Comparison of Protein Requirements Based on the Nitrogen Balance and Indicator Amino Acid Oxidation Methods: An Umbrella Review and Meta-analysis.

Background: An accurate understanding of protein requirements helps prevent health risks caused by deficiency. No statistical comparison exists between the nitrogen balance (NB) method, the standard method for estimating protein requirements, and the indicator amino acid oxidation (IAAO) method, which has been increasingly studied.

Objectives: This study aimed to quantitatively compare the protein requirements of the NB and IAAO methods through meta-analyses.

Methods: Studies estimating protein requirements in healthy individuals using the NB or IAAO methods were reviewed. First, previous reviews were systematically searched to identify original NB articles up to 2012 and IAAO articles up to 2023 from their references. Original articles published after each review's search period, up to 11 January, 2025, were systematically searched using PubMed and Ichushi-Web. Manual searches were performed through citation tracking of included literature and gray literature. This study followed PRISMA guidelines. Differences in protein requirements between assessment methods were compared using Welch's t test.

Results: A total of 43 NB articles (777 participants) and 17 IAAO articles (186 participants) were included. In nonathletes, protein requirements were significantly higher by 36% with IAAO [mean: 0.88 g/kg/d; 95% confidence interval (CI): 0.85, 0.90] than with NB (mean: 0.64 g/kg/d; 95% CI: 0.61, 0.68). In athletes, protein requirements were significantly higher by 27% with IAAO (mean: 1.61 g/kg/d; 95% CI: 1.44, 1.78) than with NB (mean: 1.27 g/kg/d; 95% CI: 1.06, 1.47). In nonathletes, IAAO values remained significantly higher than NB values across age and sex subgroups.

Conclusions: The protein requirements calculated using the IAAO method were ∼30% higher than those obtained using the NB method. The quantitative findings of this study provide important information for scientific consideration of protein requirements. This trial was registered at PROSPERO as CRD42025636735.