Nitrogen Requirements in Healthy Adults: A Systematic Review and Meta-Analysis of Nitrogen Balance Studies

Background: Nitrogen balance studies have traditionally been used to estimate protein requirements in adults. However, ethical and practical constraints have made new studies increasingly difficult to conduct. This systematic review and meta-analysis aimed to compile and analyze the most comprehensive individual-level dataset to date. Methods: We included 31 studies that reported nitrogen intake and excretion data for healthy adults. Studies were selected based on strict eligibility criteria that required data from at least three intake levels per individual. Nitrogen requirements were estimated using regression analysis. In total, data from 395 individuals were analyzed. We used a random effects model for the meta-analysis. Subgroup comparisons and meta-regression were conducted based on sex, age, climate, and protein source. Results: The overall mean nitrogen requirement was 104.2 mg N/kg/day. No significant differences were found by sex, age group (<60 vs. ≥60 years), climate (temperate vs. tropical), or protein source (animal, plant, or mixed). Subgroup and meta-regression analyses did not reveal consistent moderator effects. Substantial heterogeneity was observed (I² > 90%). Conclusions: This analysis provides the most extensive compilation of individual-level nitrogen balance data to date. While the average nitrogen requirement was consistent with previous estimates, the high heterogeneity limits definitive conclusions. Nonetheless, this dataset provides a valuable foundation for revising protein intake recommendations and guiding future research on human nitrogen metabolism in the absence of new studies.