METGES, CORNELIA C., EL-KHOURY, ANTOINE E., SELVARAJ, AMBALINI B., TSAY, RITA H., ATKINSON, ALAN, REGAN, MEREDITH M., BEQUETTE, BRIAN J., and YOUNG, VERNON R.
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