and broken down as needed. The body’s assimilation of aminoacids into proteins and its release of amino acids via protein degradation and excretion can be trackedby measuring nitrogen balance, which should be positive during growth and steady in adulthood. Anenergy deficit or an inadequate protein intake may force the body to use amino acids as fuel, creating anegative nitrogen balance. Protein eaten in excess of need is degraded and stored as body fat.REVIEW ITCHAPTER 6 PROTEIN: AMINO ACIDS 197Eleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.High-quality proteinsA high-quality protein contains all the essential amino acids in relatively the sameamounts and proportions that human beings require; it may or may not containall the non-essential amino acids. Proteins that are low in an essential amino acidcannot, by themselves, support protein synthesis. Generally, foods derived fromanimals (meat, fish, poultry, cheese, eggs, yoghurt and milk) provide high-qualityproteins, although gelatin is an exception. (It lacks tryptophan and cannot supportgrowth and health as a diet’s sole protein.) Proteins from plants (vegetables, nuts,seeds, grains and legumes) have more diverse amino acid patterns and tend to belimiting in one or more essential amino acids. Some plant proteins are notoriouslylow quality (for example, corn protein). A few others are high quality (for example,soy protein).Researchers have developed several methods for evaluating the quality of foodproteins and identifying high-quality proteins; Appendix D provides details.Complementary proteinsIn general, plant proteins are lower quality than animal proteins, and plants also offerless protein (per weight or measure of food). For this reason, many vegetarians improvethe quality of proteins in their diets by combining plant-protein foods that have differentbut complementary amino acid patterns. This strategy yields complementary proteinsthat together contain all the essential amino acids in quantities sufficient to supporthealth. The protein quality of the combination is greater than for either food alone(see Figure 6.15).Many people have long believed that combining plant proteins at every meal is criticalfor protein nutrition. For most healthy vegetarians, though, it is not necessary to balanceamino acids at each meal if protein intake is varied and energy intake is sufficient.2Vegetarians can receive all the amino acids they need over the course of a day by eating avariety of whole grains, legumes, seeds, nuts and vegetables. Protein deficiency will develop,however, when fruits and certain vegetables make up the core of the diet, severely limitingboth the quantity and quality of protein. Highlight 2 describes how to plan a nutritiousvegetarian diet.PROTEIN REGULATIONS FOR FOOD LABELSAll food labels must state the quantity of protein in grams. The ‘% RDI per serve’ for protein isnot mandatory on labels but is increasingly used to highlight the contribution a food productcan make to overall daily protein intake.PUTTINGCOMMONSENSE TOTHE TESTFoods derivedfrom animals areconsidered highquality proteins.TRUELys Met Trp LegumesGrainsTogether IleFIGURE 6.15 Complementary proteinsIn general, legumes provide plentyof isoleucine (Ile) and lysine (Lys) butfall short in methionine (Met) andtryptophan (Trp). Grains have theopposite strengths and weaknesses,making them a perfect match for legumes.A diet that supplies all the essential amino acids in adequate amounts ensures protein synthesis. Thebest guarantee of amino acid adequacy is to eat foods containing high-quality proteins or mixtures offoods containing complementary proteins that can each supply the amino acids missing in the other. Inaddition to its amino acid content, the quality of protein is measured by its digestibility and its ability tosupport growth. Such measures are of great importance in dealing with malnutrition worldwide, but inAustralia and New Zealand, where protein deficiency is not common, protein quality scores of individualfoods deserve little emphasis.REVIEW IT198 UNDERSTANDING NUTRITIONEleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.HEALTH EFFECTS AND RECOMMENDEDINTAKES OF PROTEINLEARN ITIdentify the health benefits of, and recommendations for, protein.Protein is indispensable to life. Consequently it should come as no surprise that protein deficiencycan have devastating effects on people’s health. But, like the other nutrients, protein in excess canalso be harmful. This section examines the health effects and recommended intakes of protein.PROTEIN–ENERGY MALNUTRITIONWhen people are deprived of protein, energy or both, the result is protein–energy malnutrition(PEM). Although PEM touches many adult lives, it most often strikes early in childhood. It isone of the most prevalent and devastating forms of malnutrition in the world, afflicting one ofevery four children worldwide. In 2013 alone, the estimated number of children that died fromhunger was 3.1 million.3Inadequate food intake leads to poor growth in children and to weight loss and wasting inadults. Children who are thin for their height may be suffering from acute PEM (recent severefood deprivation), whereas children who are short for their age have experienced chronic PEM(long-term food deprivation). Poor growth due to PEM is easy to overlook because a small childmay look quite normal; however, it is the most common sign of malnutrition.PEM is most prevalent in Africa, Central America, South America and East and South-East Asia.In Australia and New Zealand, homeless people and those living in substandard housing in innercities and rural areas have been diagnosed with PEM. In addition to those living in poverty, elderlypeople who live alone and adults who are addicted to drugs and alcohol are frequently victims of PEM.PEM can develop in young children when parents mistakenly provide ‘health-food beverages’ thatlack adequate energy or protein instead of milk, most commonly because of nutritional ignorance,perceived milk intolerance or food faddism. Adult PEM is also seen in people hospitalised withinfections such as AIDS or tuberculosis; these infections deplete body proteins, demand extraenergy, induce nutrient losses and alter metabolic pathways. Furthermore, poor nutrient intakeduring hospitalisation worsens malnutrition and impairs recovery, whereas nutrition interventionoften improves the body’s response to other treatments and the chances of survival. PEM is alsocommon in those suffering from the eating disorder anorexia nervosa (discussed in Highlight 8).Prevention emphasises frequent, nutrient-dense, energy-dense meals and, equally importantly,resolution of the underlying causes of PEM – poverty, infections and illness.Classifying PEMPEM occurs in two forms: marasmus and kwashiorkor, which differ in their clinical features (seeTable 6.3). The following paragraphs present three clinical syndromes – marasmus, kwashiorkorand the combination of the two.MarasmusAppropriately named from the Greek word meaning ‘dying away’, marasmus reflects a severedeprivation of food over a long time (chronic PEM). Put simply, the person is starving and sufferingfrom an inadequate energy and protein intake (and inadequate essential fatty acids, vitamins andminerals as well). Marasmus occurs most commonly in children from six to 18 months of agein all the overpopulated and impoverished areas of the world. Children in impoverished nationssimply do not have enough to eat and subsist on diluted cereal drinks that supply scant energy andprotein of low quality; such food can barely sustain life, much less support growth. Consequently,marasmic children look like little old people – just skin and bones.Rice drinks are oftensold as milk alternatives,but they fail to provideadequate protein,vitamins and minerals.CHAPTER 6 PROTEIN: AMINO ACIDS 199Eleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.TABLE 6.3 Features of marasmus and kwashiorkor in childrenSeparating PEM into two classifications oversimplifies the condition, but at the extremes,marasmus and kwashiorkor exhibit marked differences. Marasmus-kwashiorkor mix presentssymptoms common to both marasmus and