Gut proteolysis contributes essential amino acids during exercise.

Arteriovenous difference and tracer dilution techniques were utilized to determine the effect of exercise on whole body, gut, liver, and splanchnic leucine kinetics. Five postabsorptive dogs were infused with [1-13C]leucine and studied during rest, 90 min of moderate-intensity treadmill exercise (1st 45 min, early; last 45 min, late exercise), and 90 min of recovery. The whole body leucine rate of appearance (Rai; mumol.min 1.kg-1) increased from rest (3.33 +/- 0.11) during early (3.68 +/- 0.14) and late (4.24 +/- 0.27, P < 0.05) exercise and was 3.41 +/- 0.19 during recovery. Gut Ra increased from rest (0.64 +/- 0.08) during early (0.92 +/- 0.12) and late (1.30 +/- 0.20, P < 0.05) exercise and was 0.77 +/- 0.16 during recovery. Liver leucine Ra did not significantly change (P > 0.05). The whole body leucine rate of disappearance (Rd) paralleled whole body leucine Ra throughout. Leucine Rd across the gut, liver, and splanchnic bed, however, did not significantly change (P > 0.05), indicating an increase in leucine uptake outside of these regions. Because active skeletal muscle is likely the principal consumer of these amino acids, the data suggest that gut protein-derived amino acids are utilized for the attenuation of net muscle protein catabolism during and immediately following exercise.