LEUCINE METABOLISM DURING FASTING AND EXERCISE

Whole body leucine kinetics were examined in seven healthy young men while in a 14-h postabsorptive state (PAS) and after a 3.5-day fast (FS). Subjects received a primed constant intravenous infusion of L-[1-C-13]leucine while resting for 3 h and then while exercising on a cycle ergometer at 45% maximal O2 uptake to exhaustion. Blood samples drawn during isotopic steady state were analyzed for C-13 enrichment of leucine and alpha-ketoisocaproic acid, and expired gas samples were analyzed for CO-2-(C-13). Resting leucine flux was higher in the FS, and there was a slight increase in leucine oxidation. During exercise, leucine flux did not differ between PAS and FS but leucine oxidation rose markedly. In the FS, leucine oxidation was 25 +/- 7 (SD)-mu-mol.kg-1.h-1 at rest and rose to 75 +/- 21-mu-mol.kg-1.h-1 during exercise; in the PAS, oxidation was 20 +/- 5-mu-mol.kg-1.h-1 at rest and 52 +/- 17-mu-mol.kg-1.h-1 during exercise. These data indicate that the high rate of leucine oxidation previously found during exercise was increased further by a 3.5-day fast.