Resistance training exerts high contractile forces on skeletal muscle which results in an anabolic response that is characterised by muscle protein synthesis possibly via activation of the insulin signalling mTOR kinase pathway. This period of protein synthesis can last for up to 48 hours and can be maximised by the consumption of branched chain amino acids, particularly leucine. Because the contractile forces in endurance exercise are lower, protein synthesis is not stimulated to the same extent as with resistance training, and this explains the lower levels of skeletal muscle seen in endurance athletes compared to those involved in classic strength sports. The lower levels of muscle protein synthesis seen in endurance exercise are associated with a fall in plasma levels of leucine, which may be attributed to the increased demand of branched chain amino acids to provide energy for muscle contraction.
It may be therefore, that providing supplemental leucine during or after endurance exercise is able to enhance protein synthesis by sparing endogenous stores, which are contained mainly in skeletal muscle tissue. Researchers1 have investigated the effects of 2 different concentrations of leucine on the muscle protein synthesis seen after bouts of endurance exercise (60% VO2 max). Subjects consumed 10g essential amino acid drink with either 3.50 or 1.87g leucine during exercise in a randomised crossover design study. Muscle protein synthesis and whole body protein turnover were assessed by using a primed continuous infusion of [2H5]phenylalanine and [1-13C]leucine, and mTOR complex 1 signalling was assessed with multiplex and immunoblot analyses. The results showed that protein synthesis was 33% greater after consumption of the high leucine drink compared to the low leucine drink.
These data would suggest that supplemental leucine is able to spare endogenous protein stores. This supports other studies that show that high quality protein during or after exercise is able to prevent whole body proteolysis and improve nitrogen retention. Because the high leucine drink was more effective than the low leucine drink at stimulating protein synthesis, leucine appears to be a key component that facilitates this the anti-proteolytic effect. There was no difference in the stimulation of mTOR phosphorylaton irrespective of which drink was consumed, despite differences in the amounts of muscle protein synthesis detected. Therefore intracellular signalling via the mTOR pathway appears not to be a definitive predictor of muscle protein synthesis in endurance exercise. The exact amount of supplementary protein necessary to stimulate optimal amounts of protein synthesis is not known, but high quality whey protein has been shown to be effective because of its high leucine concentrations.
RdB
