American Journal of Clinical Nutrition, Vol 63, 532-545, Copyright © 1996 by The American Society for Clinical Nutrition, Inc

ORIGINAL RESEARCH COMMUNICATIONS

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

M Sanchez, AE El-Khoury, L Castillo, TE Chapman, AB Filho, L Beaumier and VR Young
Laboratory of Human Nutrition, Massachusetts Institute of Technology, Cambridge, 02142, USA.

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. 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 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.

This article has been cited by other articles:

				A. V Kurpad, M. M Regan, T. D. Raj, V. N Rao, J. Gnanou, and V. R Young The daily phenylalanine requirement of healthy Indian adults Am. J. Clinical Nutrition, June 1, 2006; 83(6): 1331 - 1336. [Abstract] [Full Text] [PDF]

				J. W-C Hsu, L. A Goonewardene, M. Rafii, R. O Ball, and P. B Pencharz Aromatic amino acid requirements in healthy men measured by indicator amino acid oxidation Am. J. Clinical Nutrition, January 1, 2006; 83(1): 82 - 88. [Abstract] [Full Text] [PDF]

				W. Kriengsinyos, L. J. Wykes, R. O. Ball, and P. B. Pencharz Oral and Intravenous Tracer Protocols of the Indicator Amino Acid Oxidation Method Provide the Same Estimate of the Lysine Requirement in Healthy Men J. Nutr., August 1, 2002; 132(8): 2251 - 2257. [Abstract] [Full Text] [PDF]

				A. V Kurpad, T. Raj, A. El-Khoury, L. Beaumier, R. Kuriyan, A. Srivatsa, S. Borgonha, A. Selvaraj, M. M Regan, and V. R Young Lysine requirements of healthy adult Indian subjects, measured by an indicator amino acid balance technique Am. J. Clinical Nutrition, May 1, 2001; 73(5): 900 - 907. [Abstract] [Full Text] [PDF]

				F. J van Spronsen, M. van Rijn, J. Bekhof, R. Koch, and P. G. Smit Phenylketonuria: tyrosine supplementation in phenylalanine-restricted diets Am. J. Clinical Nutrition, February 1, 2001; 73(2): 153 - 157. [Abstract] [Full Text] [PDF]

				S. A Roberts, J. M Thorpe, R. O Ball, and P. B Pencharz Tyrosine requirement of healthy men receiving a fixed phenylalanine intake determined by using indicator amino acid oxidation Am. J. Clinical Nutrition, February 1, 2001; 73(2): 276 - 282. [Abstract] [Full Text] [PDF]

				A. E El-Khoury, P. C. Pereira, S. Borgonha, A. Basile-Filho, L. Beaumier, S. Y Wang, C. C Metges, A. M Ajami, and V. R Young Twenty-four-hour oral tracer studies with L-[1-13C]lysine at a low (15 mg{middle dot}kg-1{middle dot}d-1) and intermediate (29 mg{middle dot}kg-1{middle dot}d-1) lysine intake in healthy adults Am. J. Clinical Nutrition, July 1, 2000; 72(1): 122 - 130. [Abstract] [Full Text] [PDF]

				V. R. Young and S. Borgonha Nitrogen and Amino Acid Requirements: : The Massachusetts Institute of Technology Amino Acid Requirement Pattern J. Nutr., July 1, 2000; 130(7): 1841S - 1849. [Abstract] [Full Text]

				C. Bos, C. Gaudichon, and D. Tome Nutritional and Physiological Criteria in the Assessment of Milk Protein Quality for Humans J. Am. Coll. Nutr., April 1, 2000; 19(90002): 191S - 205. [Abstract] [Full Text] [PDF]

				C. A Raguso, M. M Regan, and V. R Young Cysteine kinetics and oxidation at different intakes of methionine and cystine in young adults1 Am. J. Clinical Nutrition, February 1, 2000; 71(2): 491 - 499. [Abstract] [Full Text] [PDF]