kwashiorkor. In all cases, children are likely todevelop diarrhoea, infections and multiple nutrient deficiencies. MARASMUSKWASHIORKORInfancy (younger than 2 years)Older infants and young children (1 to 3 years)Severe deprivation, or impaired absorption,of protein, energy, vitamins and mineralsInadequate protein intake or, more commonly,infectionsDevelops slowly; chronic PEMRapid onset; acute PEMSevere weight lossSome weight lossSevere muscle wasting, with no body fatSome muscle wasting, with retention of somebody fatGrowth: ,60% weight-for-ageGrowth: 60 to 80% weight-for-ageNo detectable oedemaOedemaNo fatty liverEnlarged fatty liverAnxiety, apathyApathy, misery, irritability, sadnessGood appetite possibleLoss of appetiteHair is sparse, thin and dry; easily pulled outHair is dry and brittle; easily pulled out;changes colour; becomes straightSkin is dry, thin and easily wrinklesSkin develops lesions Without adequate nutrition, muscles – including the heart – wasteand weaken. Because the brain normally grows to almost its fulladult size within the first two years of life, marasmus impairs braindevelopment and learning ability. Reduced synthesis of key hormonesslows metabolism and lowers body temperature. There is little or nofat under the skin to insulate against cold. Hospital workers find thatchildren with marasmus need to be clothed, covered and kept warm.Because these children often suffer delays in their mental and behaviouraldevelopment, they also need loving care, a stimulating environment andparental attention.The starving child faces this threat to life by engaging in as little activityas possible – not even crying for food. The body musters all its forces tomeet the crisis, so it cuts down on any expenditure of energy not needed forthe functioning of the heart, lungs and brain. Growth ceases; the child isno larger at age four than at age two. Enzymes are in short supply and theGI tract lining deteriorates. Consequently, the child can’t digest and absorbwhat little food is eaten.KwashiorkorKwashiorkor typically reflects a sudden and recent deprivation of food(acute PEM). Kwashiorkor is a Ghanaian word that refers to the birthorder of a child and is used to describe the illness a child develops whenthe next child is born. When a mother who has been nursing her firstGetty Images/Wesley BocxeThe extreme loss of muscle and fat characteristic ofmarasmus is apparent in this child’s ‘matchstick’ arms.200 UNDERSTANDING NUTRITIONEleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.child bears a second child, she weans the first child and puts the secondone on the breast. The first child, suddenly switched from nutrient-dense,protein-rich breast milk to a starchy, protein-poor cereal, soon beginsto sicken and die. Kwashiorkor typically sets in between 18 months and2 years.Kwashiorkor usually develops rapidly as a result of protein deficiencyor, more commonly, is precipitated by an illness such as measles or otherinfection.4 Other factors, such as aflatoxins (a contaminant sometimesfound in mouldy grains), may also contribute to the development of, orsymptoms that accompany, kwashiorkor.The loss of weight and body fat is usually not as severe in kwashiorkoras in marasmus, but some muscle wasting may occur. Proteins andhormones that previously maintained fluid balance diminish, and fluidleaks into the interstitial spaces. The child’s limbs and abdomen becomeswollen with oedema, a distinguishing feature of kwashiorkor. A fattyliver develops due to a lack of the protein carriers that transport fat out ofthe liver. The fatty liver lacks enzymes to clear metabolic toxins from thebody, so their harmful effects are prolonged. Inflammation in responseto these toxins and to infections further contributes to the oedema thataccompanies kwashiorkor. Without sufficient tyrosine to make melanin,the child’s hair loses its colour, and inadequate protein synthesis leavesthe skin patchy and scaly, often with sores that fail to heal. The lack ofproteins to carry or store iron leaves iron free. Unbound iron is commonin children with kwashiorkor and may contribute to their illnesses anddeaths by promoting bacterial growth and free-radical damage.Marasmus–kwashiorkor mixThe combination of marasmus and kwashiorkor is characterised by theoedema of kwashiorkor with the wasting of marasmus. Most often, the childsuffers the effects of both malnutrition and infections. Some researchersbelieve that kwashiorkor and marasmus are two stages of the same disease. They point outthat kwashiorkor and marasmus often exist side by side in the same community wherechildren consume the same diet. They note that a child who has marasmus can later developkwashiorkor. Some research indicates that marasmus represents the body’s adaptation tostarvation and that kwashiorkor develops when adaptation fails.InfectionsIn PEM, antibodies to fight off invading bacteria are degraded to provide amino acids forother uses, leaving the malnourished child vulnerable to infections. Blood proteins, includinghaemoglobin, are no longer synthesised, so the child becomes anaemic and weak. Dysentery,an infection of the digestive tract, causes diarrhoea, further depleting the body of nutrientsand fluids. In the marasmic child, once infection sets in, kwashiorkor often follows, and theimmune response weakens further.The combination of infections, fever, fluid imbalances and anaemia often leads toheart failure and occasionally to sudden death. Infections combined with malnutrition areresponsible for two-thirds of the deaths of young children in developing countries. Measles,which might make a healthy child sick for a week or two, kills a child with PEM within twoor three days.For this reason,kwashiorkor issometimes referredto as ‘wet’ PEM andmarasmus as ‘dry’ PEM.The oedema characteristic of kwashiorkor is apparentin this child’s swollen belly. Malnourished childrencommonly have an enlarged abdomen from parasitesas well.amanaimages/Stephen Morrison/epaCHAPTER 6 PROTEIN: AMINO ACIDS 201Eleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.HEALTH EFFECTS OF PROTEINWhile many of the world’s people struggle to obtain enough food energy and protein, inindustrialised countries both are so abundant that problems of excess are seen. Overconsumptionof protein offers no benefits and may pose health risks. High-protein diets have been implicatedin several chronic diseases, including heart disease, cancer, osteoporosis, obesity and kidneystones, but evidence is insufficient to establish an upper level.6Researchers attempting to clarify the relationships between excess protein and chronicdiseases face several obstacles. Population studies have difficulty determining whether diseasescorrelate with animal proteins or with their accompanying saturated fats,for example. Studies that rely on data from vegetarians must sort out themany lifestyle factors, in addition to a ‘no-meat diet’, that might explainrelationships between protein and health.Heart diseaseA high-protein diet may contribute to the progression of heart disease.As Chapter 5 mentions, foods rich in animal protein also tend tobe rich in saturated fats. Consequently, it is not surprising to finda correlation between animal-protein intake (red meats and dairyproducts) and heart disease.7 On the other hand, substituting vegetableprotein for animal protein improves blood lipids and decreases heartdisease mortality.8Research suggests that elevated levels of the amino acid homocysteinemay be an independent risk factor for heart disease, heart attacks andsudden death in patients with heart disease.9 Researchers do not yet fullyunderstand the many factors – including a high-protein diet – that canraise homocysteine in the blood or whether elevated levels are a causeor an effect of heart disease.10 Until they can determine the exact rolethat homocysteine plays in heart disease, researchers are following severalleads in pursuit of the answers. Elevated homocysteine levels are amongthe many adverse health consequences of smoking cigarettes and drinkingalcohol.11 Homocysteine is also elevated with inadequate intakes of Bvitamins and can usually be lowered with