Short-term studies of the effects of macronutrients on energy intake have often used a preload/test-meal paradigm. Subjects are given a 'preload' of known or unknown composition and after a fixed interval are offered one or more 'test-meals', at which they can eat ad libitum, but consumption is monitored. During the intervening periods they may also be asked to complete visual analogue scales indicating hunger or satiety. Similar studies have also been conducted to assess food intake over 1-3 days after a specific macronutrient manipulated meal or meals.
There are relatively few studies to address the specific effect of protein on subsequent intake. However, most data would suggest that in isoener-getic quantities, protein is probably the most satiating macronutrient (15-17). Whether this leads to differences in prospective consumption is more debatable. For example in a study of breakfasts high in protein, fat or carbohydrate followed by ad libitum consumption at lunch 5 hours later and thereafter, Stubbs et al. found that the high protein breakfast suppressed self-reported hunger to a greater extent over the entire 24-hour period (18). However, it was not translated into any difference in 24-hour energy intake or balance. Similarly in an extended 3-day study there was no difference in intake on days 2 and 3 following a high protein diet on day 1, relative to isoenergetic diets high in fat or carbohydrate (19). However, in most diets, protein is a much smaller constituent than fat or carbohydrate, which probably limits its overall contribution to the regulation of energy intake.
Instead most of the preload studies have focused on the relative roles of fat and carbohydrate by providing a constant proportion of protein and thus allowing reciprocal changes in fat and carbohydrate. For example, in a study where subjects were given a standard breakfast as a preload, or the same meal supplemented with 1520 kJ of fat or carbohydrate, ad libitum intake was measured at a subsequent test meal or snack (20). Visual analogue scores showed that the carbohydrate supplement suppressed hunger and the desire to eat, whereas the fat supplement had no effect relative to the control. By 4 hours later this difference had disappeared and there was no difference in energy intake at the test lunch consumed 4.5 hours after breakfast. In a subsequent study in which preloads were followed by an ad libitum snack 90 minutes later, when the difference in the hunger scores between preloads was still evident, there was a significant suppression of the snack intake after the carbohydrate-supplemented preload, whilst intake followed the fat-supplemented preload did not differ from the control.
The different results obtained by altering the interval between preload and test meal (20,21) or by varying the size or composition of foods (22) preclude total consensus in the interpretation of these studies. A further confounding factor in the analysis of these studies is the effect of the subjects' state of knowledge. Overt manipulations of the fat to carbohydrate content of foods may produce contradictory results to covert studies. For example, Shide and Rolls showed that following a covert low fat preload, subjects consumed less at an ad libitum lunch than following a high fat preload. However, when information about the fat content of the preload was provided, the intake at lunch was greater following the low fat preload (23). In some cases the individual characteristics of the subjects may further distort the interpretation. Some studies have suggested that the degree of dietary restraint will influence intake at a test meal (24), although others have failed to find a difference between restrained and unrestrained eaters (23). However, it is generally agreed that dietary carbohydrate initiates a much stronger satiety signal than dietary fat and in so doing may limit prospective food consumption.
The concept that dietary carbohydrate may be more satiating than fat is consistent with the hypothesis of Flatt (7). Flatt argues that the quantity of carbohydrate oxidized each day is similar to the body's storage capacity for carbohydrate, whereas the storage capacity for fat is considerably greater than day-to-day consumption or oxidation of this macronutrient. Thus day-to-day fluctuations in carbohydrate stores are proportionally very much larger than for fat. This confers a much greater sensitivity to changes in the pool size for carbohydrate, than for fat, and Flatt argues that this modulates later consumption in order to restore the equilibrium. Based on this hypothesis the status of the body's carbohydrate store is critical in determining intake and implies that individuals eat sufficient food to defend their carbohydrate stores. Thus on a diet with a low carbohydrate: fat ratio the total amount of energy consumed in order to provide sufficient carbohydrate will be greater than when consuming a diet with a high carbohydrate: fat ratio. This scenario must inevitably lead to fat deposition which will persist until such time as the substrate mixture being oxidized matches that of the habitual diet (i.e. RQ = FQ). Fat accumulation is thus interpreted as a response to a high fat diet (25).
Evidence for this theory comes from prolonged feeding trials in mice (7). Specifically there was a negative correlation between changes in carbohydrate stores and the subsequent day's ad libitum intake, yet no association between net energy balance on one day and intake the next. However, extensive testing of this model in human studies has largely failed to support this early conjecture. This is clearly demonstrated in a study where glycogen stores were perturbed by feeding isoenergetic extremes of fat to carbohydrate intake (9 vs. 79% carbohydrate) on a 'manipulation' day and observing ad libitum intake on the following 'outcome' day (3). These studies were conducted within a whole-body calorimeter such that macronutrient balance could be ascertained relative to a nominal zero at the start of each phase of the study. Despite a difference in carbohydrate balance of 327 g between the high and low carbohydrate manipulations there was no significant difference in intake the following day. Subsequent studies have examined the impact of macronutrient manipulations which also include changes in energy balance on subsequent energy intake (26). Here an energy deficit of approximately 15% was created by the removal of either dietary fat or carbohydrate. The macronutrient manipulations produced significant differences in substrate oxidation, which were predictable from the oxidative hierarchy, but there was no evidence of any macro-nutrient-specific effects on subsequent intake. In each case, energy balance was restored after 1 day of ad libitum eating. These studies suggest that the net flux of all macronutrients may be better able to explain the pattern of subsequent energy intake than nutrient-specific models. In the light of this, Friedman has proposed a theoretical framework in which the stimulus to food intake is derived at the level of oxidative phosphorylation and adenosine triphosphate (ATP) production (27). However, to date there is relatively little direct experimental evidence in support of this model.
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