The thermic effect of food and the thermic effect of exercise are often overlooked when energy balance is considered. Obesity is considered by many to be a simple balance between energy consumed and energy expended, but this does not take into account the many ways in which the body is able to waste or preserve energy. Consuming food causes a thermic effect, called the thermic effect of food, and this can vary depending on the type of food consumed. In addition, exercise not only increases energy output, but a thermic effect of exercise occurs following exercise, and this expends additional energy that is not considered in energy balance calculations. Thermogenesis can account for a considerable amount of the daily energy expenditure and this is turn is dependent on the type and quantity of food consumed as well as the type and amount of exercise performed. Eating less and exercising more is therefore no guarantee that energy balance will be tipped favourably towards weight loss.
For example, in one study1, researcher investigated the effects of two meals of different size on the energy expenditure of healthy individuals during exercise. Subjects consumed a meal containing either 1000 or 3000 kcal for breakfast, and then performed exercise on a cycle ergometer. The meals increased the thermic effect of the exercise by 10 % compared to subjects that had not eaten. This may suggest that the additional energy allowed the metabolism of the subjects to become less efficient. However, there was no difference in the thermic effect between the 1000 and 3000 kcal meals. Following exercise there was also an enhanced thermic effect observed in those subjects that had consumed breakfast, and this thermogenic effect was of the same magnitude as the thermogenic effect caused by ingesting the food itself. However, long term thermic effects were not seen when subjects overeat 4000 kcal daily for 30 days and consumed high or low protein diets.
These results are interesting as they illustrate the absurdity of ascribing a particular amount of energy to performance of a particular activity. Clearly from this study is it observable that eating food prior to exercise not only increases energy expenditure during the exercise but also increases the thermic effect following exercise. Not taking into account the complex interactions of energy systems is one of the reasons for the poor success rate of weight loss strategies that centre of increased exercise and decreased energy expenditure. This study also shows that eating food may actually increase the productivity of exercise, by food consumption having a positive effect on the possible improvements in body composition. This may be the reason that some individuals actually lose body fat following increases in energy intake. Of course this assumes that the energy ingested is on high quality, as low quality food containing metabolic poisons would not be expected to benefit thermogenic responses.