Myofibrillar protein synthesis following ingestion of soy protein isolate at rest and after resistance exercise in elderly men

Abstract

Background: Increased amino acid availability stimulates muscle protein synthesis, however, aged muscle appears less responsive to the anabolic effects of amino acids when compared to the young. We aimed to compare changes in myofibrillar protein synthesis (MPS) in elderly men at rest and after resistance exercise following ingestion of different doses of soy protein and compare the responses to those we previously observed with ingestion of whey protein isolate. Methods. Thirty elderly men (age 715 y) completed a bout of unilateral knee-extensor resistance exercise prior to ingesting no protein (0g), or either 20g or 40g of soy protein isolate (0, S20, and S40 respectively). We compared these responses to previous responses from similar aged men who had ingested 20g and 40g of whey protein isolate (W20 and W40). A primed constant infusion of L-[1- 13C]leucine and L-[ring- 13C 6]phenylalanine and skeletal muscle biopsies were used to measure whole-body leucine oxidation and MPS over 4h post-protein consumption in both exercised and non-exercised legs. Results: Whole-body leucine oxidation increased with protein ingestion and was significantly greater for S20 vs. W20 (P=0.003). Rates of MPS for S20 were less than W20 (P=0.02) and not different from 0g (P=0.41) in both exercised and non-exercised leg muscles. For S40, MPS was also reduced compared with W40 under both rested and post-exercise conditions (both P<0.005); however S40 increased MPS greater than 0g under post-exercise conditions (P=0.04). Conclusions: The relationship between protein intake and MPS is both dose and protein source-dependent, with isolated soy showing a reduced ability, as compared to isolated whey protein, to stimulate MPS under both rested and post-exercise conditions. These differences may relate to the lower postprandial leucinemia and greater rates of amino acid oxidation following ingestion of soy versus whey protein. © 2012 Yang et al.; licensee BioMed Central Ltd.