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J Appl Physiol 70: 43-47, 1991;
8750-7587/91 $5.00
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Journal of Applied Physiology, Vol 70, Issue 1 43-47, Copyright © 1991 by American Physiological Society

ARTICLES

Leucine metabolism during fasting and exercise

J. Knapik, C. Meredith, B. Jones, R. Fielding, V. Young and W. Evans
US Army Research Institute of Environmental Medicine, Natick 01760-5007.

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-[1-13C]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 13C enrichment of leucine and alpha-ketoisocaproic acid, and expired gas samples were analyzed for 13CO2. 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) mumol.kg-1.h-1 at rest and rose to 75 +/- 21 mumol.kg-1.h-1 during exercise; in the PAS, oxidation was 20 +/- 5 mumol.kg-1.h-1 at rest and 52 +/- 17 mumol.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.


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