fortified foods or supplementsAPPLICATIONS OF NUTRITIONAL RESEARCHamanaimages/Faisal Isse/Xinhua PressDonated food saves some people from starvation,but it is usually insufficient to meet nutrient needs oreven to defend against hunger.Protein and rehabilitationIf caught in time, the life of a starving child may be saved with rehydration and nutritionintervention. In severe cases, diarrhoea will have caused dramatic fluid and mineral lossesthat need to be replaced during the first 24 to 48 hours to help raise the blood pressureand strengthen the heartbeat. After that, protein and food energy may be given in smallquantities several times a day, with intakes gradually increased as tolerated.5 Severelymalnourished people, especially those with oedema, recover better with an initial dietthat is relatively low in protein (10 per cent of energy intake).Experts assure us that we possess the knowledge, technology and resources to endhunger. Programs that tailor interventions to the local people and involve them in theprocess of identifying problems and devising solutions have the most success. To winthe war on hunger, those who have the food, technology and resources must makefighting hunger a priority.202 UNDERSTANDING NUTRITIONEleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.of vitamin B12, vitamin B6 and folate.12 Lowering homocysteine, however, may not help preventheart attacks. Supplements of the B vitamins do not always benefit those with heart diseaseand, in fact, may actually increase the risks.In contrast to homocysteine, the amino acid arginine may help protect against heart diseaseby lowering blood pressure and homocysteine levels.13 Additional research is needed to confirmthe benefits of arginine. In the meantime, it is unwise for consumers to use supplements ofarginine, or any other amino acid (as explained on page 205). Physicians, however, may find itbeneficial to add arginine supplements to their heart patients’ treatment plan.CancerAs in heart disease, the effects of protein and fats on cancers cannot be easily separated.Population studies suggest a correlation between high intakes of animal proteins and some typesof cancer (notably, cancers of the colon, breast, kidneys, pancreas and prostate); in particular,red meat and processed meats with cancer of the colon.Adult bone loss (osteoporosis)Chapter 12 presents calcium metabolism, and Highlight 12 elaborates on the main factorsthat influence osteoporosis. This section briefly describes the relationships between proteinintake and bone loss. When protein intake is high, calcium excretion increases. Whetherexcess protein depletes the bones of their chief mineral may depend upon the ratio ofcalcium intake to protein intake. After all, bones need both protein and calcium. An idealratio has not been determined, but a young woman whose intake meets recommendationsfor both nutrients has a calcium-to-protein ratio of more than 20 to 1 (milligrams tograms), which probably provides adequate protection for the bones. For most women inAustralia and New Zealand, however, average calcium intakes are lower and protein intakesare higher, yielding a 9 to 1 ratio, which may produce calcium losses significant enough tocompromise bone health. In other words, the problem may reflect too little calcium, nottoo much protein.14Some (but not all) research suggests that animal protein may be more detrimental to calciummetabolism and bone health than vegetable protein. Importantly, inadequate intakes of proteinmay compromise bone health. Osteoporosis is particularly common in elderly women and inadolescents with anorexia nervosa – groups who typically receive less protein than they need.For these people, increasing protein intake may be just what they need to protect their bones.Weight controlResearch on the associations between protein intake and body weight has revealed someinteresting, although often inconsistent, findings. One study suggests that inconsistent findingsmay reflect differences between animal and vegetable proteins, with animal proteins having apositive association with overweight and vegetable proteins having a negative one.15Another study examined people who were deliberately overfed by 1000 kcalories daily.16Not too surprisingly, they all gained weight, but those receiving a low-protein diet gainedabout half as much weight as those receiving an adequate- or high-protein diet. A look attheir body composition revealed that the low-protein group stored almost all their excesskcalories as fat and lost a little lean body tissue. By comparison, the other protein groupsstored about half the excess kcalories as fat and gained lean body tissue. Importantly, theexcess kcalories increased total body fat similarly for all groups; the different amountsof dietary protein affected changes in lean body mass.17 These findings highlight theimportance of distinguishing between body weight and body fat – a point revisited inChapters 8 and 9.Fad weight-loss diets that encourage a high-protein, low-carbohydrate diet may be effective,but only because they are low-kilojoule diets. Diets that provide adequate protein, moderatefat and sufficient energy from carbohydrates can better support weight loss and good health.Processed meatsinclude ham, bacon,corned beef, pastrami,salami, sausage,bratwurst, and hotdogs; they have beenpreserved by smoking,curing, salting or addingpreservatives.CHAPTER 6 PROTEIN: AMINO ACIDS 203Eleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.Including protein at each meal may help with weight loss by providing satiety.18 Selecting toomany protein-rich foods, such as meat and milk, may crowd out fruits, vegetables and wholegrains, making the diet inadequate in other nutrients.Kidney diseaseExcretion of the end products of protein metabolism depends, in part, on an adequate fluidintake and healthy kidneys. A high-protein intake does not cause kidney disease, but it doesincrease the work of the kidneys and accelerate kidney deterioration in people with chronickidney disease.19 Restricting dietary protein may help to slow the progression of kidney diseasein people who have this condition.20RECOMMENDED INTAKES OF PROTEINThe body continuously breaks down and loses some protein and it cannotstore amino acids. To replace protein, the body needs dietary protein for tworeasons. First, food protein is the only source of the essential amino acids,and second, it is the only practical source of nitrogen with which to buildthe non-essential amino acids and other nitrogen-containing compoundsthe body needs.Given recommendations that people’s fat intakes should contribute20 to 35 per cent of total food energy, and carbohydrate intakes shouldcontribute 45 to 65 per cent, that leaves 10 to 35 per cent for protein. In an8000-kilojoule diet, that represents 800 to 2800 kilojoules from protein, or50 to 165 grams. Average intakes in Australia and New Zealand fall withinthis range.Protein RDIThe protein RDI for adult males is 0.84 grams per kilogram of body weight per day; for femalesit is 0.75 grams per kilogram of body weight. For infants, children, pregnant women during thesecond and third trimesters, breastfeeding women and people over 70 years of age, the RDI isslightly higher.The RDI generously covers the needs for replacing worn-out tissue, so it increases for largerpeople; it also covers the needs for building new tissue during growth, so it increases for infants,children, and pregnant and lactating women.21 The protein RDI is the same for athletes as forothers, even though athletes may need more protein and many fitness authorities recommend ahigher range of protein intakes for athletes pursuing different activities (seeTable 14.5 for details).In setting the RDI, the committee responsible assumes that people arehealthy and do not have unusual metabolic needs for protein, that theprotein eaten will be of mixed quality (from both high- and low-qualitysources) and that the body will use the protein efficiently. In addition, thecommittee assumes that the protein is consumed along with sufficientcarbohydrate and fat to provide adequate energy, and that other nutrientsin the diet are adequate.Protein