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Food Lipids: Chemistry, Nutrition, and Biotechnology

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C. Influence of Dietary Fat on Fatty Acid Oxidation <strong>and</strong><br />

Energy Expenditure<br />

It has been known since the early days of calorimetry that diets high in fat lower<br />

the respiratory quotient, an indicator of increased fatty acid oxidation (164). Fatty<br />

acid oxidative rates are dependent in part on chain length <strong>and</strong> degree of unsaturation.<br />

In normal-weight men, fatty acid oxidation rates are highest for lauric acid, followed<br />

by polyunsaturated <strong>and</strong> monounsaturated fatty acids <strong>and</strong> least for longer chain saturated<br />

fatty acids for which oxidation decreases with increasing chain length (165).<br />

Dietary lipid stimulation of fatty acid oxidation is thought to act through the sympathetic<br />

nervous system (166) <strong>and</strong> stimulation of carnitine palmitoyltransferase activity<br />

(167), both of which are influenced by the source of dietary fat. For example,<br />

safflower oil–fed rats have the highest sympathetic activity when compared with<br />

coconut oil– or medium-chain triglyceride–fed rats (166). Similarly, feeding fish oil<br />

causes a marked increase in carnitine acyltransferase activity in hepatic mitochondria<br />

as compared with corn oil feeding (167). Peroxisomal oxidation is also increased by<br />

diets containing fish oils when compared with vegetable oils (145). Studies in rodents<br />

have demonstrated that the n-3 fatty acids EPA <strong>and</strong> DHA both stimulate peroxisomal<br />

� oxidation in the liver, whereas EPA increases mitrochondrial � oxidation (168).<br />

Significantly higher mitochondrial <strong>and</strong> peroxisomal �-oxidative rates in fish oil–fed<br />

rats, as compared with those fed palm or safflower oil (169), are presumably due to<br />

the up-regulation of genes encoding proteins for mitochondrial <strong>and</strong> peroxisomal enzymes<br />

[see reviews by Clarke et al. (137,170)]. Preferential oxidation of n-3 fatty<br />

acids seemingly limits substrate availability for triglyceride synthesis <strong>and</strong> delivery<br />

to adipose tissue, as the oxidative rate of fatty acids is inversely proportional to<br />

storage rates (171).<br />

Energy expenditure can be influenced by dietary lipids. In general, high-fat<br />

diets have a lower heat increment than diets high in carbohydrate or protein (164).<br />

This may lead to a decrease in dietary energy utilization <strong>and</strong> an increase in body<br />

weight gain when fed a high-fat diet. While evidence exists for high-fat diets causing<br />

obesity, not all high-fat diets affect energy metabolism <strong>and</strong> body weight in the same<br />

manner. For example, diets high in essential fatty acids result in a lowering of body<br />

weight <strong>and</strong> increase in thermogenin content in rat brown adipose tissue (172). Increased<br />

levels of dietary polyunsaturated fatty acids increase hepatic carnitine palmitoyltransferase<br />

activity in Zucker obese rats <strong>and</strong> reduce liver lipid content (173).<br />

Diets containing safflower oil cause an increase in thermogenesis (174) <strong>and</strong> uncoupling<br />

protein content (175) of brown adipose tissue in comparison with diets containing<br />

beef tallow.<br />

The n-3 fatty acids, present in fish oils, may also induce the expression of<br />

uncoupling proteins. As their name implies, uncoupling proteins dissociate mitochondrial<br />

oxidative phosphorylation from energy production, leading to energy loss<br />

as heat. Three mitochondrial uncoupling proteins (UCPs) have been identified thus<br />

far. UCP-1 is expressed predominantly in brown adipose tissue (176), UCP-2 in white<br />

adipose tissue <strong>and</strong> other tissues (177), <strong>and</strong> UCP-3 in skeletal muscle (178). n-3 fatty<br />

acids have been demonstrated to increase expression of UCPs beyond that of other<br />

types of dietary fats in several studies. Hun et al. (179) reported that KK-Ay/TaJcl<br />

mice fed high-fat diets supplemented with n-3 fatty acids had increased levels of<br />

UCP-2 mRNA in white adipose tissue. Similarly, Takahashi <strong>and</strong> Ide (161) reported<br />

Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.

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