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Food Lipids: Chemistry, Nutrition, and Biotechnology

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the fat component <strong>and</strong> the other half contained 20% lard as the fat component.<br />

Animals selecting from the soybean oil diets consumed more carbohydrate <strong>and</strong> less<br />

protein than animals choosing from the lard-based diets. Grossman et al. (74) further<br />

showed that rats gavaged with either beef tallow or corn oil 2 hours before selecting<br />

from either high-carbohydrate or high-protein diets consume less carbohydrate <strong>and</strong><br />

more protein if given tallow versus corn oil. They (74) also demonstrated that the<br />

hepatic vagus must be intact for this selection to occur <strong>and</strong> that mercaptoacetate<br />

(fatty acid oxidation inhibitor) can blunt the effect.<br />

3. Metabolic Signals<br />

In an attempt to underst<strong>and</strong> the mechanisms underlying the effects of dietary fat on<br />

appetite, investigators have examined absorptive or postabsorptive responses to dietary<br />

fat. In particular, fatty acid oxidation has been studied as a possible satiety<br />

signal that is generated in response to food (i.e., fat) consumption. Scharrer <strong>and</strong><br />

Langhans (75) demonstrated that in rats, consumption of a high-fat diet can be stimulated<br />

by inhibiting fatty acid oxidation with 2-mercaptoacetate. Friedman et al. (76)<br />

also reported a stimulation of food intake when fatty acid oxidation is inhibited with<br />

methyl palmoxirate. This feeding effect, labeled ‘‘lipoprivic feeding,’’ has been<br />

shown to be impaired by hepatic vagotomy (77) <strong>and</strong> subdiaphragmatic vagotomy<br />

(78). In addition, Ritter <strong>and</strong> Taylor (79) reported that capsaicin can block this effect,<br />

implying that vagal sensory neurons appear to be involved in lipoprivic feeding.<br />

Studies utilizing brain lesions indicate that lipoprivic feeding involves the lateral<br />

parabrachial nucleus <strong>and</strong> possibly the area postrema/nucleus of the solitary tract (80).<br />

Type of fat may influence this response as Wang et al. (81) reported that food intake<br />

is stimulated by mercaptoacetate in rats given corn oil but not tallow diets.<br />

Other investigators have examined aspects of fat metabolism as potential satiety<br />

signals by administering fat metabolites centrally or peripherally. Arase et al. (82)<br />

reported that intracerebroventricular infusions of �-hydroxybutyrate (�-OHB) reduce<br />

food intake in Sprague–Dawley or Osborne–Mendel rats consuming either a highor<br />

a low-fat diet. However, such infusions do not reduce food intake in S5B/PI rats,<br />

rats that are resistant to weight gain when consuming high-fat diets. Peripheral injections<br />

of �-OHB decrease food intake in S5B/PI rats by not Obsorne–Mendel<br />

animals, <strong>and</strong> glycerol has no effect in either strain (83). Peripheral injections of<br />

glycerol in Wistar rats (84) <strong>and</strong> �-OHB in Sprague–Dawley rats (85) also decrease<br />

food intake.<br />

Fat, when infused into the intestine, can suppress hunger, induce satiety, or<br />

delay gastric emptying (16,86–88). However, as stated previously, dietary fat appears<br />

to be overconsumed <strong>and</strong> often can lead to obesity. The disparate effects of intraintestinal<br />

infusions of fat versus dietary fat have been termed the ‘‘fat paradox’’ (89).<br />

It has been suggested that high-fat foods have a very high palatability <strong>and</strong> orosensory<br />

stimulation (90–92), leading to overconsumption before the nutrients can even enter<br />

the intestine to generate satiety signals. In support of this hypothesis, satiety signals<br />

potentially arising from postabsorptive metabolic events appear to be blunted from<br />

fat in comparison with other nutrients. For example, it has been reported that carbohydrate<br />

<strong>and</strong> protein consumption is followed by an increase in their oxidation<br />

(93–95), whereas oxidation of fat is not generally stimulated until 3–7 days following<br />

consumption of a high-fat diet (96,97). Furthermore, it has been reported that<br />

fat oxidation is especially limited in obese as compared with lean individuals (34,98).<br />

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

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