Phenylalanine and tyrosine kinetics in young men throughout a continuous 24-h period, at a low phenylalanine intake.
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.
Sánchez
M
M
Laboratory of Human Nutrition, Massachusetts Institute of Technology, Cambridge 02139.
el-Khoury
A E
AE
Castillo
L
L
Chapman
T E
TE
Young
V R
VR
D000284:Administration, Oral; D000328:Adult; D001944:Breath Tests; D002153:Calorimetry, Indirect; D002149:Energy Intake; D005215:Fasting / Q000378:metabolism; D006801:Humans; D006900:Hydroxylation; D007262:Infusions, Intravenous; D007700:Kinetics; D008297:Male; D010084:Oxidation-Reduction; D010649:Phenylalanine / Q000008:administration & dosage / Q000378:metabolism*; D014443:Tyrosine / Q000378:metabolism*
D014443:Tyrosine; D010649:Phenylalanine