Three separate experiments in lean subjects confirmed that a 1.52-MJ (362-kcal) carbohydrate supplement at breakfast suppressed appetite 90 min later but had no effect on a test meal given after 270 min. A 1.52-MJ (362-kcal) fat supplement produced no detectable action on measures of appetite at any time point. Therefore, fat and carbohydrate do not have identical effects on the appetite profile. In a further study in obese subjects, a novel experimental design was used to assess the satiating efficiency and compensatory response of fat. Eating from a range of either high-fat or high-carbohydrate foods, obese subjects voluntarily consumed twice as much energy from the fat items, thereby indicating a weak action of fat on satiation. In turn, this large intake of fat exerted a disproportionately weak effect on satiety. These studies suggest that the appetite-control system may have only weak inhibitory mechanisms to prevent the passive overconsumption of dietary fat. The results indicate how this action could induce a positive energy balance and lead to a gradual upward drift in body mass index.

Three studies using healthy volunteers (n = 271) investigated the effects of caffeine, carbohydrates and carbonation in functional "energy" drinks (EDs) with the aim of determining their benefit in every-day life. The results showed caffeine to be the main ED constituent responsible for the effects found, with possible minor, relatively weak effects of carbohydrates. EDs were found to improve and/or maintain mood and performance during fatiguing and cognitively demanding tasks relative to placebo. In terms of absolute values, EDs maintained levels of arousal compared to a deterioration in arousal where placebo was consumed. These effects were found in caffeine-deprived participants, and so may be largely due to "withdrawal reversal". There were only minor differences in the effects of water vs. "sensory-matched" placebo, supporting previous findings indicating that the type of placebo does not alter the conclusions drawn about the effects of the full ED. Finally, carbonation had various effects on mood, some of which were present immediately following consumption, others were consistent with slower absorption of caffeine (and possibly carbohydrates) from carbonated drinks.

In experiment 1, normal weight male subjects were provided with three types of breakfast consumed in the Human Appetite Research Unit on separate experimental days 1 week apart. The intensity of hunger, fullness and other subjective feelings were tracked by means of visual analogue rating scales at intervals during the day. Energy and nutrient intakes were measured directly from ad libitum test meals consumed at lunch and dinner. During the rest of the day and until after breakfast the following day, food intake was measured indirectly through weighed food records. The test breakfasts comprised a basic meal 184 1 kJ (440 kcal) and the same meal supplemented with similar amounts of either fat (1515 kJ, 362 kcal) or carbohydrate (1527 kJ, 365 kcal). No differences were detected between the effects of the basic breakfast compared with the fat‐supplemented breakfast. The carbohydrate supplement suppressed hunger ratings during a limited period after consumption (the post‐ingestive window) coinciding with the expected metabolism of carbohydrate. In experiment 2, a direct test of consumption during this post‐ingestive window confirmed that the carbohydrate supplemented breakfast suppressed intake but the fat supplement did not. These results demonstrate that carbohydrate and fat can produce quite different effects on satiety. Under these experimental conditions the supplement of fat produced no detectable effect on the expression of appetite and illustrates how dietary fat could lead to passive over‐consumption of energy. However this effect may be modified by the particular pattern of food consumption during the course of a day.

The human appetite system contains central and peripheral mechanisms that interact with environmental features, especially with the physical and nutrient composition of the food supply. Foods varying in nutrient composition exert different physiologic effects, some of which function as satiety signals. High-fat diets (low food quotient) lead to high levels of energy intake. This effect is termed passive overconsumption and overcomes fat-induced physiological satiety signals. High-fat foods exert a weak effect on satiation (intra-meal satiety), and fat has a weaker effect, joule for joule, on postingestive satiety than do other macronutrients. The frequency of obesity is greater among high-fat than low-fat consumers. However, the development of obesity on a high-fat diet is not a biological inevitability. The investigation of people who resist the weight-inducing properties of high-fat diets is a key research strategy. Understanding the appetite control system suggests behavioral, nutritional, and pharmacologic strategies for modifying dietary fat intake.

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