in abundanceMost people in Australia and New Zealand receive more protein thanthey need. Even athletes in training typically don’t need to increase theirprotein intakes because the additional foods they eat to meet their highenergy needs deliver protein as well. (Chapter 14 provides full details ofthe energy and protein needs of athletes.) That protein intake is high isRDI for protein:• males (19–50 years):64 g protein per day(0.84 g/kg/day)• women (19–50 years):46 g protein per day(0.75 g/kg/day)• 10 to 35% of energyintakePolara Studios, Inc.Vegetarians obtain their protein from whole grains,legumes, nuts, vegetables and, in some cases, eggsand milk products.Shutterstock.com/Jacek ChabraszewskiFor many people, this 150 gram steak providesalmost all of the meat and much of the proteinrecommended for a day’s intake.204 UNDERSTANDING NUTRITIONEleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.not surprising considering the abundance of food eaten and the central role meats hold inour diet. Besides meat, well-fed people eat many other nutritious foods, many of which alsoprovide protein. A cup of milk provides 8 grams of protein. Grains and vegetables provide smallamounts of protein, but they can add up to significant quantities; fruits and fats provide noprotein.The optimal diet is adequate in energy (kilojoules) from carbohydrate and fat. It delivers approximately0.84 g/kg/day of protein for healthy-weight males and 0.75 g/kg/day of protein for healthy-weightfemales. The typical Australian and New Zealand diet more than adequately meets these optimalprotein needs.REVIEW ITPROTEIN AND AMINO ACID SUPPLEMENTSWebsites, health-food stores and popular magazine articles advertise a wide variety of proteinsupplements, and people take these supplements for many different reasons. Athletes take proteinpowders to build muscle. Dieters take them to spare their body’s protein while losing weight.Women take them to strengthen their fingernails. People take individual amino acids, too – tocure herpes, to make themselves sleep better, to lose weight, and to relieve pain and depression.Like many other magic solutions to health problems, protein and amino acid supplementsdon’t work these miracles. Furthermore, they may be harmful.Protein powdersBecause the body builds muscle protein from amino acids, many athletes take protein powderswith the hope of stimulating muscle growth. Muscle work builds muscle; protein supplementsmay assist this.22 (Highlight 14 presents more information on other supplements athletescommonly use.) Protein powders can supply amino acids to the body, but nature’s proteinsources – lean meat, milk, eggs and legumes – supply all these amino acids and more.Whey protein appears to be particularly popular among athletes hoping to achieve greatermuscle gain. A waste product of cheese manufacturing, whey protein is a common ingredientin many low-cost protein powders. When combined with strength training, whey supplementsmay increase protein synthesis slightly, but they do not seem to enhance athletic performance.To build stronger muscles, athletes need to eat food with adequate energy and protein to supportthe weight-training work that does increase muscle mass. Those who still think they need morewhey can drink a glass of milk; one cup provides 1.5 grams of whey.Purified protein preparations contain none of the other nutrients needed to support thebuilding of muscle, and the protein they supply is not needed by athletes who eat food. It isexcess protein, and the body dismantles it and uses it for energy or stores it as body fat. Thedeamination of excess amino acids places an extra burden on the kidneys to excrete unusednitrogen.Amino acid supplementsSingle amino acids do not occur naturally in foods and offer no benefit to the body; in fact,they may be harmful. The body was not designed to handle the high concentrations andunusual combinations of amino acids found in supplements. Large doses of amino acids causediarrhoea. An excess of one amino acid can create such a demand for a carrier that it limitsthe absorption of another amino acid, presenting the possibility of a deficiency. Those aminoacids winning the competition enter in excess, creating the possibility of toxicity. Toxicity ofsingle amino acids in animal studies raises concerns about their use in human beings. Anyoneconsidering taking amino acid supplements should be cautious not to exceed levels normallyfound in foods.Use of amino acids asdietary supplements isinappropriate, especiallyfor:• all women of childbearing age• pregnant or lactatingwomen• infants, children andadolescents• elderly people• people with inbornerrors of metabolismthat affect theirbody’s handling ofamino acids• smokers• people on low-proteindiets• people with chronicor acute mental orphysical illnesseswho take amino acidswithout medicalsupervision.CHAPTER 6 PROTEIN: AMINO ACIDS 205Eleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.Most healthy athletes eating well-balanced diets do not need amino acid supplements.Advertisers point to research that identifies the branched-chain amino acids as the mainones used as fuel by exercising muscles. What the advertisements leave out is that compared toglucose and fatty acids, branched-chain amino acids provide very little fuel and that ordinaryfoods provide them in abundance anyway. Large doses of branched-chain amino acids can raiseplasma ammonia concentrations, which can be toxic to the brain. Branched-chain amino acidsupplements may be beneficial in conditions such as liver disease, but otherwise, they are notroutinely recommended.The branched-chainamino acids are leucine,isoleucine and valine.CURRENT RESEARCH IN NUTRITIONFighting sarcopeniaSarcopenia is the wasting of muscles prevalent in the very old and frail. The mechanismdriving muscle wasting in the elderly is poorly understood; however, ageing is oftenassociated with anorexia, immobility and ill health, which together can result inaccelerated muscle loss. The benefits of a balanced protein and energy supplementationare being increasingly explored and analysed.23Protein deficiencies arise from both energy-poor and protein-poor diets and lead to the devastatingdiseases of marasmus and kwashiorkor. Together, these diseases are known as PEM (protein-energymalnutrition), a major form of malnutrition causing death in children worldwide. Excesses of proteinoffer no advantage; in fact, overconsumption of protein-rich foods may induce health problems as well.The optimal diet is adequate in energy from carbohydrate and fat and delivers approximately64 grams of protein per day (or 0.84 g/kg/day) for healthy weight males and 46 grams of proteinper day (0.75 g/kg/day) for healthy weight females. The typical Australian and New Zealand dietmore than adequately meets these optimal protein values.Healthy people never need protein or amino acid supplements. It is safest to obtain lysine,tryptophan and all other amino acids from protein-rich foods, eaten with abundant carbohydrate andsome fat to facilitate their use in the body. With all that we know about science, it is hard to improveon nature.REVIEW IT206 UNDERSTANDING NUTRITIONEleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.207ONLINE STUDY TOOLSCHAPTER ACTIVITIESPUTTING COMMONSENSE TO THE TEST: ANSWERS1 Meat is the most important source of protein in thediet. FALSEAlthough meat and other animal products are a good sourceof protein, too much focus on meat as a protein source maymean that people exclude other sources of protein thatcontribute to a well-balanced diet.2 When proteins are denatured they cease beingproteins. FALSEThe denaturing of proteins means that their shape ischanged and hence their function is lost. However, theyremain proteins. Think of an egg as a good example of thisoccurrence.3 Amino acids are the building blocks of proteins. TRUEAmino acids are the simplest structures of proteins. Combiningamino acids allows the formation of all the different proteinsand all their different functions in the human body.4 Proteins have many roles in the body, including that ofenergy provision through glucose production. TRUEAlthough the body prefers to use proteins for structure andfunction, it will also use protein for energy production.5 Foods derived from animals are considered highquality proteins. TRUEEssentially all foods contain some protein but proteins that arederived from animals are considered high-quality proteins.Foods that derive from animals – meats, fish, poultry,eggs and milk products – provide plenty of protein but areoften accompanied by fat. Those that derive from plants –whole grains, vegetables and legumes – may provide lessprotein but also less fat.