Dietary protein intake impacts human skeletal muscle protein fractional synthetic rates after endurance exercise

Departments of 1 Nutritional Sciences and 2 Kinesiology, University of Connecticut, Storrs, Connecticut; 3 Metabolism Unit, Shriners Burns Institute, Department of Surgery, University of Texas Medical Branch, Galveston, Texas; and 4 Department of Biology, Northern Ontario School of Medicine, Sudbury, Ontario, Canada Submitted 14 February 2005 ; accepted in final form 19 May 2005 This investigation evaluated the physiological impact of different dietary protein intakes on skeletal muscle protein synthesis postexercise in endurance runners. Five endurance-trained, 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 wk. Diets were designed to be eucaloric with carbohydrate, fat, and protein approximating 60, 30, and 10%; 55, 30, and 15%; and 40, 30, and 30% for LP, MP, and HP, respectively. Substrate oxidation was assessed via indirect calorimetry at 3 wk of the dietary interventions. Mixed-muscle protein fractional synthetic rate (FSR) was measured after an endurance run (75 min at 70% O 2 peak ) using a primed, continuous infusion of [ 2 H 5 ]phenylalanine. Protein oxidation increased with increasing protein intake, with each trial being significantly different from the other ( P < 0.01). FSR after exercise was significantly greater for LP (0.083%/h) and MP (0.078%/h) than for HP (0.052%/h; P < 0.05). There was no difference in FSR between LP and MP. This is the first investigation to establish that habitual dietary protein intake in humans modulates skeletal muscle protein synthesis after an endurance exercise bout. Future studies directed at mechanisms by which level of protein intake influences skeletal muscle turnover are needed. amino acids; fractional synthetic rate; humans Address for reprint requests and other correspondence: N. Rodriguez, Dept. of Nutritional Sciences, 3624 Horsebarn Hill Rd. Ext., U-4017, Univ. of Connecticut, Storrs, CT 06269 (e-mail: nancy.rodriguez{at}uconn.edu )