EFFECTS OF PROTEIN INTAKE ON LEUCINE KINETICS, GLUCOSE RATE OF APPEARANCE, AND SUBSTRATE OXIDATION IN RUNNERS

Abstract

Recent promotion of high protein diets for enhanced performance necessitates evaluation of the metabolic responses to major changes in macronutrient intake in competitive athletes. This study compared the effects of increased dietary protein intake on whole body leucine kinetics, glucose rate of appearance (GluRa), and substrate oxidation in endurance runners. Six endurance trained athletes (mean ± SEM, 20 ± 0.6 yrs., 70 ± 1.2kg, 9 ± 0.5% body fat, 180 ± 5 cm, VO2peak 71 ± 1.9 ml/kg/min) were randomly assigned to either a high (3.6 g/kg, HP) or moderate protein (1.8 g/kg, MP) diet in a cross over design experiment. Diets were isocaloric, with the percentages of carbohydrate, fat, and protein approximating 55%, 30%, 15%, and 40%, 30%, 30%, for MP and HP respectively. Leucine kinetics (rate of appearance, oxidation, and nonoxidative leucine disposal) and GluRa were determined using primed, continuous infusions of 1[13C] leucine and 6,6 [2H] glucose, respectively. Substrate oxidation was estimated via indirect calorimetry. Plasma concentrations of insulin, glucagon, and free fatty acids were also determined. Dietary protein intake did not influence leucine kinetics. GluRa was lower for HP compared to MP (p = 0.08, 13.7 vs 15.1 umol/kg/min). Protein oxidation was significantly increased (p < 0.01) for HP (52%) vs MP (25%). These differences were paralleled by changes in carbohydrate oxidation (p < 0.01, 25% vs 51% for HP and MP, respectively). Insulin levels were greater (p = 0.05) for MP than HP while no differences for glucagon and free fatty acids were noted. Limitations associated with whole body leucine methodology coupled with the noted difference in glucose turnover make it premature to conclude that increased dietary protein provided no benefit for protein utilization in endurance runners. However, the lack of change in oxidation of the essential amino acid leucine in the presence of increased protein oxidation via indirect calorimetry indicates an effect of increased protein intake on protein metabolism. Thus, further studies are therefore needed to delineate the mechanisms by which these changes occurred and to evaluate the effects of similar dietary interventions on skeletal muscle protein metabolism in endurance runners. Supported by the National Cattlemen's Beef Association