Every organ undergoes protein synthesis—the building of new protein in the body. Muscle protein synthesis refers specifically to the building of new muscle protein. Virtually every tissue and organ in our bodies depends on muscle protein synthesis. Here’s why.
Tissues and organs are able to sustain a balance between protein synthesis and breakdown, even if you aren’t consuming enough dietary protein, because they can draw from amino acids circulating in the blood. Blood amino acid concentrations remain stable because of muscle protein breakdown.
During muscle protein breakdown amino acids are released into the bloodstream. Other tissues and organs use these amino acids to strike a balance between protein synthesis and breakdown, even when you aren’t absorbing amino acids through your diet.
In a way, muscle can be considered a reservoir of amino acids for the rest of the body. Muscle is the only tissue in the body that can afford to lose some of its mass without impairment of health. The result is a net loss of skeletal muscle (the catabolic state) in the absence of dietary protein intake.
When you ingest dietary protein the situation is reversed. An anabolic state is created in muscle, whereby dietary amino acids are absorbed and sufficient skeletal muscle protein is produced to offset the muscle protein lost during the post-absorptive state (when the GI tract is empty and energy comes from the breakdown of our body’s reserves). An anabolic state thus occurs in muscle after you eat a meal.
The importance of muscle as a source of amino acids in the blood in the absence of dietary intake can be illustrated by the IRA hunger strikes in the 1980s, when protestors starved themselves to death in British prisons. In order for some medical benefit to be derived from their sacrifice, protesters requested that blood samples be taken daily. Throughout the entire fasting period they maintained reasonably good health with the maintenance of normal blood amino acid concentrations. The point at which muscle mass became so depleted that insufficient amino acids were released to sustain normal blood amino acid concentrations signalled that death was imminent. These individuals were normal weight at the outset and survived approximately 40 days of fasting.
In contrast, severely obese individuals generally have an increased muscle mass as well as increased fat mass. There are documented cases of obese individuals surviving for more than one year of fasting with the intake of only micronutrients and water. The difference in survival time is due to the fact that even after one year of fasting obese individuals were able to maintain normal amino acid levels in the blood because of their expanded muscle mass. While these examples depict extreme circumstances, they clearly highlight the central role of muscle mass in maintaining normal amino acid availability during periods of the day in which food is not being absorbed.
Since the principle role of dietary amino acids is restoration of muscle protein mass, optimal protein and amino acid nutrition should focus on the stimulation of muscle protein synthesis, even in sedentary individuals who are unconcerned about physical function. Learn more about amino acid nutrition here.