title: Kinetics of L-[1-(13)C]leucine when ingested with free amino acids, unlabeled or intrinsically labeled casein.
abstract: 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.
authors.affiliation: Clinical Research Center and Laboratory of Human Nutrition, School of Science, Massachusetts Institute of Technology, Cambridge, Massachusetts 02142, USA. metges@www.dife.de
authors.forename: C C
authors.initials: CC
authors.lastname: Metges
authors.forename: A E
authors.initials: AE
authors.lastname: El-Khoury
authors.forename: A B
authors.initials: AB
authors.lastname: Selvaraj
authors.forename: R H
authors.initials: RH
authors.lastname: Tsay
authors.forename: A
authors.initials: A
authors.lastname: Atkinson
authors.forename: M M
authors.initials: MM
authors.lastname: Regan
authors.forename: B J
authors.initials: BJ
authors.lastname: Bequette
authors.forename: V R
authors.initials: VR
authors.lastname: Young
mesh_terms: D000328:Adult; D000596:Amino Acids / Q000008:administration & dosage*; D002245:Carbon Dioxide / Q000032:analysis; D002247:Carbon Isotopes; D002364:Caseins / Q000008:administration & dosage*; D003903:Deuterium; D004032:Diet; D004044:Dietary Proteins / Q000008:administration & dosage*; D005260:Female; D006801:Humans; D007328:Insulin / Q000097:blood; D007651:Keto Acids / Q000097:blood; D007700:Kinetics; D007930:Leucine / Q000008:administration & dosage* / Q000097:blood / Q000493:pharmacokinetics*; D008297:Male; D008643:Mesentery / Q000378:metabolism; D010084:Oxidation-Reduction
chemical_list: D000596:Amino Acids; D002247:Carbon Isotopes; D002364:Caseins; D004044:Dietary Proteins; D007328:Insulin; D007651:Keto Acids; D002245:Carbon Dioxide; C013082:alpha-ketoisocaproic acid; D003903:Deuterium; D007930:Leucine