• Calculate your daily protein needs and compare themwith your protein intake. Consider whether you receiveenough, but not too much, protein daily.• Describe your dietary sources of proteins and whetheryou use mostly plant-based or animal-based proteinfoods in your diet.Look at the intake vs goals report:• How do your protein needs compare with your proteinintake? Consider whether you receive enough, butnot too much, protein daily. Remember, 100 per centmeans your intake is meeting your needs based onyour intake and profile information.• If your protein intake exceeds 100 per cent, considerthe possible negative consequences of a high proteinintake over many years. Debate the risks and benefitsof taking protein or amino acid supplements.NUTRITION PORTFOLIOVisit http://login.cengagebrain.com and use the accesscode that comes with this book for 12 months’ access tothe CourseMate resources and study tools for this chapter:• Complete your Nutrition portfolio• Take the revision quiz• Try out the interactive Nutrition calculations• Watch the Animations• Revisit the chapter with the integrated eBook• Try out an interactive version of the ‘How to’ activities,and more!Eleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.Multiple choice questionsAnswers can be found at the back of the book.1 Which part of its chemical structure differentiatesone amino acid from another? abcdits side groupits acid groupits amino groupits double bonds2The connection of amino acids to each other occursthrough what kind of reaction?abcdredoxnuclearcondensationdehydration 3 In the stomach, hydrochloric acid:a denatures proteins and activates pepsinb hydrolyses proteins and denatures pepsinc emulsifies proteins and releases peptidased condenses proteins and facilitates digestion4 Proteins that maintain the acid–base balance of theblood and body fluids by accepting and releasinghydrogen ions are: abcdbuffersenzymeshormonesantigens5When proteins are deaminated, what substance isproduced?abcdureaketonesammonianitrogen 6 Protein turnover describes the amount of protein: abcdfound in foods and the bodyabsorbed from the dietsynthesised and degradedused to make glucose7Which of the following foods provides the highestquality protein? abceggcorngelatind whole grainsMarasmus develops from:8abcdtoo much fat clogging the livermegadoses of amino acid supplementsinadequate protein and energy intakeexcessive fluid intake causing oedema 9The strategy of combining plant-protein foods thathave different amino acids patterns is known as:aessential protein combiningb vegetarian protein combiningcdcomplementary proteinsprotein matching 10 Which of these foods has the least protein per ½ cup? abcdricebroccolikidney beansorange juiceReview questions1How does the chemical structure of proteins differfrom the structures of carbohydrates and fats?(p. 181) 2 Describe the structure of amino acids, and explainhow their sequence in proteins affects the proteins’shapes. (pp. 181–182)3 What are essential amino acids? Can humans produceessential amino acids? When might an amino acid be‘conditionally essential’? (pp. 182–183)4 Describe protein digestion and absorption. (pp. 185–187)5 Describe protein synthesis. (pp. 187–190)6 What are enzymes? What roles do they play inchemical reactions? Describe the differencesbetween enzymes and hormones. (pp. 187–189)7 How does the body use proteins as a regulator of fluidbalance? As an acid–base regulator? (pp. 191–192)8 What is the body’s preferred use of amino acids? Canamino acids be used to make glucose? What is thisprocess called? (p. 193)9 How can vegetarians meet their protein needswithout eating meat? (p. 198)10 What are the health consequences of ingestinginadequate protein and energy? Describe marasmusand kwashiorkor. How can the two conditions bedistinguished, and in what ways do they overlap?(pp. 199–200)11 How might protein excess, or the type of proteineaten, influence health? (pp. 202–204)12 What factors are considered in establishingrecommended protein intakes? (pp. 204–205)13 What are the benefits and risks of taking protein andamino acid supplements? (pp. 205–206)STUDY QUESTIONS208 UNDERSTANDING NUTRITIONEleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.These problems will give you practice in doing simplenutrition-related calculations using hypotheticalsituations (answers can be found in the Answers sectionat the back of this book). Once you have mastered theseexamples, you will be prepared to examine your ownprotein needs. Be sure to show your calculations for eachproblem.1 Compute recommended protein intakes for peopleof different sizes. The intake for a woman who weighs65 kilograms is computed for you as an example:0.75 g/kg 3 65 kg 5 48.8 g protein per day. aba woman who weighs 70 kilogramsa man (25 years old) who weighs 83 kilograms 2 Recommendations for protein based on percentageof energy intake are not appropriate, especially forpeople on low-energy diets. Consider a woman whois 26 years old and weighs 52 kilograms. Her dietprovides 5200 kJ per day with 50 grams carbohydrateand 100 grams fat. aWhat is this woman’s protein intake? Show yourcalculations.Is her protein intake appropriate? Justify youranswer.Are her carbohydrate and fat intakes appropriate?Justify your answer.bc This exercise should help you develop a perspectiveon protein recommendations.SEARCH ME! NUTRITIONKeyword: protein supplementsProtein supplements are taken by many recreational and elite athletes. Read the article Dietary supplements and sportsperformance: amino acids. Is there any merit in athletes’ preferencing protein supplements over a normal diet?NUTRITION CALCULATIONSNUTRITION ON THE NETAnalyse the nutrient composition of foods online: Tolearn more about the nutrient content of the foods youeat, you can access the full NUTTAB Food CompositionDatabase provided by Food Standards Australia NewZealand from http://www.foodstandards.gov.au/science/monitoringnutrients/nutrientables• Learn more about sickle-cell anaemia from the USNational Heart, Lung and Blood Institute or the Sickle CellDisease Association of America: http://www.nhlbi.nih.govor http://www.sicklecelldisease.org• Learn more about protein–energy malnutrition andworld hunger from the World Health OrganizationNutrition Programme or the US National Institute ofChild Health and Human Development: http://www.who.int/nut or http://www.nichd.nih.govChapter 20 offers many more websites on malnutritionand world hunger.CHAPTER 6 PROTEIN: AMINO ACIDS 209Eleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.NUTRITIONAL GENOMICSLEARN ITExplain how nutrients influence gene activity (nutrigenomics)and how genes influence the activities of nutrients(nutrigenetics).Imagine this scenario: A physician scrapes a sample ofcells from inside your cheek and submits it to a genomicslab. The lab returns a report based on your genetic profilethat reveals which diseases you are most likely to developand makes recommendations for specific diet and lifestylechanges that can help you maintain good health. You mayalso be given a prescription for a dietary supplement thatwill best meet your personal nutrient requirements. Such ascenario may one day become reality as scientists uncoverthe genetic relationships between diet and disease. (Untilthen, however, consumers need to know that currentgenetic test kits commonly available on the Internet areunproven and quite likely fraudulent.)Can your specific diet and lifestyle needs be decided in a laboratory?Shutterstock.com/Darren BakerHIGHLIGHT 6210A over the past several decades, for example. Biochemistryrevealed vitamin A’s three chemical