The synthesis of muscle protein is essential to the body's ongoing growth, repair, and maintenance of its skeletal muscle groups. The other types of human muscle tissue, cardiac muscle and the smooth muscles that are part of internal organ structure, are constituted through different cellular processes.
Proteins are the compounds comprised of amino acids—the building blocks of tissue formation within the body. The synthesis of protein is the method by which muscles are constructed. The human body synthesizes protein from diet at a rapid rate while the body is growing through adolescence and into young adulthood. The rate at which protein is synthesized slows significantly after age 20. It is for this reason that even among active, highly trained adults, the actual rate of muscle growth will be far less in relative terms to that of a healthy teenager.
In an adult athlete, the synthesis of muscle protein is also related to how the muscles are being exercised. Other than the ongoing repair and maintenance of existing muscle tissues that may be damaged through the course of daily living, human skeletal muscle will never get larger or stronger through either sedentary activities or the consumption of particular foods or supplements. Muscular activity is a prerequisite of meaningful muscle development, built on protein synthesis.
All forms of physical activity will direct specific stress into a muscle. In a sport such as distance running or cycling, the stresses are cumulative, the combined effect of repetitive movements that are at a relatively lower level of intensity. In activities that involve explosive and powerfully focused movements (such as weight training) the forces directed against the muscle are much more significant, and they occur over a much shorter time span.
In each circumstance, the muscle will naturally break down, a process known as "catabolism." The breakdown includes the physical separation of the fibers that comprise the muscle structure. The subsequent repair of the damaged muscle is "anabolism," the building up and the growth of the existing and previously damaged fiber. Anabolic steroids take their name due to their contribution to the building up of muscles. Protein synthesis is the mechanism by which the body affects this repair and muscle growth: as a very general proposition, when the body produces more synthesized protein than it consumes through its catabolic processes, muscle will be developed.
There are measurable ways to determine the balance in the body on an ongoing basis between its catabolic and its anabolic states. The essential amino acid, leucine, is used as an indicator of the state of this balance in more sophisticated sports science analyses. A positive leucine balance is evidence that this acid is present in the cells, a condition consistent with protein anabolism. Leucine is a component of numerous commercial protein supplements taken to stimulate further protein synthesis in the body.
Muscle protein synthesis is also considered in the context of sore, damaged, or overused muscles. Sports research in this area has focused not so much on whether the ingestion of protein is likely to be useful to the athlete, but at what point after the exercise should the protein be consumed so that the body can derive a maximum restorative and muscle-building effect. The combined effects of carbohydrate consumption and protein consumption have also been thoroughly considered in recent years.
The sports science community supports the usefulness of carbohydrate replacement immediately after exercise. Such replacement tends to deliver glycogen to the affected muscles more quickly; in addition, the entire body has a replenished supply of carbohydrates from which the whole of the musculoskeletal system can be restored. When proteins are consumed along with carbohydrates immediately after exercise, the catabolic process is not stopped within the affected muscles; the process of protein synthesis is immediately stimulated (a kick-start is an expression commonly employed in the research to describe the effect). This action leads to the prevention of further protein loss in the muscle. As the degradation of the muscle due to strenuous exercise will not reach its peak for approximately three days after the exercise that affected the muscle, it is important to continue the ingestion of protein. The maintenance of consistent dietary practices is essential to the body's ability to respond on an ongoing basis to the demand for muscle protein synthesis.
The body has a need to ensure effective muscle protein synthesis throughout the course of an athletic career. With the rise in masters level participation in a wide variety of sports (generally defined as competitions for athletes aged 40 years and older), older athletes are affected by catabolic and anabolic processes. The body's response to the increased consumption of protein after exercise does not significantly vary with age, for either men or women.