Overfeeding is considered by many to be a cause of weight gain, However, it is often overlooked that overfeeding can increase energy expenditure. The relationship between energy expenditure and excess calorie intake is not fully understood but may relate to two factors. Firstly, protein can directly stimulate energy expenditure when present in excess. This may explain the weight loss effects of protein. Secondly, other macronutrients may cause an increase in body mass, which in turn increases energy expenditure. In relation to the latter, this may explain the higher resting metabolic rates in the overweight and obese. A number of studies have investigated the effects of overfeeding protein on the resting metabolic rates of healthy normal weight individuals. Often such experiments involve the use of metabolic chambers, where energy expenditure can be measured very accurately over a number of hours. One such recent study involved an incremental overfeeding of protein over a number of weeks.
In a randomised controlled trial1, 25 healthy normal weight subjects consumed around 40 % excess calories for 56 days with amounts of protein that included 5, 15 or 25 % protein. In the low protein group the excess energy came from fat. This group did not experience an increase in energy expenditure in the first 24 hours of overfeeding. However, as the protein content of the diet increased and the excess energy was provided by more protein, an increase in both 24 hour energy expenditure and sleeping energy expenditure was observed. When the researchers analysed the data, it was found that over the course of 8 weeks, the excess energy expenditure came from the protein rather than the excess energy. In addition, it was found that protein and fat were reciprocally related during overfeeding. Therefore protein appears to stimulate energy expenditure, and this increased energy expenditure may continue in the long term. This increased energy expenditure may explain some of the weight loss effects of high protein diets.
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