The resting metabolic rate (RMR) is the amount of calories oxidised at rest. The thermic effect of food (TEF) is the amount of heat produced postprandially. The RMR and the TEF are interesting because evidence suggests that alterations to these parameters may be a cause of weight gain. Generally, the RMR is dependent on the amount of muscle mass an individual has. Skeletal muscle is metabolically very active because of protein turnover and so a high muscle mass requires considerable energy to maintain. Athletes therefore tend to have higher RMR than sedentary individuals. However, not all of the increase in RMR can be explained by skeletal muscle. Athletes also tend to have a lower TEF compared to sedentary individuals. This may be because calorie intake is usually required for rebuilding energy stores and so there is less requirement to oxidise the food to produce heat.
The differences in the RMR and TEF between athletes and sedentary individuals was illustrated in a study published in the American Journal of Clinical Nutrition in 19881. The researchers measured the RMR rate of a group of male endurance athletes and compared them to a group of similarly aged untrained males. The RMR of the athletes was higher than the sedentary individuals as might be expected. However, the increased RMR persisted ever after the researchers adjusted the model to take account of percentage body fat and lean mass. Therefore highly trained individuals have a higher RMR that cannot be explained solely by an increase in skeletal muscle mass. In order to assess if this result was the result of thyroid hormone levels, triiodothyronine and thyroxine concentrations were measured in the subjects. However, there was no difference between the thyroid hormones of the trained and untrained individuals.
When the researchers measured the TEF, they found that the athletes had a blunted thermogenic response to a test meal. The reduction in the TEF was around 30 % between the untrained and trained individuals, supporting the hypothesis that intense training increases the economy of energy. This may be a result of depleted energy stores in trained skeletal muscles and the liver. Interestingly, obese individuals also show an blunted TEF, which may occur for the same reason. This paradox is likely the result of leptin resistance that causes the hypothalamus to receive inadequate feedback regarding the amount of body fat. In response the hypothalamus, decreases the TEF to paradoxically increase body fat. The increase in RMR in this study between the trained endurance athletes and the sedentary subjects was around 10 %. Obese individuals also have an increased RMR which is believed to be due to the increased skeletal muscle required to support their excessive weight.
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