Estimation of the protein requirements of young men and women using the indicator amino acid oxidation method

The indicator amino acid oxidation (IAAO) method is an alternative method of assessing protein requirements. However, there have been few studies of its use in healthy young adults, and the potential for a sex difference has not been evaluated. The primary purpose of the present study was to evaluate the protein requirements of healthy young men and women using the IAAO method and a basic study diet including a crystalline amino acid mixture, and the secondary purpose was to identify any sex difference in the protein requirement.

Healthy young men (n = 7) and women (n = 6) were randomly allocated to consume a study diet containing differing levels of protein (0.24-1.58 g/kg/day) in the form of a crystalline amino acid mixture, along with L-[1-¹³C] phenylalanine. Their mean protein requirements were determined using a biphase linear regression crossover analysis of the exhaled ¹³CO₂, which identified a breakpoint at the minimum rate of appearance of ¹³CO₂ for the various protein intakes. This protocol was preregistered with the UMIN Clinical Trials Registry as UMIN000050212.

The breakpoints were estimated to be 1.17 (95% confidence interval (CI), 0.95-1.39) g/kg body mass/day for the male participants and 1.04 (95% CI, 0.84-1.24) g/kg body mass/d for the female participants. When their protein intake was expressed per unit fat-free mass, the breakpoints were estimated to be 1.50 (95% CI, 1.20-1.80) g/kg fat-free mass/day for the men and 1.35 (95% CI, 1.11-1.60) g/kg fat-free mass/day for the women. There was no significant difference in the breakpoint between the sexes.

The estimated breakpoints obtained during the present study was higher than requirements obtained using the results of nitrogen balance studies. There was no sex difference in the breakpoints using protein intake per unit body mass or fat-free mass for healthy young adults.