Up to 40% of the protein found in the body exists as muscle tissue. The turnover and replacement of this protein mass is called protein metabolism. Protein metabolism in muscle accounts for about one-half to one-third of the total protein turnover in the body.
The basic building block of muscle is protein. The balance between protein production versus protein breakdown determines whether muscles grow or shrink. Physical activity or exercise, particularly in the form of resistance exercise, results in net synthesis of protein, and muscle growth. However, the ability of exercise to change protein metabolism involves a complex interplay between the type of physical activity (intensity, frequency, duration), dietary considerations, and hormonal input.
Muscle mass is governed by a use-disuse principle. With increased physical activity, muscles adapt by growing larger (muscle hypertrophy), whereas in sedentary states the loss of mechanical activity leads to muscle breakdown (atrophy).
A key determinant of the protein synthesis rate in muscle is the availability of free amino acids. This availability is controlled by the levels of amino acids in the bloodstream that, in turn, is dependent on diet. Dietary consumption of protein in the post-recovery period is critical for the ability of muscle to increase its mass in response to exercise. In fact, fasting after exercise leads to muscle atrophy. Diet also modulates the anabolic (growth) response of muscle to exercise by changing the level of the hormone insulin. Insulin and insulin-like growth-factor are known to decrease the rate of protein degradation in muscle, and can help tilt the balance of protein metabolism in favor of protein synthesis.
Due to the adaptive nature of protein metabolism in muscle, increased protein synthesis rate in the exercise recovery period may lead simply to homeostasis (maintenance of existing tissue) rather than muscle growth. The anabolic response of muscle to exercise follows a threshold rule, wherein future growth often requires the mechanical stress on the muscles to exceed the levels previously experienced. Hence, body builders must continually lift heavier loads to grow larger muscles. Moreover, this explains why dynamic or low-impact exercise regimens, while beneficial to overall (especially cardiovascular) health, do not promote robust muscle growth. Research studies on the anabolic effects of dynamic exercise often produce conflicting conclusions depending on whether they study trained subjects (whose muscles may have already maximized their responses) or untrained subjects. While the details of how muscles establish the threshold for this anabolic response are incompletely understood, it appears to be due in part to the fact that trained muscles also show decreased protein degradation during the acute phases of exercise.
Because of the pivotal role of protein metabolism in maintaining and building muscle mass, there is great interest in trying to use nutritional supplements to enhance muscle production in athletes. Evidence suggests that protein types can affect muscle anabolism through increased protein synthesis. Milk protein in particular has shown some beneficial effects for athletes. Diets that meet the energetic demands of the individual and derive 12%–15% of their total calories from protein are considered sufficient to support the metabolic needs of muscle. The timing and quantity of protein consumption are important in influencing protein synthesis. Approximately 20 g of protein should be consumed after exercise and at subsequent meals following training.
See also Nutritional supplements .
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Daniel M. Cohen, PhD