Substrates are powerful modulators of amino acid and protein turnover in vivo
(Table 4). Intravenous infusions of amino acids exert a protein-anabolic effect,
because they directly inhibit endogenous protein degradation and stimulate
protein synthesis at the whole-body level. A stimulation of protein synthesis has
been observed also at the forearm level. These changes resulted in an improvement
of body and tissue protein balance, which is the ultimate goal of any nutritional
intervention aimed at preserving body protein stores. In humans acute intravenous
infusions of carbohydrates do not appear to affect either protein degradation or
leucine oxidation. However, animal studies support the view that glucose
availability spares essential amino acids at least in the fetus. The effects of
hypercaloric refeeding with high-carbohydrate diets may, however, result in
increased protein turnover. Lipids, in the form of long-chain fatty acids,
inhibit endogenous protein breakdown and may suppress leucine oxidation in the
whole body. They do not affect protein synthesis. In contrast, medium-chain fatty
acids apparently increased leucine oxidation, and therefore increased net protein
catabolism. Ketone bodies may be anabolic provided that fatty acid concentrations
are not concurrently decreased
