he thermic effect of food (TEF) is the rise in body temperature in response to energy intake. Following a meal, body temperature rises as the oxidation of lipids and glucose progresses. This increase in temperature can be measured directly to assess the efficiency that food in being oxidised as an energy source. It is known that obese individuals generally have higher resting metabolic rates (RMR) when compared to lean individuals. This has been hypothesised to be due to the increased body mass, particularly the larger muscle mass required to support the extra weight. The RMR cannot therefore explain why obese individuals gain weight when compared to lean individuals as their daily energy requirement is actually higher. However, evidence suggests that the TEF is blunted in obese individuals, and it is this that might contribute to incremental weight gain, or be a sign of other unidentified metabolic abnormalities.
For example, one group of researchers1 assessed the response of obese and lean individuals to overfeeding of 1000 kcal per day above maintenance calories. In response to the overfeeding the lean subjects had an increase in RMR, but the obese subjects did not. Only lean subjects responded to overfeeding with an increase in glucose oxidation at the expense of lipid oxidation. When the obese subjects were then placed on a calorie restrictive diet of 589 kcal per day, their RMR dropped in the last 3-days of the 18-day diet. The researchers then investigated the TEF from a mixed meal during the under- and overfeeding periods and found that the obese subjects had an impaired thermic response when compared to their weight maintenance phase. This reduced TEF was reflected in a reduced circulating level of noradrenaline in the obese subjects, which was significantly lower than in their lean counterparts.
These results support other data and show that obese individuals have a reduced thermic response to food. This may be causes by a reduction in the levels of circulating noradrenaline, which would normally stimulate β-adrenergic receptors and induce thermogenesis following a meal. The inability of obese subjects to oxidise energy likely results from the associated insulin resistance that accompanies abdominal weight gain. The blunted thermic effect of food seen in obese subjects reflects the increase diversion of energy from skeletal muscle to adipose tissue and concomitant accumulation of body fat. Forced energy restriction proponents claim that undernutrition is of benefit to obese individuals, but from this study it can be seen that such a treatment results in an impaired TEF and a reduction in RMR. This suggests that any initial weight loss would have been followed by weight re-gain and would therefore be counter-productive.
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