Effects of four weeks of daily soy or dairy milk ingestion on the exercise-induced proteolytic responses in plasma and skeletal muscle in a post-menopausal female population.

Sarcopenia is defined as a loss of muscle mass and muscle strength associated with aging. Changes in muscle function can occur as early as the fourth decade of life. Evidence indicates that the calpain and ubiquitin proteolytic degradation pathways account for the majority of myofibrillar protein degradation seen with muscle atrophy. The purpose of this study was to investigate the effects of daily consumption of soy or dairy milk in older females on systemic plasma and local skeletal muscle markers of muscle proteolysis before and after a single bout of eccentric exercise. Thirty one female participants consumed three servings per day of either soy or dairy milk for four weeks, followed by a 45 minute downhill run. Testing sessions were complete pre and post supplementation, as well as at 0, 4, 24, and 48 hrs post exercise. Each testing session included blood collection and vastus lateralis muscle biopsy, except no biopsy was performed immediately or 48 hrs post exercise. The blood was analyzed for plasma creatine kinase and the muscle tissue for mRNA expression of ubiquitin, E2, E3, atrogin-1, muRF1, calpain 1, and calpain 2. Although no significant group x time interactions were observed, a significant time effect was observed for plasma CK (p<0.0001) and muscle mRNA expression of E3 (p=0.04), atrogin-1 (p=0.02), and calpain 2 (p=0.04). Further, evaluation revealed that these differences resulted after the acute exercise bout. The results of this study showed that soy milk ingestion was not influential in attenuating the expression of the ubiquitin and calpain proteolytic pathways compared to dairy milk; however, the eccentric exercise bout was effective at altering expression of markers within both pathways. Future studies should address potential confounding dietary factors by utilizing an inert control group to determine if both soy and dairy milk could be protective with regard to exercise induced proteolysis.