What do the trials say — and why did the field outgrow its own RDA?
Protein's evidence base is enormous — decades of nitrogen-balance studies, muscle-protein-synthesis tracer work, satiety trials, and the aging-muscle prevention literature — and its defining modern story is the field outgrowing its own reference number: the 0.8 g/kg RDA that was set to prevent deficiency, being quietly superseded by the 1.2–2.0 g/kg consensus for health, aging, and performance. This chapter reads the MPS tracer studies, the PROT-AGE synthesis, the kidney-myth nulls, and the satiety literature.
The field teaches dose-response and population-conditional reading in their clearest forms:
Gerardus Mulder isolates the nitrogen-rich substance and names it 'of the first rank' — believing it a single fundamental material. The single-substance theory fails; the name's ambition stands.
The 0.8 g/kg recommendation is derived from nitrogen-balance studies in young adults — the deficiency-prevention floor that will define the field for eighty years.
Radioactive-isotope tracer studies begin mapping amino-acid metabolism — muscle protein synthesis becomes measurable, and the field acquires its sharpest tool.
Lucas and colleagues identify leucine as the amino-acid trigger for MPS — the threshold concept that reorganizes dosing advice around per-meal amounts.
Systematic reviews of high-protein diets in healthy adults find no kidney harm — the most-repeated nutrition caution of the century, contradicted by its own trials.
The international expert group recommends 1.2–1.5 g/kg for older adults — the RDA outgrown for the population that needs it most, and the field's modern turning point.
POPULATION · Young adults, labeled-amino-acid infusion protocols
FINDING · Leucine alone (independent of the full amino-acid mixture) activates the mTOR-MPS pathway — the threshold concept established, with ~2.5 g per serving as the practical trigger.
LIMITATION · Acute tracer studies measure the synthetic response, not long-term muscle gain — the threshold's validation at the whole-muscle level came from the subsequent training trials.
POPULATION · Expert synthesis of the older-adult protein literature
FINDING · Older adults should consume 1.2–1.5 g/kg daily (vs. the 0.8 RDA) and 25–40 g per meal — because anabolic resistance raises the per-meal threshold with age.
LIMITATION · An expert consensus, not a trial itself — but it synthesizes the RCT base that has since grown to support it, and it is the field's most-cited aging-protein document.
POPULATION · Older adults across combined-intervention trials
FINDING · Protein supplementation plus resistance training slows or partially reverses age-related muscle loss — the combination outperforming either alone.
LIMITATION · Adherence is the real-world variable (both the training and the protein need consistency); the effect sizes are functional rather than dramatic — maintaining independence, not bodybuilding.