• phytochemicals• moderate alcohol consumption• physical activity.Chapter 18 provides many more details.Not too surprisingly, numerous genes influence how the body handles the synthesis,transport, and degradation of lipids and lipoproteins. Much current research is focused on hownutrient–gene interactions may direct the progression of heart disease.LIPIDS IN THE BODYIn the body, lipids provide energy, insulate against temperature … Continue reading “the body handles the synthesis | My Assignment Tutor”
• phytochemicals• moderate alcohol consumption• physical activity.Chapter 18 provides many more details.Not too surprisingly, numerous genes influence how the body handles the synthesis,transport, and degradation of lipids and lipoproteins. Much current research is focused on hownutrient–gene interactions may direct the progression of heart disease.LIPIDS IN THE BODYIn the body, lipids provide energy, insulate against temperature extremes, protect againstshock, and maintain cell membranes. This section provides an overview of the roles oftriglycerides and fatty acids and then of the metabolic pathways they can follow withinthe body’s cells.ROLES OF TRIGLYCERIDESFirst and foremost, the triglycerides – either from food or from the body’s fat stores – provide thebody with energy. When people dance all night, the triglycerides from their dinner provide someof the fuel that keeps them moving. When people lose their appetites, their stored triglyceridesfuel much of their body’s work until they can eat again.Adipose tissue is more than just a storage depot for fat. Adipose tissue actively secretesseveral hormones known as adipokines – proteins that help regulate energy balance andinfluence several body functions.7 When body fat is markedly reduced or excessive, thetype and quantity of adipokine secretions change, with consequences for the body’s health.Researchers are exploring how adipokines influence the links between obesity and chronicdiseases such as type 2 diabetes, hypertension, and heart disease.8 Obesity, for example,increases the release of the adipokine resistin that promotes inflammation and insulinresistance – factors that predict heart disease and diabetes. Similarly, obesity decreases therelease of the adipokine adiponectin that protects against inflammation, diabetes, and heartdisease.Fat serves other roles in the body as well. Because fat is a poor conductor of heat, the layerof fat beneath the skin insulates the body from temperature extremes. Fat pads also serve asnatural shock absorbers, providing a cushion for the bones and vital organs. Fat provides thestructural material for cell membranes and participates in cell signalling pathways.PUTTINGCOMMONSENSE TOTHE TESTTriglycerides arethe major form offats in the humandiet.TRUELEARN ITOutline the major roles of fats in the body, including a discussion of essential fatty acids and theomega fatty acids.The body makes special arrangements to digest and absorb lipids. It provides the emulsifierbile to make them accessible to the fat-digesting lipases that dismantle triglycerides, mostly tomonoglycerides and fatty acids, for absorption by the intestinal cells. Four types of lipoproteinstransport all classes of lipids (triglycerides, phospholipids, and cholesterol), but the chylomicronsare the largest and contain mostly triglycerides from the diet; VLDL are smaller and are about halftriglycerides; LDL are smaller still and contain mostly cholesterol; and HDL are the densest and are richin protein. High LDL cholesterol indicates increased risk of heart disease, whereas high HDL cholesterolhas a protective effect.REVIEW IT154 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 05:41:03.Copyright © 2016. Cengage Australia. All rights reserved.ESSENTIAL FATTY ACIDSThe human body needs fatty acids, and it can make all but two of them – linoleicacid (the 18-carbon omega-6 fatty acid) and linolenic acid (the 18-carbonomega-3 fatty acid). These two fatty acids must be supplied by the diet and aretherefore essential fatty acids. The cells do not possess the enzymes to makeany of the omega-6 or omega-3 fatty acids from scratch, nor can they convertan omega-6 fatty acid to an omega-3 fatty acid or vice versa. Cells can, however,start with the 18-carbon member of an omega family and make the longer fattyacids of that family by forming double bonds (desaturation) and lengtheningthe chain two carbons at a time (elongation), as shown in Figure 5.18. Thisis a slow process because the omega-3 and omega-6 families compete for thesame enzymes. Too much of a fatty acid from one family can create a deficiencyof the other family’s longer fatty acids, which is critical only when the dietfails to deliver adequate supplies. Therefore, the most effective way to maintainbody supplies of all the omega-6 and omega-3 fatty acids is to obtain themdirectly from foods – most notably, from vegetable oils, seeds, nuts, fish andother seafood.Linoleic acid and the omega-6 familyLinoleic acid is an essential fatty acid and the primary member of the omega-6 family. Whenthe body receives linoleic acid from the diet, it can make other members of the omega-6 family –such as the 20-carbon polyunsaturated fatty acid, arachidonic acid (as shown in Figure 5.18).Should a linoleic acid deficiency develop, arachidonic acid, and all other omega-6 fatty acidsthat derive from linoleic acid, would also become essential and have to be obtained from thediet. Normally, vegetable oils and meats supply enough omega-6 fatty acids to meet thebody’s needs.Linolenic acid and the omega-3 familyLinolenic acid is the essential fatty acid and the primary member of the omega-3 family .Like linoleic acid, linolenic acid cannot be made in the body and must be supplied by foods.Given this 18-carbon fatty acid, the body can make small amounts of the 20- and 22-carbonmembers of the omega-3 series, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA).These