The regulation of body and skeletal muscle protein metabolism by hormones and amino acids.

For many decades, it has been recognized that insulin, growth hormone, glucocorticoids, insulin-like growth factor 1, thyroid hormones, and other hormones regulate body protein metabolism. It has been more recently recognized, but not understood, that humor factors present in states of acute and chronic inflammation could have a strong impact on protein turnover. Most recently, the role of amino acids, acting as signaling molecules, has become increasingly clarified. In aggregate, these factors (together with neuromuscular activity) determine the balance of body protein mass. We will review some of these data, particularly focusing on amino acids, insulin, and the growth hormone axis and their actions in muscle and how these relate to whole-body protein metabolism.