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

Am J Physiol Endocrinol Metab. 2000 Jun;278(6):E1000-9. doi: 10.1152/ajpendo.2000.278.6.E1000.


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 micromol. 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-(13)C]- and [(2)H(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.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Adult
  • Amino Acids / administration & dosage*
  • Carbon Dioxide / analysis
  • Carbon Isotopes
  • Caseins / administration & dosage*
  • Deuterium
  • Diet
  • Dietary Proteins / administration & dosage*
  • Female
  • Humans
  • Insulin / blood
  • Keto Acids / blood
  • Kinetics
  • Leucine / administration & dosage*
  • Leucine / blood
  • Leucine / pharmacokinetics*
  • Male
  • Mesentery / metabolism
  • Oxidation-Reduction


  • Amino Acids
  • Carbon Isotopes
  • Caseins
  • Dietary Proteins
  • Insulin
  • Keto Acids
  • Carbon Dioxide
  • alpha-ketoisocaproic acid
  • Deuterium
  • Leucine