omega-3 fatty acids play critical roles in the optimal structure and function of cells.9 Foundabundantly in the eyes and brain, the omega-3 fatty acids are essential for normal growth, visualacuity, and cognitive development.10 They may also play an important role in the prevention andtreatment of heart disease, as later sections explain.EicosanoidsThe body uses arachidonic acid and EPA to make substances known as eicosanoids. Eicosanoidsare a diverse group of more than 100 compounds. Sometimes described as ‘hormone-like’,eicosanoids differ from hormones in important ways. For one, hormones are secreted in onelocation and travel to affect cells all over the body, whereas eicosanoids appear to affect onlythe cells in which they are made or nearby cells in the same localised environment. For another,hormones elicit the same response from all their target cells, whereas eicosanoids often havedifferent effects on different cells.The actions of various eicosanoids sometimes oppose each other. For example, one causesmuscles to relax and blood vessels to dilate, whereas another causes muscles to contract andblood vessels to constrict. Certain eicosanoids participate in the immune response to injuryand infection, producing fever, inflammation and pain. One of the ways aspirin relieves thesesymptoms is by slowing the synthesis of these eicosanoids.Linoleic aciddesaturation(18:3)(18:2)elongation(20:3)(20:4)desaturationArachidonic acidThe first number indicates the number ofcarbons and the second, the number ofdouble bonds. Similar reactions occurwhen the body makes the omega-3 fattyacids EPA and DHA from linolenic acid.FIGURE 5.18 The pathway from oneomega-6 fatty acid to anotherA non-essentialnutrient (such asarachidonic acid) thatmust be supplied bythe diet in specialcircumstances (asin a linoleic aciddeficiency) is consideredconditionallyessential.This omega-3linolenic acid is knownas alpha-linolenicacid and is the fattyacid referred to inthis chapter. Anotherfatty acid, also with18 carbons and threedouble bonds, belongs tothe omega-6 family andis known as gammalinolenic acid.CHAPTER 5 THE LIPIDS: TRIGLYCERIDES, PHOSPHOLIPIDS AND STEROLS 155Eleanor, 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 05:41:03.Copyright © 2016. Cengage Australia. All rights reserved.Eicosanoids that derive from omega-3 fatty acids differ from those that derive from omega-6fatty acids, with the omega-3 family providing greater health benefits. The omega-3 eicosanoidshelp lower blood pressure, prevent blood clot formation, protect against irregular heartbeats,and reduce inflammation, whereas the omega-6 eicosanoids tend to promote clot formation,inflammation, and blood vessel constriction.11Omega-6 to omega-3 ratioBecause omega-6 and omega-3 fatty acids compete for the same enzymes and their actionsoften oppose each other, researchers have studied whether there is an ideal ratio that bestsupports health.12 Suggested ratios range from 4:1 to 10:1. Although some researcherssupport such recommendations, others find the ratio of little value in improving healthor predicting risk.13 Increasing the amount of omega-3 fatty acids in the diet is clearlybeneficial, but reducing the amount of omega-6 fatty acids in the diet to improve the ratiomay not be helpful. Omega-6 fatty acids protect heart health by lowering LDL cholesteroland improving insulin resistance.14Fatty acid deficienciesMost diets in Australia and New Zealand meet the minimum essential fatty acid requirements.Historically, deficiencies have developed only in infants and young children who have been fedfat-free milk and low-fat diets, or in hospital clients who have been mistakenly fed formulas thatprovided no polyunsaturated fatty acids for long periods of time. Classic deficiency symptomsinclude growth retardation, reproductive failure, skin lesions, kidney and liver disorders, andsubtle neurological and visual problems.A PREVIEW OF LIPID METABOLISMThe blood delivers triglycerides to the cells for their use. This is a preview of how the cells storeand release energy from fat; Chapter 7 provides details.Storing fat as fatWhen meals deliver more energy than the body needs, the excess is stored as fat in the adiposecells for later use. An enzyme – lipoprotein lipase (LPL) – hydrolyses triglycerides from circulatinglipoproteins, releasing fatty acids, diglycerides, and monoglycerides into the adipose cells.Enzymes inside the adipose cells reassemble these fatty acids, diglycerides and monoglyceridesinto triglycerides again for storage. Triglycerides fill the adipose cells, storing a lot of energy in arelatively small space.Using fat for energyAfter meals, the blood delivers chylomicrons and VLDL loaded with triglycerides to the body’scells for energy. Fat supplies about 60 per cent of the body’s ongoing energy needs during rest.During prolonged light to moderately intense exercise or extended periods of food deprivation,fat may make a slightly greater contribution to energy needs. During energy deprivation, severallipase enzymes (particularly hormone-sensitive lipase) inside the adipose cells respond bydismantling stored triglycerides and releasing the glycerol and fatty acids directly into the blood.Energy-hungry cells anywhere in the body can then capture these compounds and take themthrough a series of chemical reactions to yield energy, carbon dioxide, and water. A personwho fasts (drinking only water) will rapidly metabolise body fat. Even with abundant body fat,the person has to obtain some energy from lean protein tissue because the brain, nerves andred blood cells need glucose – and without carbohydrate, only protein and the small glycerolmolecule of a triglyceride can be converted to glucose; fatty acids cannot be. Still, in times of156 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 05:41:03.Copyright © 2016. Cengage Australia. All rights reserved.