structures.Immunology identified the anti-infective propertiesof one of these structures, while physiology focused onanother structure and its role in vision. Epidemiology hasreported improvements in the death rates and vision ofmalnourished children given vitamin A supplements, andbiology has explored how such effects might be possible.The process was slow as researchers collected informationon one gene, one action and one nutrient at a time.Today’s research in nutritional genomics involves all ofthe sciences, coordinating their multiple findings, andexplaining their interactions among several genes, actionsand nutrients in relatively little time. As a result, nutritionknowledge is growing at an incredibly fast pace.The recent surge in genomics research grew fromthe Human Genome Project, an international effortby industry and government scientists to identify anddescribe all of the genes in the human genome – that is,all the genetic information contained within a person’scells. Completed in 2003, this project developed manyof the research technologies needed to study genes andgenetic variation. Scientists are now working to identifythe individual proteins made by the genes, the genesassociated with diseases, and the dietary and lifestylechoices that most influence the expression of those genes.Such information will have major implications for societyin general, and for healthcare in particular.1A GENOMICS PRIMERFigure H6.1 shows the relationships among the materialsthat comprise the genome. As the discussion of proteinsynthesis in Chapter 6 points out, genetic information isencoded in DNA molecules within the nucleus of cells.The DNA molecules and associated proteins are packedwithin 46 chromosomes. The genes are segments of a DNAstrand that can eventually be translated into one or moreproteins. The sequence of nucleotide bases within eachgene determines the amino acid sequence of a particularprotein. Scientists currently estimate that there are between20000 and 25000 genes in the human genome.How nutrients influence gene activity and how genesinfluence the activities of nutrients is the focus of anew field of study called nutritional genomics. Unlikesciences in the 20th century, nutritional genomics takesa comprehensive approach in analysing informationfrom several fields of study, providing an integratedunderstanding of the findings. Consider how multipledisciplines contributed to our understanding of vitaminEleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.HIGHLIGHT 6 NUTRITIONAL GENOMICSAs Figure 6.7 (page 188) explained, when cells makeproteins, a DNA sequence is used to make messengerRNA. The nucleotide sequence in messenger RNA thendetermines the amino acid sequence to make a protein.This process – from genetic information to proteinsynthesis – is known as gene expression. Gene expressioncan be determined by measuring the amounts ofmessenger RNA in a tissue sample. Microarray technologyallows researchers to detect messenger RNA and analysethe expression of thousands of genes simultaneously.Simply having a certain gene does not determine thatits associated trait will be expressed; the gene has to beactivated. (Similarly, owning lamps does not ensure youwill have light in your home unless you turn them on.)Nutrients are among many environmental factors that playkey roles in either activating or silencing genes. Switchinggenes on and off does not change the DNA itself, but it canhave dramatic consequences for a person’s health.The area of study that examines how environmentalfactors influence gene expression without changing theDNA is known as epigenetics. To turn genes on, enzymesattach proteins near the beginning of a gene. If enzymesattach a methyl group (CH3) instead, the protein isblocked from binding to the gene and the gene remainsswitched off. Other factors influence gene expression aswell, but methyl groups are currently the best understood.They also are known to have dietary connections.The photo of two mice illustrates epigenetics and howdiet can influence genetic traits such as hair colour andA chromosome is made of DNAand associated proteins.The human genome is a completeset of genetic material organisedinto 46 chromosomes, locatedwithin the nucleus of a cell.The double helical structure of aDNA molecule is made up of twolong chains of nucleotides. Eachnucleotide is composed of aphosphate group, a 5-carbonsugar and a base.The sequence of nucleotidebases (C, G, A, T) determinesthe amino acid sequence ofproteins. These bases areconnected by hydrogen bondingto form base pairs—adenine (A)with thymine (T) and guanine (G)with cytosine (C).A gene is a segment of DNA thatincludes the information needed tosynthesise one or more proteins.112 3 4 52345NucleusChromosomeDNACellGeneCCCCG GGGTTTTTTAAAAAAAFIGURE H6.1 The human genomeAdapted from US Department of Energy Office of Science, A Primer: From DNA to Life, available athttp://www.ornl.gov/sci/techresources/Human_Genome/publicat/primer2pager.pdf.Both of these mice have the gene that tends to produce fat yellowpups, but their mothers had different diets. The mother of the mouseon the right received a dietary supplement, which silenced the gene,resulting in brown pups with normal appetites.Jirtle and Waterland211Eleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.body weight. Both mice have a gene that tends to producefat yellow pups, but their mothers were given differentdiets. The mother of the mouse on the right was given adietary supplement containing the B vitamins folate andvitamin B12. These nutrients silenced the gene for ‘yellowand fat’, resulting in brown pups with normal appetites.As Chapter 10 explains, one of the main roles of theseB vitamins is to transfer methyl groups. In the case of thesupplemented mice, methyl groups migrated onto DNAand shut off several genes, thus producing brown coatsand protecting against the development of obesity andsome related diseases. Keep in mind that these changesoccurred epigenetically. In other words, the DNA sequencewithin the genes of the mice remained the same.Whether silencing or activating a gene is beneficialor harmful depends on what the gene does. Silencing agene that stimulates cancer growth, for example, wouldbe beneficial, but silencing a gene that suppresses cancergrowth would be harmful. Similarly, activating a genethat defends against obesity would be beneficial, butactivating a gene that promotes obesity would be harmful.Much research is under way to determine which nutrientsactivate or silence which genes.GENETIC VARIATION AND DISEASEExcept for identical twins, no two persons are geneticallyidentical. The variation in the genomes of any two persons,however, is only about 0.1 per cent, a difference of onlyone nucleotide base in every 1000. Yet it is this incrediblysmall difference that makes each of us unique and explainswhy, given the same environmental influences, some of usdevelop certain diseases and others do not. Similarly, geneticvariation explains why some of us respond to interventionssuch as diet and others do not. For example, following adiet low in saturated fats will significantly lower LDLcholesterol for most people, but the degree of change variesdramatically among individuals, with some people havingonly a small decrease or even a slight increase. In otherwords, dietary factors may be more helpful or more harmfuldepending on a person’s particular genetic variations.2(Such findings help to explain some of the conflictingresults from research studies.) The goal of nutritionalgenomics is to custom design specific recommendationsthat fit the needs of each individual. Such personalisedrecommendations are expected to provide more effectivedisease prevention and treatment solutions.Diseases characterised by a single-gene disorder aregenetically predetermined, usually exert their effects earlyin life and greatly affect those touched by them, but arerelatively rare. The cause and effect of single-gene disordersis clear – those with the genetic defect get the disease andthose without it don’t. In contrast, the more commondiseases, such as heart disease and cancer, are influencedby many genes and