Energy restriction is problematic because while it is able to induce weight loss in most people, much of the weight lost is often muscle tissue. The resting metabolic rate is proportional to the amount of skeletal muscle possessed by an individual, and this explains why energy restriction diets cause a reduction in resting metabolic rate. The fall in resting metabolic rate following energy restriction is the reason that most energy restriction diets eventually fail, and the reason why weight gain is highly likely after following such a diet. Moreover, the weight regain following weight loss on an energy restricted diet is comprised largely of body fat, and this creates a situation whereby cycles of energy restriction often cause a deterioration in body composition. Preventing the loss of skeletal muscle mass that occurs during energy restriction is therefore something that is interesting to those who wish to improve body weight. One way to achieve this is to increase the protein intake of the diet.
Dietary protein is effective at preventing skeletal muscle loss primarily because it can help maintain a positive nitrogen balance. It is the negative energy balance in combination with a negative nitrogen balance that is particularly damaging to skeletal muscle mass during energy restriction. The quality of protein is also important, because particular amino acids within the protein are required by skeletal muscle in order to maintain protein synthesis. Of the amino acids in protein, leucine, isoleucine and valine, the branched chain amino acids, are particularly useful to skeletal muscle for protein synthesis. Of these, leucine is perhaps the most important. Whey protein is a rich source of branched chain amino acids and as a result is particularly effective at maintaining nitrogen balance and at inducing skeletal muscle protein synthesis rates. Consuming whey protein while restricting energy has been shown to result in a greater preservation of skeletal muscle compared to energy restriction alone.
For example, in one study1, researchers assessed the effects of a whey protein supplement containing added leucine and micronutrients on the skeletal muscle mass of obese individuals undergoing energy restriction. The subjects followed the -600 kcal energy restriction for 13 weeks and also performed resistance training 3 times per week during this time. Subjects receiving the protein supplement consumed 21 grams of protein per day with an additional 21 grams after performing resistance training. This increased protein intake from 0.85 to 1.1 kg per kg body weight in the protein supplement group. The high protein group lost more body weight compared to the control group (3.4 versus 2.8 kg) and more fat mass (3.2 versus 2.4 kg). The high protein group also gained 0.4 kg of appendicular muscle mass, whereas the control group lost 0.5 kg. Therefore whey protein may be beneficial to those wishing to improve body composition through an energy restriction and resistance training programme.