Twenty-four-hour intravenous and oral tracer studies with L-[1-¹³C]phenylalanine and L-[3,3-²H₂]tyrosine at a tyrosine-free, generous phenylalanine intake in adults

The daily rates of whole-body phenylalanine oxidation and hydroxylation were determined in young men receiving [1-¹³C]phenylalanine and [²H₂]tyrosine via primed, constant intravenous (n = 3) or oral (n = 5) infusion for 24 consecutive hours (12-h fast followed by 12-h fed period), and given a generous phenylalanine (100 mg·kg^-1·d^-1), tyrosine-free, but otherwise adequate L-amino acid-based diet for 6 d before the tracer study. Our hypothesis was that subjects would be in whole-body phenylalanine equilibrium. Estimates of the daily rates of phenylalanine oxidation (phe-ox) and hydroxylation (phe-OH) were significantly higher for the subjects receiving the oral compared with intravenous tracer (P < 0.01 for both comparisons), with the estimates of phe-ox obtained with 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). The precision of the agreement between the measured 24-h rates of phe-ox and phe-OH compared with the predicted daily rates by extrapolation from the last hour of the 12-h fast and fifth hour of the fed period was poor. From the 24-h data, daily phenylalanine balances were estimated to be positive for both the intravenous and oral tracer protocols, although it was less positive for the oral tracer group. These results imply that the [¹³C]phenylalanine probe underestimated whole-body irreversible loss of phenylalanine, and suggest that daily phenylalanine balance in earlier 24-h phenylalanine-tyrosine tracer studies (Am J Clin Nutr 1995;61:555-70) at low phenylalanine intakes may have been overestimated. Studies involving [¹³C]tyrosine as tracer will be required to further assess whole-body aromatic amino acid balance.