PHENYLALANINE AND TYROSINE KINETICS IN YOUNG MEN THROUGHOUT A CONTINUOUS 24-H PERIOD, AT A LOW PHENYLALANINE

We determined the daily rates of whole-body phenylalanine oxidation (p he-Ox) and hydroxylation (phe-OH) in young men receiving [1-C-13]pheny lalanine and [H-2(2)]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 receivi ng the oral tracer, with estimates of phe-Ox obtained with the oral tr acer during the 12-h fast period being close to those predicted from s imilar 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. O ur results question the adequacy of current international recommendati ons for aromatic amino acid requirements in healthy adults.