Although most recent research has focused on the effects of individual amino acids such as leucine and glutamine on protein synthesis, other work has shown that mixtures of amino acids can also stimulate protein synthesis. Dogs were subjected to treadmill running (150 minutes at 10 km/hr and 12% incline) and intravenously infused with a solution containing amino acids and glucose (AAG), amino acids (AA), glucose (G), or saline (S) in a randomized order. Both AAG and AA increased leucine uptake by the gut, while G and S resulted in a net release. Moreover, AAG resulted in greater uptake of leucine versus AA. The AAG treatment was the only condition that did not result in a significant release of phenylalanine from skeletal muscle.
The results indicate that AA supplementation may reduce exercise-induced proteolysis in the gut, and this effect may be enhanced by the simultaneous administration of G. Another important observation was that both AA and G might be required to prevent net protein degradation in skeletal muscle during exercise.
In another study using six healthy untrained men, an intravenous infusion of an amino acid mixture (about 0.15 g/kg-1.h-1 for 3 hours) was administered at rest and after a leg resistance exercise routine. The intravenous amino acid infusion resulted in similar elevations in arterial amino acid concentrations at rest and after exercise, whereas leg blood flow was greater after exercise than at rest. The increases in amino acid transport above baseline were greater after exercise than at rest during the hyperaminoacidemia. Although muscle protein synthesis was elevated after the infusion and was greater after exercise than at rest, muscle protein breakdown was not significantly affected either at rest or after exercise.
The researchers concluded that the stimulatory effect of exogenous amino acids on muscle protein synthesis might be enhanced by prior exercise because of enhanced blood flow. Furthermore, the results implied that protein intake immediately after exercise may be more anabolic than when ingested at some later time.
The anabolic actions of amino acids may also be independent of exercise. Research on elderly individuals (age 71 ± 2 yrs.) also used an intravenous infusion of an amino acid mixture. Muscle protein synthesis was stimulated by the increased amino acid availability whereas protein breakdown did not change. This resulted in a positive net balance of amino acids across the muscle. The researchers concluded that although muscle mass is decreased in the elderly, muscle protein anabolism might be stimulated by an increase in amino acid availability.