The Phenylalanine Requirement of Elderly Men and Women Measured by Direct 13C Carbon Oxidation Method Is Similar to That of Young Adults

Background: The phenylalanine requirement of the elderly is not known. Current recommendations are based on studies in young adults and are derived from a combined estimate of the total aromatic amino acids, phenylalanine, and tyrosine. Objectives: The purpose of this study was to determine the dietary phenylalanine requirement of adults aged >65 y, using the direct amino acid oxidation method, by measuring the oxidation of L-[1-C-13]phenylalanine to (CO2)-C-13 in response to graded phenylalanine intakes in the presence of excess tyrosine. Methods: Twelve subjects (6 men, 6 women), aged 73.8 +/- 6.7 y (mean +/- SD) and with a BMI (in kg/m(2)) of 26.4 +/- 4.8 and 25.2 +/- 4.4 for men and women, respectively, were randomized to phenylalanine intakes ranging from 7.20 to 40.0 mg.kg(-1).d(-1) for a total of 66 studies. Study diets were isocaloric and isonitrogenous, providing protein and energy at 1.0 g.kg(-1).d(-1) and 1.5 x resting energy expenditure (REE), respectively. Protein was provided as an amino acid mixture patterned after egg protein, with an excess of tyrosine and alanine to balance the nitrogen as phenylalanine intakes were varied. Two days prior to the study day, subjects were adapted to a milkshake diet providing protein at 1.0 g.kg(-1).d(-1) and energy at 1.7 x REE. The mean phenylalanine requirement was determined using biphase linear regression analysis, which identified a breakpoint in the (FCO2)-C-13 in response to graded phenylalanine intakes. Results: The mean and upper 95% CIs (approximating the recommended dietary allowance) of phenylalanine requirements were estimated to be 9.03 and 15.9 mg.kg(-1).d(-1), respectively. Conclusion: These results are similar to previously derived estimates of 9.1 and 13.6 mg.kg(-1).d(-1) in young adult men and suggest that higher protein needs of the elderly to stimulate similar muscle protein synthesis rates as young adults are not driven by an increased requirement for phenylalanine.