workload leads to an increase in dietary protein needs. Industrial workers were and are served bigger steaks the more demanding their daily workload is. Milk is promoted as the "white motor" in advertisements. For these reasons, tons of protein are sold each year to strength athletes, as they believe they can only make their muscles grow when they ingest protein supplemental to their usual diet, and to endurance athletes, because they believe they break down muscle protein during training and competition.
The high-purity crystalline amine acids now available have created an easy new health food and supplementation market among the athletes who already believed in the beneficial effect of protein. Advertisements claim that free amine acids and small peptides are a better source of amine acids to support high net rates of protein deposition because they are more readily absorbed than whole dietary protein. The rapid absorption of amine acids leads to high plasma and tissue amine acid concentrations, In fact, it seems to drive the amine acids primarily into the oxidation pool, since the Michaelis constant (Km) of the oxidative enzymes is much higher than the Km of the enzymes involved in protein synthesis. By definition, increased oxidation means a reduced net rate of protein deposition, at least in the first hours after ingestion. Apart from serving as building blocks for protein synthesis, several amine acids have unique functions in human physiology, and they are marketed as supplements to support these functions. However, the scientific evidence to support the claim that they are needed in addition to the habitual ingestion of dietary proteins is usually lacking.
This chapter focuses on several aspects of protein and amine acid metabolism in endurance exercise and on the effect of amine acid and protein supplementation on performance-related aspects of metabolism. First presented will be the author's opinion on whether endurance exercise leads to net protein catabolism, increased amine acid oxidation, and therefore an increased protein requirement. A detailed overview is given elsewhere in this volume (see also Rennie, 1996). Subsequently, the rationale for supplementation with branched-chain amine acids (BCAAs) and glutamine and the effects on metabolism and performance will be evaluated. Finally, suggestions will be presented that the glycogen resynthesis rates in muscle following exercise can be accelerated by the combined ingestion of carbohydrate and protein (Zawadzki et al., 1992) and by glutamine infusion (Varnier et al., 1995).
Recent stable isotope tracer studies have reopened the discussion on whether net protein breakdown (protein synthesis < protein degradation) and increased amino acid oxidation occur during prolonged endurance exercise at the whole body level. If the answer is yes, the next question is whether net protein breakdown occurs in muscle or in the gut and liver area. Stable isotope