Phenylalanine and tyrosine kinetics in young men throughout a continuous 24-h period, at a low phenylalanine intake

Melchor Sánchez, Antoine E. El-Khoury, Leticia Castillo, Thomas E. Chapman, Vernon R. Young

Research output: Contribution to journalArticlepeer-review

39 Scopus citations


We determined the daily rates of whole-body phenylalanine oxidation (phe- Ox) and hydroxylation (phe-OH) in young men receiving [1-13C]phenylalanine and [2H2]tyrosine via primed, constant intravenous (n = 5) or oral (n = 7) infusions for a consecutive 24 h (12-h fast followed by 12-h fed period), and given a low-phenylalanine (21.9 mg · kg-1 · d-1), no-tyrosine, but otherwise adequate L-amino acid-based diet for 6 d before the tracer study. Estimates of the daily rates of phe-Ox and phe-OH were significantly higher (P < 0.001) for the subjects receiving the oral tracer, with estimates of phe-Ox obtained with the oral tracer during the 12-h fast period being close to those predicted from similar 24-h leucine kinetic studies (Am J Clin Nutr 1994;59:1000-11). There was generally poor agreement between the measured 24- h rates of phe-Ox and phe-OH compared with the daily rates as predicted from the last hour of the 12-h fast and 5th hour of feeding. From the 24-h data, daily phenylalanine balances were estimated to be positive with the intravenous-tracer protocol and negative with the oral-tracer group. Our results question the adequacy of current international recommendations for aromatic amino acid requirements in healthy adults.

Original languageEnglish (US)
Pages (from-to)555-570
Number of pages16
JournalAmerican Journal of Clinical Nutrition
Issue number3
StatePublished - Mar 1995
Externally publishedYes

ASJC Scopus subject areas

  • Medicine (miscellaneous)
  • Nutrition and Dietetics


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