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An experiment was conducted show that moderate exercise at energy balance does not affect 24-hr leucine oxidation or nitrogen retention in healthy men. A continuous 24-h intravenous [I-13C]leucine/[15N] lurea tracer study in eight healthy adult men was carried out. Their diet supplied 1 g protein-kg-1.day-1 and exercise was for 90 min during the 12-hr fast and 12-hr fed periods of the day. The diet and exercise regimen were adapted for six days and tests were carried out on the seventh day. Results show that moderate exercise does not affect whole body leucine kinetics.

Metges, Cornelia C., Antoine E. El-Khoury, Ambalini B. Selvaraj, Rita H. Tsay, Alan Atkinson, Meredith M. Regan, Brian J. Bequette, and Vernon R. Young. Kinetics of L-[1-[sup.13]C]leucine when ingested with free amino acids, unlabeled or intrinsically labeled casein. Am J Physiol Endocrinol Metab 278: E1000-E1009, 2000.--In two groups of five adults, each adapted to two different dietary regimens for 6 days, the metabolic fate of dietary [1-[sup.13]C]leucine was examined when ingested either together with a mixture of free amino acids simulating casein (extrinsically labeled; condition A), along with the intact casein (extrinsically labeled; condition B), or bound to casein (intrinsically labeled; condition C). Fed state leucine oxidation (Ox), nonoxidative leucine disposal (NOLD), protein breakdown, and splanchnic uptake have been compared using an 8-h oral [1-[sup.13]C]leucine and intravenous [[sup.2][H.sub.3]]leucine tracer protocol while giving eight equal hourly mixed meals. Lower leucine Ox, increased NOLD, and net protein synthesis were found with condition C compared with condition A (19.3 vs. 24.9; 77 vs. 55.8; 18.9 vs. 12.3 [micro]mol [multiplied by] [kg.sup.-1] [multiplied by] 30 [min.sup.-1]; P [is less than] 0.05). Ox and NOLD did not differ between conditions B and C. Splanchnic leucine uptake calculated from [1-[sup.13]C]- and [[sup.2][H.sub.3]]leucine plasma enrichments was between 24 and 35%. These findings indicate that the form in which leucine is consumed affects its immediate metabolic fate and retention by the body; the implications of these findings for the tracer balance technique and estimation of amino acid requirements are discussed. leucine oxidation; leucine flux; amino acid mixture; intrinsic label