American Journal of Clinical Nutrition, Vol 61, 555-570, Copyright © 1995 by The American Society for Clinical Nutrition, Inc

ORIGINAL RESEARCH COMMUNICATIONS

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

M Sanchez, AE el-Khoury, L Castillo, TE Chapman and VR Young
Laboratory of Human Nutrition, Massachusetts Institute of Technology, Cambridge 02139.

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.

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