Forced calorie restriction is the current treatment for being overweight. The main problem with this approach however, is that it does not provide successful long term results. Initially, forced calorie restriction causes weight loss, but this has been shown to be composed of significant amounts of skeletal muscle mass. As skeletal muscle is catabolised, the resting metabolic rate falls and the maintenance energy requirement of the individuals falls concomitantly. The end result of a phase of forced calorie restriction is an increased risk of regaining any lost fat when normal eating resumes. Further, obesity is a perceived state of starvation. When the obese individual restricts energy intake, the hypothalamus counteracts the reduced caloric intake by reducing the thermic effect of food (TEF), the thermic effect of activity (TEA) and resting metabolic rate, in order to conserve energy. In addition appetite is upregulated in an effort to return energy intake to its previous balance point. Forced calorie restriction therefore causes adaptations that detrimentally affects its efficiency.
Addressing these counterregulatory mechanisms might be effective at improving the benefits of energy restriction diets. Two strategies that could be of use here are thermogenic foods and foods that diminish appetite. Capsaicin is a component of cayenne pepper that has thermogenic properties in humans and high protein diets are known to increase satiety. Researchers have tested the effects of capsaicin and a high protein intake on the diminished energy expenditure and increased appetite during forced calorie restriction, both individually and in combination1. Subjects with 25 % body fat spent 36 hours in a respiratory chamber. This sequence was repeated 8 times and each time the subjects were given a different treatment. The treatments consisted of maintenance calories or a 20 % calorie restriction with either 1030 mg of capsaicin from 2 red pepper, increased protein (from 10 % to 25 % of energy, at the expense of carbohydrate which was reduced from 60 to 45 % of energy), or both capsaicin and the high protein treatment in combination.
As expected, calorie restriction caused a negative energy balance compared to the maintenance calorie group and this was reflected in a decreased energy expenditure, a diminished TEF and increased appetite. However, these effects were diminished with the addition of capsaicin and ameliorated completely by the addition of a high protein intake and a combination of capsaicin and high protein intake. Addition of capsaicin or high protein and a combination of the two to the energy restrictive diet also increased fat oxidation and the respiratory quotient compared to energy restriction alone. Addition of a higher protein intake to the energy restricted diet also prevented a negative nitrogen balance. These results therefore show that protein and capsaicin can prevent some of the negative consequences of forced energy restriction in the short-term. Interestingly, the protein and capsaicin cause increased energy expenditure in the maintenance calorie diet suggesting that calorie restriction is not necessary to cause weight loss.