Title

Skeletal muscle protein turnover in runners and endurance-trained adults consuming the RDA for protein

Date of Completion

January 2003

Keywords

Biology, Animal Physiology|Health Sciences, Nutrition|Health Sciences, Recreation

Degree

Ph.D.

Abstract

Dietary protein needs of physically active individuals are a highly debated topic. However, the relationship between endurance exercise, protein intake, and skeletal muscle protein metabolism has received little attention in the scientific literature. This investigation consists of two studies which provide insight to this area. In the first investigation, five male runners participated in a randomized, crossover design diet intervention, where they consumed either a low (0.8 g/kg; LP), moderate (1.8 g/kg; MP), or high (3.6 g/kg; HP) protein diet for 4 weeks. Diets were isocaloric, with carbohydrate, fat and protein approximating 60%, 30%, 10%; 55%, 30%, 15; and 40%, 30%, 30% for LP, MP, and HP respectively. Mixed muscle protein fractional synthetic and breakdown rates (FSR/FBR) were assessed at week 4 following a 75 min run at 70% VO2peak using primed continuous infusions of [2H5]phenylalanine (FSR) and [15N]phenylalanine (FBR). Postexercise FSR was significantly higher for LP and MP compared to HP. Preliminary findings demonstrated a trend for postexercise FBR to decrease with increasing protein intake, thereby resulting in a less negative NET balance (FSR-FBR). This is the first study to demonstrate that dietary protein intake can impact protein utilization postexercise in the fasted state. ^ In the second study, the impact of 6-weeks of endurance training on FSR and FBR was examined in 12 untrained individuals fed eucaloric diets containing 0.8 g protein /kg/d. Subjects were divided into an egg and no egg group to examine the impact of protein source on protein utilization. Resting FSR and FBR significantly increased and NET balance became more negative (p < 0.05) following 4 weeks of training (p < 0.05). After a 45min run at 65% VO 2max, FBR significantly increased compared to rest (p < 0.05). Consumption of the egg based diet did not significantly impact protein metabolism with the exception of the egg group demonstrating a significantly higher FBR postexercise compared to the no egg group (p < 0.05). These findings are the first to demonstrate that routine endurance training significantly impacts skeletal muscle protein metabolism at rest and following a single exercise session. The impact of dietary protein source on these parameters was less clear and warrants further investigation. ^