The Nitrogen Balance Myth — Why We Got Protein Requirements Wrong

The method used to set your protein requirement was known to be flawed before the ink was dry.

Nitrogen balance measures the difference between nitrogen consumed (from dietary protein) and nitrogen excreted (in urine, feces, sweat, and skin). If intake exceeds output, you’re in “positive balance” — presumably building protein. If output exceeds intake, you’re losing it.

The method has been used since the 19th century and remains the basis of the current protein RDA. The problem, as Rand et al. (2003) documented in a meta-analysis in the American Journal of Clinical Nutrition, is that nitrogen balance systematically overestimates intake and underestimates losses — meaning it makes protein requirements look lower than they actually are.

Elango et al. (2010) explained the mechanistic reasons in Current Opinion in Clinical Nutrition and Metabolic Care: nitrogen losses through sweat, skin, hair, and breath are poorly captured; fecal nitrogen is difficult to attribute accurately; and subjects adapt to low protein intakes by reducing oxidation, which creates apparent “balance” at intakes that are actually suboptimal for long-term health.

The Indicator Amino Acid Oxidation (IAAO) technique, developed by Pencharz and Ball (2003), avoids these problems by measuring the metabolic fate of a labeled amino acid at varying protein intakes. When protein intake is inadequate, all amino acids including the indicator are oxidized rather than incorporated into protein. The “breakpoint” where oxidation decreases identifies the requirement.

Using IAAO, protein requirements consistently come in 40–50% higher than nitrogen balance estimates — suggesting the current RDA of 0.8 g/kg/day is meaningfully below optimal intake for most adults.

References

1. Rand WM, Pellett PL, Young VR. Meta-analysis of nitrogen balance studies for estimating protein requirements in healthy adults. Am J Clin Nutr. 2003;77(1):109-127. PubMed
2. Elango R, Humayun MA, Ball RO, Pencharz PB. Evidence that protein requirements have been significantly underestimated. Curr Opin Clin Nutr Metab Care. 2010;13(1):52-57. PubMed
3. Pencharz PB, Ball RO. Different approaches to define individual amino acid requirements. Annu Rev Nutr. 2003;23:101-116. PubMed