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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
[1-13C]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
[1-13C]leucine and intravenous
[2H3]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 < 0.05). Ox and NOLD did not
differ between conditions B and C. Splanchnic leucine
uptake calculated from [1-13C]- and
[2H3]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
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