Amino acids, hormones and physical activity appear to be the key short-term physiological regulators of muscle mass. In this article we will investigate their effects on musculoskeletal mass.
The machinery of the body, i.e. muscle, tendon, ligament and bone, accounts for the overwhelming amount of protein bound amino acid in the body, with the collagen in connective tissue alone accounting for approx. 3 kg of body weight.
One of the interesting features of working in whole human beings is the extent to which extrapolations from previous work in animals are not necessarily accurate, and I will point out some of these later.
However, the stimulation of muscle protein synthesis by amino acids is not infinite and appears to be time-limited; in other words, there is tachyphylaxis of the process in terms
of amino acids supplied. This phenomenon, previously recognized by Joe Millward and given the term ‘muscle full’, appears to limit the extent to which muscles protein synthesis can be stimulated by a single meal on a given occasion.
One feature of human muscle that is different from that in rodents, lagomorphs and fowl is that there appears to be no effect of fibre type on the rates of muscle protein synthesis.
In animals, the redder the muscle the faster the rate of protein synthesis, but we can find no biologically significant effect at rest or after feeding of fibre proportions in human soleus, triceps or quadriceps.
Although there are occasional reports to the contrary, I think that the balance of evidence suggests that insulin has no stimulatory effect on muscle protein synthesis in adult human muscle, which will come as a surprise to many who work on rodents or who read current textbooks.
It seems that the insulin stimulation of protein synthesis may be a phenomenon associated with growth or immaturity, and indeed most studies showing the phenomenon in rats
are with animals that are still growing.
There is no doubt, however, that insulin markedly inhibits human muscle protein breakdown. This process seems to be exquisitely sensitive, with most of the inhibition breakdown occurring at plasma levels of insulin between those seen in an overnight fast and after a small meal, i.e. in the range of 0–15 m-units/l.
Muscle and tendon collagen synthesis are unresponsive to alterations in nutrient availability, but surprisingly bone collagen synthesis is.
It has been known for many years that exercise causes major alterations in whole-body amino acid metabolism, and, although it is not true as Liebig suggested that muscle burns protein as a major fuel, there are marked increases in the oxidation of amino acids as a result of exercise, with the branched chain amino acids showing linear increase in their oxidation with energy expenditure.
In the post-exercise period after strenuous exercise, there are marked increases in muscle protein synthesis, with a 2-fold increase apparent within 12–24 h followed by a slow decrease over the next 24–72 h. In less strenuous dynamic exercise (walking, cycling or running at low power outputs), there appears to be some stimulation of general muscle synthesis but this lasts for a relatively short period of a few hours.