24-HOUR INTRAVENOUS AND ORAL TRACER STUDIES WITH L-[1-C-13]-2-AMINOADIPIC ACID AND L-[1-C-13]LYSINE AS TRACERS AT GENEROUS NITROGEN AND LYSINE INTAKES IN HEALTHY-ADULTS

Background: This is a continuation of investigations of the relations between amino acid kinetics and amino acid dietary requirements in hea lthy adults. Objective: The aim was to investigate the 24-h pattern an d rate of the metabolism of an L-[1-C-13]-2-aminoadipic acid ([C-13]AA A) tracer and of whole-body L-[1-C-13]lysine ([C-13]lysine) oxidation and balance in healthy, young adults receiving a generous intake of ly sine. Design: Thirteen healthy adults were given an adequate, L-amino acid-based diet supplying 77 mg lysine.kg(-1).d(-1) for 6 d before the tracer studies. Two subjects received [C-13]AAA intravenously and 2 r eceived it orally; 3 subjects received [C-13]lysine intravenously and 6 received it orally. We measured (CO2)-C-13 output, plasma [C-13]AAA and [C-13]lysine enrichment, and urinary [C-13]AAA. Results: [C-13]AAA oxidation was estimated to be higher after the orally administered th an after the intravenously administer tracer; plasma [C-13]AAA was sim ilar to urinary [C-13]AAA. Whole-body lysine oxidation showed a rhythm that was induced by meal feeding. The intravenous [C-13]lysine tracer gave mean estimates of lysine balances (lysine intake minus oxidation ) that apparently were too low (-15.7 mg.kg(-1).d(-1)) or too high (16 .6 mg.kg(-1).d(-1), P < 0.05 from zero balance) on the basis of urinar y [C-13]AAA or plasma [C-13]lysine estimates of oxidation, respectivel y. For the orally administered tracer and plasma [C-13]lysine enrichme nt, the mean balance was slightly positive (8.7 mg.kg(-1).d(-1), P < 0 .05 from zero). Conclusions: Use of urinary [C-13]AAA as an index of t he enrichment of the precursor pool did not appear to significantly im prove the estimate of the fasting and feeding components of daily lysi ne balance. For estimates of daily, whole-body lysine oxidation, we pr opose use of plasma [C-13]lysine with a 24-h, orally administered trac er protocol.