he thermic effect of food (TEF) is the increase in body heat produced following a meal. This can be measured in a laboratory setting and gives some indication as to the amount of energy that the body is prepared to waste postprandially. Generally the hypothalamus can control postprandial thermogenesis to moderate the oxidation of energy and in this way control energy balance. Obese individuals have a blunted TEF and this can be attributed to a reduction in circulating catecholamines following ingestion of food. The reduced activity of the sympathetic nervous system can be measured in obese subjects and this supports evidence that obesity is a condition of perceived starvation whereby consumed energy is efficiently retained in an effort to increase adiposity. Interestingly vegetarians my also have a lower TEF compared to non-vegetarians, despite vegetarianism being associated with a lower body weight.
For example, in one study1, researchers fed vegetarian and non-vegetarian subjects, who had been matched for body composition and aerobic fitness, a liquid meal consisting of 24 % protein, 55 % carbohydrate and 21 % fat. In response, the vegetarians produced a significantly lower TEF when compared to the non-vegetarian group. In the vegetarians the thermic response postprandially was 55.8 kcal per 180 min, whereas the non-vegetarians thermic response was 76.4 kcal per 180 min. The vegetarians therefore had a 25 % lower TEF. However, the resting metabolic rate (RMR), triiodothyronine (T3), insulin and glucose concentrations of the two groups did not differ. Vegetarians generally have lower energy intakes than non-vegetarians and so the requirement to oxidise excess calories postprandially may be less in vegetarians. Or put another way, vegetarians may have increased metabolic efficiency.
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