Exercise has been shown to have a profound influence on statural, hypertrophic, and reparative growth throughout the human life span in both men and women. Exercise can facilitate statural growth and provides the necessary mechanical and metabolic stimuli (through the secretion of GH and other anabolic hormones) that are necessary for hypertrophy of the musculoskeletal system and for reparative growth. Exercise reverses the ultra-structural changes seen with immobilization (loss of myofilaments and shrinkage of muscle fibers), and is such a potent anabolic stimulus that muscle hypertrophy can occur even under nutritionally unfavorable conditions or in the presence of wasting diseases such as AIDS and malignancy. It appears that increased tension development (either passive or active) is
the critical event in initiating compensatory growth and that this process appears to be independent of GH and insulin, as well as testosterone and thyroid hormones.
One study looked at the combined effects of exercise and energy restriction on muscle mass. The findings of this study suggest that protein synthesis is stimulated by exercise even with energy-restricted diets and that energy intake and exercise have independent effects on the regulation of muscle mass and protein synthesis.
Another study looked at protein turnover rates of two human subjects during an unassisted crossing of Antarctica. During the Austral summer of 1992–1993, two men walked 2300 km across Antarctica in 96 days, unassisted by other men, animals, or machines. During the journey they ate a high-energy diet of freeze-dried rations containing 56.7% fat, 35.5% carbohydrate, and 7.8% protein. Despite this high-energy intake both men lost more than 20 kg in body weight due to their extremely high energy expenditures. Studies of protein turnover using [15N]glycine by the
single-dose end-product method were made before, during, and after the journey, and these demonstrated considerable differences in the metabolic responses of the two men to the combined stresses of exercise, cold, and under nutrition. However, both men maintained high and relatively stable levels of protein synthesis during the expedition.
Thus even when one does not take in enough calories (usually intentionally while trying to lose weight) one can increase or at least maintain functioning muscle mass through resistance exercise. Exercise, or more correctly, the right kind of exercise is absolutely essential in producing muscle hypertrophy, and for increasing lean body mass and strength. For example, in one study two groups of obese females were compared; one group exercised while the other group did not. Both groups were on a calorie-reduced diet that contained 80 g of protein. The weight-training group showed a significant muscle hypertrophy in the muscles exercised although both groups lost the same weight. Although muscle hypertrophy in certain muscles can occur under hypocaloric conditions, significant muscle hypertrophy cannot occur if there is an inadequate intake of protein. Dietary protein levels must be sufficiently high in order to provide the substrate needed for muscular hypertrophy. In general, in order to increase muscle mass, increased exercise intensity must be accompanied by an increased dietary protein intake. When intensity of effort is high and the body is stimulated to adapt by increasing muscle mass, dietary protein intake must also be high.
Moreover, once a certain threshold of work intensity is crossed, dietary protein, and certain protein and amino acid supplements, become extremely important in augmenting the anabolic effects of exercise, not only by providing necessary substrates but also by directly influencing protein synthesis and catabolism and by influencing the hormonal milieu.
