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Food Lipids: Chemistry, Nutrition, and Biotechnology

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Adipose tissue from animals fed diets high in polyunsaturated fatty acids generally<br />

exhibit a greater lipolytic response to catecholamines <strong>and</strong> synthetic �-adrenoceptor<br />

agonists as compared with tissue from animals fed diets high in saturated fat<br />

(187,203–208). The decreased responsiveness of fat cells from rats fed saturated fat<br />

diets is associated with reductions in adenylate cyclase, cyclic AMP phosphodiesterase,<br />

<strong>and</strong> hormone-sensitive lipase activity (203). �-Adrenergic receptor binding is<br />

also lower in fat cell membranes from rats fed saturated fat as compared to polyunsaturated<br />

fat diets (204,209). The reduction in �-adrenergic receptor binding associated<br />

with the saturated fat diet is due to decreased binding affinity rather than<br />

to changes in receptor number (204,209). The reduced binding affinity is in turn<br />

correlated with a reduction in membrane fluidity in cells from the rats fed the high<br />

saturated fat diets (204).<br />

In contrast to the above studies, several investigators have failed to observe an<br />

effect of dietary fat type on lipolytic response (188,192,210,211). Lipinski <strong>and</strong> Mathias<br />

(188) observed that norepinephrine-stimulated lipolysis in rat adipocytes is depressed<br />

by an increase in fat calories but is unaffected by the degree of saturation<br />

of the fat. Likewise, Mersmann et al. (192,210) reported an increase in the number<br />

of �-adrenergic receptors, with no change in receptor affinity, in adipose tissue from<br />

pigs fed high levels of saturated fat. However, the observed increase in receptor<br />

number is not associated with alterations in �-adrenergic agonist–mediated function<br />

(including lipolysis). Recently, Portillo et al. (212) observed that under energy-controlled<br />

feeding conditions, various dietary fat regimes caused major changes in<br />

adipose tissue phospholipid composition; however, such modifications did not lead<br />

to important changes in lipolysis. Portillo et al. (212) suggested that some changes<br />

in adipose tissue fatty acid composition may have little effect on overall physiological<br />

function.<br />

3. Relationship to Obesity<br />

Although the evidence is not totally consistent, it appears the rate of lipolysis may<br />

be influenced to some extent by both the level <strong>and</strong> the type of dietary fat. A reduction<br />

in lipolytic rate as is commonly observed in response to high-fat (particularly saturated<br />

fat) feeding could lead to an increased retention of stored triglycerides <strong>and</strong><br />

thereby contribute to the development of obesity. However, this response may vary<br />

considerably according to species, age, adipose tissue depot site, <strong>and</strong> adipose tissue<br />

fatty acid composition. The relative contribution of lipolytic alterations to diet-induced<br />

obesity <strong>and</strong> the specific regulatory components of the lipolytic signal transduction<br />

cascade influenced by alterations in the level or type of dietary fat remain<br />

to be elucidated.<br />

E. Influence of Dietary Fat on Insulin Action<br />

An association between high-fat diets <strong>and</strong> impaired insulin action has been observed<br />

in numerous in vivo <strong>and</strong> in vitro studies. Early studies in human subjects suggested<br />

that diets high in fat lead to a reduction in glucose tolerance (213,214). More recent<br />

epidemiologic data suggest that individuals with higher fat intakes are more likely<br />

to develop disturbances of glucose metabolism, type 2 diabetes, <strong>and</strong> impaired glucose<br />

tolerance than individuals consuming lower amounts of fat, although obesity <strong>and</strong><br />

physical inactivity may be confounding factors (215). Studies in experimental ani-<br />

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

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