How Much Protein Do You “Really Need”? Meta-analysis Shows Average Adult Need Is About 0.65g/kg/day

A systematic review and meta-analysis of classic studies on nitrogen balance was published in Nutrients: in healthy adults, the average nitrogen requirement was ≈104 mg N/kg/day, which is equivalent to ≈0.65 g protein/kg/day (if converted using the standard coefficient of 6.25). No significant differences were found by gender, age (<60 vs ≥60 years), climate, or protein source (animal, plant, mixed); heterogeneity was high (I² > 90%). The authors note that new balance studies are currently almost impossible to conduct for ethical reasons, so careful processing of accumulated individual data is the basis for revising protein guidelines.

Background

• Where did the current guidelines come from? Historically, the “average requirement” (EAR) for protein has been derived from the nitrogen balance: the intake level is chosen so that the nitrogen intake (from protein) balances its losses in urine/faeces/skin. The classic RAND meta-analysis (2003) gave ≈105 mg N/kg/day, equivalent to ≈0.65 g protein/kg/day; hence the “safe level” for almost everyone (≈97.5th percentile) is ≈0.83 g/kg/day. These values formed the basis for the WHO/FAO/UNI (2007) and EFSA (2012) recommendations.
• Why the topic is back on the agenda. New full-fledged balance studies are almost impossible to implement today (ethics, duration, full excreta collection). Therefore, it makes sense to re-evaluate old individual data with modern metastatistic approaches - exactly what the new work does (the result is again about 104 mg N/kg/day ≈ 0.65 g/kg/day).
• Alternative Method and “Higher” Estimates: Over the past 15 years, IAAO (indicator amino acid oxidation), an isotopic method that allows one to estimate the requirements for essential amino acids and total protein under more “real-life” conditions, has developed. A number of IAAO studies have indicated higher requirements for some groups and in some protocols (sometimes closer to 0.9 g/kg), which fuels the “0.8 vs. 1.0+ g/kg” debate. But regulatory standards for healthy adults still rely on balance.
• Protein quality is also important. Bioavailability and amino acid profiles vary; in 2013, the FAO recommended switching from PDCAAS to DIAAS (Digestible Isolate Assessed Amino Acids). This affects the calculation of the “effective” portion of protein from different sources (animal/plant), although the Nutrients meta-analysis found no significant differences between sources.
• Why “0.65 g/kg” is not a universal target for everyone. The EAR is the minimum for the average person; the practical “recommendation for most” is higher (≈0.8–0.83 g/kg). For the elderly and those at risk of sarcopenia, ESPEN/PROT-AGE experts suggest ≥1.0–1.2 g/kg/day (unless contraindicated), distributing protein over meals. Sports, illness, and weight loss are additional reasons to increase target levels.
• Methodological subtleties and sources of variation. The balance is affected by energy consumption (energy deficit underestimates the “threshold”), the accuracy of accounting for “other” nitrogen losses, and the duration of adaptation to the diet - hence the high heterogeneity between studies and caution in transferring conclusions to all populations. This is why an updated set of individual data with modern analysis is valuable for revising the standards.

What exactly did they do?

The researchers collected a set of 31 studies on nitrogen balance at the individual participant level (a total of ≈405 people) and conducted a meta-analysis with a random effects model. The nitrogen balance method looks for the zero balance point (how much nitrogen must be eaten so that its losses in urine/faeces/through the skin are compensated). The value in mg nitrogen/kg is then converted to g protein/kg (multiplying by 6.25, since protein contains ~16% nitrogen).

Key figures

• Total for all: 104.2 mg N/kg/day → ≈0.65 g protein/kg/day.
• Men: 109.1 mg N/kg/day → ≈0.68 g/kg/day (95% CI 103.0–115.1).
  Women: 102.4 mg N/kg/day → ≈0.64 g/kg/day (95% CI 92.3–112.5).
• Protein source (animal/plant/mixed) did not significantly alter the estimated requirement; no differences were found for climate or age, although the range of results between studies was very large (I² 85–99%).

Context: How does this relate to the recommendations?

The obtained average requirement is close to previous meta-analyses (≈105 mg N/kg/day) and forms the basis of reference values in Europe and Japan. A typical calculation method is: 105 mg N/kg/day × 6.25 = ≈0.66 g protein/kg/day — this is the average requirement (EAR). To cover almost everyone (≈97.5% of the population), the guidelines are increased to a “safe level” of about 0.8–0.83 g/kg/day. The authors emphasize: nitrogen balance provides the minimum for maintaining equilibrium in people with normal activity; for those who actively train, the requirements are higher, so target recommendations for physically active people are formed based on other considerations (e.g., the share of protein in energy intake).

Why is this important?

• New balance studies are practically not carried out. Due to the tightening of ethical rules (Helsinki Declaration, national regulations), long-term protocols with low-protein diets and complete excreta collection have become almost impossible. Therefore, careful compilation and open publication of historical individual data is the best way to refine the base numbers.
• Translation into “plate language.” The estimate of 0.65 g/kg/day is the “minimum-for-balance” for the average adult. For a 70-kg person, this is ≈45 g protein/day as an average requirement; the practical “safe” level for most is ≈0.8 g/kg/day (≈56 g/day). For the elderly, those losing weight, those sick, or those who exercise vigorously, the target levels are usually higher—and this is beyond the limits of the nitrogen balance method.

Restrictions

The heterogeneity of the data is high (I² > 90%), which reflects the spread of protocols and methods for accounting for "other" nitrogen losses; in some subgroups, isolated high values were observed in older women. The authors therefore do not speak of a "new norm", but of the most complete set of individual data to date, which confirms previous guidelines and will be useful for future revisions of the norms.

Source: Suzuki D. et al. Nitrogen Requirements in Healthy Adults: A Systematic Review and Meta-Analysis of Nitrogen Balance Studies. Nutrients 17(16):2615, August 12, 2025. https://doi.org/10.3390/nu17162615