Humans do not require carbohydrates because they are not essential dietary components and withdrawal from the diet causes no obvious effects. However, they are a preferred source of fuel for the brain, and skeletal muscles must oxidise glucose during exercise in order to maintain high intensity contractions. Because of the usefulness of carbohydrate foods in supplying abundant glucose to the tissues as an energy source, it could be expected that mechanisms exist to ensure carbohydrate intake maintains blood sugar and muscle glycogen stores. In fact, evidence suggests that carbohydrates are involved in the regulation of feeding behaviour, which can be modified in order to accommodate the physiological desire to use carbohydrates as a fuel. Fat oxidation increases as carbohydrate oxidation decreases, and this would suggests that the respiratory exchange ratio may have an influence on the desire for carbohydrate sources of food.
Researchers1 have investigated the substrate oxidation on the desire for various macronutrients using a randomised study involving 16 healthy men. A breakfast consisting of 300g cottage cheese (13.2g carbohydrate, 0.3g fat, 23.1g protein) was fed to the subjects and was supplemented with either 90g sucrose (high carbohydrate, 505kcal), 6g aspartame (low carbohydrate, 152kcal), 40g vegetable oil (high fat, 505kcal), or 40g of paraffin oil (low fat, 152kcal). Following a 2 hour period, the energy expenditure and respiratory exchange ratio of the subjects were measured. In addition, the olfactory desire for a number of foods, and the ad libitum intake of a range of snacks, was assessed. As would be expected, subjects had a higher energy expenditure and a lower energy intake from snacks following the high calorie breakfasts, suggesting that satiety had been increased by the high energy intake.
Both carbohydrate breakfasts increased the respiratory exchange ratios of the subject, suggesting that oxidation of glucose had increased. In contrast, the respiratory exchange ratio decreased in response to the high fat and low fat meals. This suggests that dietary macronutrients were able to influence substrate oxidation short-term. In addition, the high and low carbohydrate breakfast induced the lowest olfactory liking for sweet foods, and resulted in the lowest proportion of high carbohydrate bread consumed. Similarly, the high fat breakfast induced the lowest olfactory desire for the fat foods, and the lowest proportion of fat rich bread consumed. The authors concluded that there was a negative correlation between the respiratory exchange ratio and the olfactory liking for sweet foods, and that this may influence the subsequent macronutrient intake. Energy intake may therefore be partially controlled by carbohydrates in order to modulate substrate oxidation.
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