Protein and energy metabolism during prolonged exercise in trained athletes

Eight elite triathlon athletes participated in a laboratory study of the effects of endurance exercise on protein and energy metabolism. The study consisted of 3 h of cycling and 5 h of treadmill running; 3.5 h before beginning the exercise, a primed constant infusion of 1-¹³C leucine and 6,6^-2H glucose was begun. Serial blood samples were collected during the rest and exercise periods for isotopic analysis. Respiratory gas exchange was determined every half hour. Results: the subjects exercised at an average of 53% ± 3% of peak V̇O₂. During the 8-h exercise period there was a decline in glucose utilization and an increase in lipid oxidation. For the first part of the exercise, most of the glucose oxidized was of muscle origin. Hepatic glucose production increased with exercise from 20 g/h to a maximum of about 60 g/h after 4 h of exercise and then decreased toward the pre-exercise rate. The plasma urea concentration was unchanged during the study. The leucine flux decreased during the first 4 h of exercise and then attained a new plateau about 20% lower than the pre-exercise value indicating an adaptive reduction in protein turnover.