typically develop over several decades.These chronic diseases have multiple genetic componentsthat predispose the prevention or development of a disease,depending on a variety of environmental factors (such assmoking, diet and physical activity).3 Both types are ofinterest to researchers in nutritional genomics.Single-gene disordersSome disorders are caused by mutations in single genesthat are inherited at birth. The consequences of amissing or malfunctioning protein can seriously disruptmetabolism and may require significant dietary or medicalintervention. A classic example of a diet-related, singlegene disorder is phenylketonuria (PKU).Approximately one in every 15000 infants inindustrialised countries is born with PKU. PKU arisesfrom mutations in the gene that codes for the enzyme thatconverts the essential amino acid phenylalanine to theamino acid tyrosine. Without this enzyme, phenylalanineand its metabolites accumulate and damage the nervoussystem, resulting in mental retardation, seizures andbehaviour abnormalities. At the same time, the bodycannot make tyrosine or compounds made from it (suchas the neurotransmitter adrenaline). Consequently,tyrosine becomes an essential amino acid – because thebody cannot make it, the diet must supply it.Although the most debilitating effect of PKU is onbrain development, other symptoms become evident if thecondition is left untreated. Infants with PKU may havepoor appetites and grow slowly. They may be irritable orhave tremors or seizures. Their bodies and urine may havea musty odour. Their skin colouring may be unusuallypale, and they may develop skin rashes.The effect of nutrition intervention in PKU isremarkable. In fact, the only current treatment forPKU is a diet that restricts phenylalanine and suppliestyrosine to maintain blood levels of these amino acidswithin safe ranges. Because all foods containing proteinprovide phenylalanine, the diet must depend on aformula to supply a phenylalanine-free source of energy,protein, vitamins and minerals. If the restricted diet isconscientiously followed, the symptoms can be prevented.Because phenylalanine is an essential amino acid, the dietcannot exclude it completely. Children with PKU needphenylalanine to grow, but they cannot handle excesseswithout detrimental effects. Therefore, their diets mustprovide enough phenylalanine to support normal growthand health but not enough to cause harm. The diet mustalso provide tyrosine. To ensure that blood concentrationsof phenylalanine and tyrosine are close to normal,children and adults who have PKU must have blood testsperiodically and adjust their diets as necessary.Multigene disordersIn multigene disorders, each of the genes can influencethe progression of a disease, but no single gene causes thedisease on its own. For this reason, genomics researchers212 UNDERSTANDING NUTRITIONEleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.HIGHLIGHT 6 NUTRITIONAL GENOMICSmust study the expression and interactions of multiplegenes. Because multigene disorders are often sensitive tointeractions with environmental influences, they are notas straightforward as single-gene disorders. Heart diseaseprovides an example of a chronic disease with multiplegene and environmental influences. Consider that majorrisk factors for heart disease include elevated bloodcholesterol levels, obesity, diabetes and hypertension, yetthe underlying genetic and environmental causes of any ofthese individual risk factors is not completely understood.Genomic research can reveal details about each of theserisk factors. For example, tests could determine whetherblood cholesterol levels are high due to increasedcholesterol absorption or production, or because ofdecreased cholesterol degradation. This information couldthen guide physicians and dietitians to prescribe the mostappropriate medical and dietary interventions from amongmany possible solutions. Today’s dietary recommendationsadvise a low-fat diet, which helps people with a small typeof LDL but not those with the large type. In fact, a low-fatdiet is actually more harmful for people with the large type.Finding the best option for each person will be a challengegiven the many possible interactions between genes andenvironmental factors and the millions of possible genevariations in the human genome that make each individualunique.4The results of genomic research are helping to explainfindings from previous nutrition research. Considerdietary fat and heart disease, for example. As Highlight 5explains, epidemiological and clinical studies have foundthat a diet high in unsaturated fatty acids often helps tomaintain a healthy blood lipid profile. Now genetic studiesoffer an underlying explanation of this relationship:diets rich in polyunsaturated fatty acids activate genesresponsible for making enzymes that break down fats andsilence genes responsible for making enzymes that makefats.5 Both actions change fat metabolism in the directionof lowering blood lipids.To learn more about how individuals respond to diet,researchers examine the genetic differences between people.The most common genetic differences involve a change ina single nucleotide base located in a particular region ofa DNA strand – thymine replacing cytosine, for example.Such variations are called single nucleotide polymorphisms(SNPs), and they commonly occur throughout thegenome. Many SNPs (commonly pronounced ‘snips’)have no effect on cell activity. In fact, SNPs are significantonly if they affect the amino acid sequence of a proteinin a way that alters its function and if that function iscritical to the body’s wellbeing. Research on a gene thatplays a key role in lipid metabolism reveals differences ina person’s response to diet depending on whether the genehas a common SNP. People with the SNP have lower LDLwhen eating a diet rich in polyunsaturated fatty acids –and higher LDL with a low intake – than those withoutthe SNP.6 These findings clearly show how diet (in thiscase, polyunsaturated fat) interacts with a gene (in thiscase, a fat metabolism gene with a SNP) to influence thedevelopment of a disease (changing blood lipids implicatedin heart disease). The quest now is to identify the geneticcharacteristics that predict various responses to dietaryrecommendations.CLINICAL CONCERNSBecause multigene chronic diseases are common, anunderstanding of the human genome will have widespreadramifications for healthcare. This new understanding ofthe human genome is expected to change healthcare by:• providing knowledge of an individual’s geneticpredisposition to specific diseases• allowing physicians to develop ‘designer’ therapies –prescribing the most effective schedule of screening,behaviour changes (including diet) and medicalinterventions based on each individual’s genetic profile• enabling manufacturers to create new medications foreach genetic variation so that physicians can prescribethe best medicine in the exact dose and frequency toenhance effectiveness and minimise the risks of sideeffects• providing a better understanding of the non-geneticfactors that influence disease development.Enthusiasm surrounding genomic research needs tobe put into perspective, however, in terms of the presentstatus of clinical medicine as well as people’s willingnessto make difficult lifestyle choices. Critics have questionedwhether genetic markers for disease would be more usefulthan simple clinical measurements, which reflect bothgenetic and environmental influences. In other words,knowing that a person is genetically predisposed to havehigh blood cholesterol is not necessarily more usefulthan knowing the person’s actual blood cholesterol level.Furthermore, if a disease has many genetic risk factors,each gene that contributes to susceptibility may have littleinfluence on its own, so the benefits of identifying anindividual genetic marker might be small. The long-rangepossibility is that many genetic markers will eventually beidentified, and the hope is that the combined informationwill be a useful and accurate predictor of disease.Having the knowledge to prevent disease and actuallytaking action do not always coincide. Despite theabundance