Background: The current estimated protein requirements are based on the nitrogen balance method, which has many limitations. An alternate approach is needed to permit a reevaluation of protein requirements.

Objective: The objective was to determine protein requirements in men by using the indicator amino acid oxidation technique.

Design: Eight healthy men randomly received graded protein intakes (0.10, 0.30, 0.60, 0.90, 1.2, 1.5 and 1.8 g kg(-1) d(-1)) as a crystalline amino acid mixture along with L-[1-(13)C]phenylalanine. The mean protein requirement was determined by applying a biphase linear regression crossover analysis on F(13)CO(2) data, which identified a breakpoint at the minimal rate of appearance of (13)CO(2) to graded protein intakes.

Results: The mean and population-safe (recommended dietary allowance; RDA) protein requirements were found to be 0.93 and 1.2 g kg(-1) d(-1), respectively. These requirements are comparable with those estimated by the application of a biphase linear regression model to the data from nitrogen balance studies (0.91 and 1.0 g kg(-1) d(-1), respectively). These requirements are 41% and 50% higher than the current recommendations for the estimated average requirement (EAR) of 0.66 g kg(-1) d(-1) and the RDA of 0.80 g kg(-1) d(-1), as determined by applying a linear regression model where it intersects the zero balance line.

Conclusion: The indicator amino acid oxidation technique defined a protein requirement that is comparable with that estimated by the application of a biphase linear regression model to nitrogen balance data in the literature. Our data and the reanalysis of the preexisting nitrogen balance data suggest that the current recommended protein requirements are too low and require reassessment.