Nutritional supplementation throughout endurance exercise: Effects on protein utilization during recovery

Changes in protein synthesis and degradation occur in the endurance athlete in response to prolonged submaximal exercise. Limited data is available regarding advantages to be gained by the endurance athlete from supplementation with complete protein. Provision of dietary protein during exercise could ameliorate the catabolism of visceral and muscle protein that occurs in response to demand for energy substrate. Alternatively, a supplement providing indispensable amino acids could assist in tissue repair and muscle recovery following exercise. Few studies have used isotopic tracer techniques to validate inferences regarding improved protein utilization during exercise recovery in athletes following nutritional supplementation. This study investigated protein metabolism during recovery in trained runners following carbohydrate and mixed carbohydrate-protein supplementation throughout endurance exercise utilizing stable isotope methodology. Endurance athletes ingested either carbohydrate (CHO), mixed carbohydrate-protein, (PTN) or no nutrients (PLA) during a 120 minute treadmill run at 65% VO$\rm\sb{2max}.$ Plasma concentrations of hormones (insulin, glucagon, growth hormone, catecholamines, and cortisol) and substrates (amino acids, glucose, lactate, and free fatty acids) were determined pre- and post-exercise and throughout the recovery period. Changes in protein synthesis and proteolysis during recovery were assessed using a primed constant infusion of L-(1-$\sp{13}$C) leucine. Supplementation did not alter the insulin or glucose response following exercise but PTN elicited an increase in glucagon and certain amino acids post-exercise. CHO attenuated increases in growth hormone and free fatty acids that were observed following the PLA trial. Greater utilization of carbohydrate throughout CHO-supplemented running was indicated by a significantly higher respiratory exchange ratio. During recovery, substrates and hormones did not differ between trials except for glucose and insulin which were increased with CHO supplementation. Protein utilization during recovery as determined by leucine rate of appearance (Ra) and oxidation (Ox) was not affected by supplementation. Consequently, supplementation did not enhance protein synthesis, estimated from the difference between Ra and Ox. In summary, CHO and PTN supplementation during endurance exercise appeared to shift reliance away from endogenous substrates but did not profoundly alter the endocrine response to endurance exercise and did not influence protein metabolism.