of current dietary recommendations, peopleseem unwilling to make behaviour changes known toimprove their health. For example, it has been estimatedthat heart disease and type 2 diabetes are 90 per centpreventable when people adopt an appropriate diet,maintain a healthy body weight and exercise regularly.7Yet these two diseases remain among the leading causesof death. Given the difficulty that people have withcurrent recommendations, it may be unrealistic toexpect that many of them will enthusiastically adopt aneven more detailed list of lifestyle modifications. Then213Eleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.again, compliance may be better when it is supported byinformation based on a person’s own genetic profile.The debate over nature versus nurture – whether genesor the environment are more influential – has quieted. Thefocus has shifted. Scientists acknowledge the importantroles of each and understand that the real answers liewithin the myriad interactions. Current research issorting through how nutrients (and other dietary factors)and genes confer health benefits or risks. Answers fromgenomic research may not become apparent for years tocome, but the opportunities and rewards may prove wellworth the efforts.HIGHLIGHT ACTIVITIESCRITICAL THINKING QUESTIONSA Your friend has decided that they are going toadopt a vegan diet, devoid of all animal products.Your nutrition knowledge tells you that there area number of nutrients of concern when it comesto vegan diets, one of which is protein. You don’tthink your friend has educated themselves aboutthe various nutrients that can be affected and youwant to help them. What is your advice? How do youexplain the concept of high quality proteins? Whatare you going to say when it comes to explainingabout complementary proteins?NUTRITION ON THE NETAnalyse the nutrient composition of foods online: Tolearn more about the nutrient content of the foods youeat, you can access the full NUTTAB Food CompositionDatabase provided by Food Standards Australia NewZealand from http://www.foodstandards.gov.au/consumerinformation/nuttab2010• Get information about human genomic discoveriesand how they can be used to improve health fromthe Public Health Genomics site of the US Centersfor Disease Control and Prevention: http://www.cdc.gov/genomics214 UNDERSTANDING NUTRITIONEleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.REFERENCESCHAPTERHIGHLIGHT1 M. Brown, Managing the acutely ill adult with sickle cell disease,British Journal of Nursing 21 (2012): 90–96.2 Position of the American Dietetic Association and Dietitiansof Canada: Vegetarian diets, Journal of the American DieteticAssociation 109 (2009): 1266–1282.3 World Hunger Education Services (WHES), Hunger notes (2015),available at http://www.worldhunger.org/articles/Learn/child_hunger_facts.htm4 H. Kismul and co-authors, Diet and kwashiorkor: a prospectivestudy from rural DR Congo, PeerJ 15 (2014): e350 https://dx.doi.org/10.7717/peerj.3505 World Health Organization, Guideline: Updates on the managementof severe acute malnutrition in infants and children (2013).6 S. R. Preis and co-authors, Dietary protein and risk of ischemicheart disease in middle-aged men, American Journal of ClinicalNutrition 92 (2010): 1265–1272.7 S. R. Preis and co-authors, Dietary protein and risk of ischemicheart disease in middle-aged men, American Journal of ClinicalNutrition 92 (2010): 1265–1272.8 C. T. McEvoy and co-authors, Vegetarian diets, low-meatdiets and health: a review, Public Health Nutrition 15 (2012):2287–2294.9 J. B. J. vanMeurs and co-authors, Common genetic loci influencingplasma homocysteine concentrations and their effect on risk ofcoronary artery disease, American Journal of Clinical Nutrition 98(2013): 668–676.10 P. Berstad and co-authors, Dietary fat and plasma totalhomocysteine concentrations in 2 adult age groups: the HordalandHomocysteine Study, American Journal of Clinical Nutrition 85(2007): 1598–1605.11 P Heese and co-authors, Alterations of homocysteine serumlevels during alcohol withdrawal are influenced by folateand riboflavin: results from the German Investigation onNeurobiology in Alcoholism (GINA), Alcohol and alcoholism47 (2012): 497–500.12 P Heese and co-authors, Alterations of homocysteine serumlevels during alcohol withdrawal are influenced by folateand riboflavin: results from the German Investigation onNeurobiology in Alcoholism (GINA), Alcohol and alcoholism47 (2012): 497–500.13 J. Pernow and C. Jung, Arginase as a potential target in thetreatment of cardiovascular disease: reversal of arginine steal?Cardiovascular Research 98 (2013): 334–343.14 K. M. Mangano and co-authors, Dietary protein is beneficial tobone health under conditions of adequate calcium intake: anupdate on clinical research, Current Opinion in Clinical Nutritionand Metabolic Care 17 (2014) 69–74.15 D. Bujnowski and co-authors, Longitudinal association betweenanimal and vegetable protein intake and obesity among men in theUnited States: The Chicago Western Electric Study, Journal of theAmerican Dietetic Association 111 (2011): 1150–1155.16 G. A. Bray and co-authors, Effect of dietary protein content onweight gain, energy expenditure, and body composition duringovereating: A randomized controlled trial, Journal of the AmericanMedical Association 307 (2012): 47–55.17 Z. Li and D. Heber, Overeating and overweight: extra caloriesincrease fat mass while protein increases lean mass, Journal of theAmerican Medical Association 307 (2012): 86–87.18 M. S. Westerterp-Plantenga and co-authors, Dietary protein – itsrole in satiety, energetics, weight loss and health, British Journal ofNutrition 108 (2012): S105–S112.19 S. M. Moe and co-authors, Vegetarian compared with meat dietaryprotein source and phosphorus homeostasis in chronic kidneydisease, Clinical Journal of the American Society of Nephrology 6(2011): 257–264.20 S. M. Moe and co-authors, Vegetarian compared with meat dietaryprotein source and phosphorus homeostasis in chronic kidneydisease, Clinical Journal of the American Society of Nephrology 6(2011): 257–264.21 National Health and Medical Research Council, Nutrient referencevalues for Australia and New Zealand, Canberra: Commonwealth ofAustralia (2006).22 N. M. Cermak and co-authors, Protein supplementation augmentsthe adaptive response of skeletal muscle to resistance-type exercisetraining: a meta-analysis, American Journal of Clinical Nutrition 96(2012): 1454–1464.23 J. E. Morley and co-authors, Nutritional recommendations forthe management of sarcopenia, Journal of the American MedicalDirectors Association 11 (2010): 391–396.1 M. Fenech and co-authors, Nutrigenetics and nutrigenomics:viewpoints on the current status and applications in nutritionresearch and practice, Journal of Nutrigenetics and Nutrigenomics 4(2011): 69–89.2 J. C. Jiménez-Chillarón and co-authors, The role of nutritionon epigenetic modifications and their implications on health,Biochimie 94 (2012): 2242–2263;3 S. W. Choi and S. Friso, Epigenetics: A new bridge betweennutrition and health, Advances in Nutrition 1 (2010): 8–16.4 M. A. Dawson, T. Kouzarides and B. J. P. Huntly, Targeting epigeneticreaders in cancer, New England Journal of Medicine 367 (2012):647–657.5 L. Bouchard and co-authors, Differential epigenomic andtranscriptomic responses in subcutaneous adipose tissue betweenlow and high responders to caloric restriction, American Journal ofClinical Nutrition 91 (2010): 309–320.6 E. S. Tai and co-authors, Polyunsaturated fatty acids interactwith PPARA-L162V polymorphism to affect plasma triglycerideapolipoprotein C-III concentrations in the Framingham HeartStudy, Journal of Nutrition 135 (2005): 397–403.7 S. Yusut and co-authors, Effect of potentially modifiable riskfactors associated with myocardial infarction in 52 countries (theINTERHEART Study): case-control study, Lancet 364 (2004): 937–952; Willett, 2002.215Eleanor, Whitney, et al. Understanding Nutrition, Cengage Australia, 2016. ProQuest Ebook Central, http://ebookcentral.proquest.com/lib/think/detail.action?docID=5024519.Created from think on 2021-04-08 05:39:12.Copyright © 2016. Cengage Australia. All rights reserved.