Reevaluation of the protein requirement in young men with the indicator amino acid oxidation technique.
Level 2 - randomized trial
Randomized crossover metabolic study in humans
PubMed 17921376 · doi:10.1093/ajcn/86.4.995
What was done
Eight healthy men were randomly assigned to receive graded protein intakes (0.10, 0.30, 0.60, 0.90, 1.2, 1.5, and 1.8 g·kg⁻¹·d⁻¹) provided as a crystalline amino acid mixture along with L-[1-¹³C]phenylalanine tracer. The indicator amino acid oxidation (IAAO) technique was used, and mean protein requirements were calculated via biphase linear regression crossover analysis on breath ¹³CO₂ excretion (F¹³CO₂) to identify the metabolic breakpoint.
What was found
The mean estimated average protein requirement was 0.93 g·kg⁻¹·d⁻¹, and the population-safe requirement (recommended dietary allowance, RDA equivalent) was 1.2 g·kg⁻¹·d⁻¹. These values were 41% and 50% higher than the standard recommendations of 0.66 g·kg⁻¹·d⁻¹ (EAR) and 0.80 g·kg⁻¹·d⁻¹ (RDA). Reanalysis of historical nitrogen balance data using a biphase linear regression model yielded estimates of 0.91 and 1.0 g·kg⁻¹·d⁻¹.
Why it matters
This study challenged long-standing dietary reference intakes derived from nitrogen balance studies, suggesting that existing official guidelines underestimate daily protein needs in adult men.
Limits
The sample size was very small (n = 8), and the study included only young, healthy men. Protein was provided as a crystalline amino acid mixture rather than intact dietary protein, and women, older adults, and clinical populations were not studied.
Cited by
- supports Alternative protein requirement assessment methods using stable isotopes indicate that human protein requirements to maintain balance are closer to 1.2 grams per kilogram of body weight per day.