Meta-analysis of nitrogen balance studies for estimating protein requirements in healthy adults.
Level 1 - systematic review of randomized trials
Meta-analysis of individual participant data from controlled nitrogen balance trials
PubMed 12499330 · doi:10.1093/ajcn/77.1.109
What was done
A meta-analysis synthesized individual-level data from 19 published nitrogen balance studies evaluating healthy adults studied at three or more test protein intake levels. The authors analyzed the statistical distribution of individual nitrogen requirements and tested for differences related to study site climate, age, sex, and dietary protein source.
What was found
Across 235 subjects, individual requirements fit a log-normal distribution. The median Estimated Average Requirement (EAR) was 105 mg N/kg/d (0.65 g good-quality protein/kg/d). The 97.5th percentile (Recommended Dietary Allowance, RDA) was 132 mg N/kg/d (0.83 g good-quality protein/kg/d) after correcting for within-individual variability. No statistically significant differences were found by climate, adult age class, sex, or protein source, though there was an indication that women might have lower requirements than men.
Why it matters
This synthesis provided the statistical basis for modern dietary reference intakes for adult protein intake, replacing earlier consensus-based guidelines with an empirical distribution model.
Limits
The findings depend entirely on nitrogen balance methodology, which is known to systematically underestimate intake losses and overestimate retention. The aggregate sample of 235 individuals across 19 studies limited statistical power to confirm potential subgroup differences in age, sex, and protein quality.
Cited by
- supports The Recommended Dietary Allowance (RDA) for protein of 0.8 g/kg was established based on nitrogen balance studies designed to determine minimum requirements to prevent deficiency, rather than to optimize muscle mass or function.