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Food Lipids: Chemistry, Nutrition, and Biotechnology

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an increase in UCP-1 mRNA in brown adipose tissue of rats fed high-fat diets containing<br />

fish oil as compared with safflower oil. Oudart et al. (180) demonstrated an<br />

increase in brown adipose tissue mitochondrial guanosine 5�-diphosphate binding<br />

with n-3 fatty acid supplementation, but no associated difference in UCP-1. Baillie<br />

et al. (170) reported an increase in skeletal muscle UCP-3 mRNA in fish oil–fed<br />

rats, which was inversely correlated with a 25% decrease in body fat mass. It has<br />

been postulated that one mechanism by which n-3 fatty acids decrease adipose tissue<br />

mass in rats <strong>and</strong> mice is by increasing thermogenesis. However, dissipation of consumed<br />

energy as heat does not explain the lack of variance in body weights in some<br />

studies.<br />

D. Influence of Dietary Fat on Adipose Tissue Lipolysis<br />

The quantity of fat stored in adipose tissue is determined by the relative rates of the<br />

simultaneously occurring metabolic processes of triglyceride synthesis <strong>and</strong> lipolysis<br />

or triglyceride breakdown. The hydrolysis of triglycerides is catalyzed by hormonesensitive<br />

lipase, an enzyme regulated by a complex cyclic AMP–dependent signal<br />

transduction cascade (Fig. 1). A variety of hormones <strong>and</strong> neurotransmitters (e.g.,<br />

adrenaline, glucagon, noradrenaline) stimulate various components of the signal<br />

transduction cascade <strong>and</strong> thereby increase lipolysis [see review by Vernon (181)].<br />

Insulin, in contrast, modulates the activity of cyclic AMP, thereby reducing lipolytic<br />

activity (181).<br />

1. Level of Dietary Fat<br />

There is considerable evidence that both hormone-stimulated lipolysis <strong>and</strong> the antilipolytic<br />

effects of insulin are influenced by the quantity <strong>and</strong>/or type of dietary fat.<br />

However, the effect of dietary fat on lipolysis varies somewhat according to the<br />

species <strong>and</strong> specific adipose tissue depot being studied. For example, feeding rats<br />

high levels of dietary fat leads to a decrease in both catecholamine- (182–190) <strong>and</strong><br />

glucagon- (182) stimulated lipolysis. The effect of high-fat feeding on adipose tissue<br />

lipolysis in the rat is believed to be due to changes in �-adrenoceptor number or to<br />

an uncoupling between the hormone receptor <strong>and</strong> adenylate cyclase, rather than to<br />

differences in hormone binding (183,184,190). In contrast, several studies have indicated<br />

an increase in cyclic AMP–dependent signal transduction <strong>and</strong>/or lipolytic<br />

response with an increase in dietary fat (187,191–195). More specifically, adenylate<br />

cyclase activity (191) <strong>and</strong> isoproterenol-stimulated lipolysis (192) are increased <strong>and</strong><br />

phosphodiesterase activity is decreased (193) in pigs fed added fat as opposed to<br />

control diets. In addition, the effect of high-fat feeding on adenylate cyclase activity<br />

<strong>and</strong> lipolytic response in pigs varies according to the adipose tissue site (191,193).<br />

In rats, high-fat feeding leads to an increase in basal <strong>and</strong> hormone-stimulated lipolytic<br />

activity that is positively correlated with fat cell size but is not associated with<br />

sympathetic nervous system activity (195). There are few studies regarding the effect<br />

of high-fat feeding on adipose tissue lipolysis in humans. However, in one shortterm<br />

study (7 days), Kather et al. (196) observed no differences in either sensitivity<br />

or response to catecholamines in adipose tissue of subjects eating fat-rich as compared<br />

with carbohydrate-rich diets.<br />

Fat feeding has been shown to influence the antilipolytic effects of insulin in<br />

adipose tissue from several species; however, the results are somewhat inconsistent.<br />

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

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