Kinetics of L-[1-13C]leucine when ingested with free amino acids, unlabeled or intrinsically labeled casein

1 Clinical Research Center and Laboratory of Human Nutrition, School of Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02142; and 2 Rowett Research Institute, Bucksburn, Aberdeen AB21 9SB, Scotland In two groups of five adults, each adapted to two different dietary regimens for 6 days, the metabolic fate of dietary [1- 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- 13 C]leucine and intravenous [ 2 H 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 µmol · kg 1 · 30 min 1 ; P