Food Lipids: Chemistry, Nutrition, and Biotechnology

fed high-fat diets. In support of this hypothesis, Emmet and Heaton (57) examined
food records from 160 subjects who had weighed their food for 4 days. They reported
that an increase in refined sugar intake is associated with a linear increase in the
intake of fat combined with carbohydrate. This suggests that refined sugar may act
as a vehicle for fat intake by increasing fat palatability.
Some have reported that there is an inverse relationship between consumption
of sugar and fat, terming it the ‘‘see-saw’’ effect. When consumption of fat and sugar
are expressed as a percentage of total intake, then this inverse relationship is present.
However, when expressed in absolute terms, there is a positive relationship between
dietary fat and carbohydrate intake (58).
B. Mechanisms for Fat-Induced Food Intake
1. Caloric Density
The higher caloric density of many high-fat diets may play a role in inducing this
hyperphagic response (36,37,59). In humans, Duncan et al. (32) reported that adult
subjects eat almost twice as many calories on a high density diet compared to a low
density diet. In studies using experimental animals, hyperphagia is typically observed
only when the caloric density of the diets is high, greater than approximately 5.8
kcal per gram of diet [for review, see Warwick and Schiffman (9)]. In addition, when
caloric density is constant, rats fed a diet high in corn oil have a caloric intake
similar to that of animals fed low-fat diets (56,60).
Several investigators have reported that a preference develops for a flavor that
is paired with a high number of calories versus one paired with a low number of
calories. A study by Johnson et al. (61) indicated that children report increased flavor
pleasantness due to association with a high density of fat calories. In rats, a flavor
associated with corn oil consumption is preferred over a flavor that is not paired
with oil (62,63). In studies in which oils or high-fat foods are given, animals often
initially do not consume greater quantities of the fat. However, over time the rats do
consume more of the fat as they learn about the associated postingestive consequences
(i.e., greater caloric value) (64–68). In contrast, others have suggested that
that dietary fat is overconsumed even when compared with an isoenergic carbohydrate
diet of similar palatability (69,70). Further, Lucas et al. (71) report that, relative
to an isocaloric high-carbohydrate diet, the postingestive effect of high-fat diets stimulates
overeating and conditions a stronger flavor preference in rats, suggesting that
some quality in fat per se may be inducing intake.
2. Stomach Distention
Differences in stomach distention due to dietary fat versus carbohydrate or protein
may also account for its hyperphagic effect (9). Warwick and Schiffman (9) suggested
that because of the greater caloric density of dietary fat, a high-fat meal has
a smaller volume than an isocaloric high-carbohydrate meal, resulting in less stomach
distention. This lesser distention would lead to an attenuation of satiety signals. In
addition, Cunningham et al. (72) reported that the rate of stomach emptying increases
when a high-fat diet is habitually consumed.
Dietary fat has also been found to influence relative consumption of carbohydrate
and protein. Crane and Greenwood (73) allowed rats to select from either highcarbohydrate
or high-protein diets. Half of the diets contained 20% soybean oil as
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