Not all protein is nutritionally equal even at the same gram count — quality matters, and nutrition science has two formal systems for measuring it, one largely superseding the other.
PDCAAS (the older standard)
Protein Digestibility-Corrected Amino Acid Score, adopted by the FAO/WHO in 1991, scores a protein by comparing its essential amino acid profile against a reference pattern, then adjusting for how digestible it is, measured at the level of the entire digestive tract. Scores are capped at 1.0 — several proteins, including whey, casein, egg, and soy protein isolate, all score a perfect 1.0 under this system, which made it hard to distinguish quality among the best sources.
DIAAS (the current, more precise standard)
Digestible Indispensable Amino Acid Score, recommended by the FAO since 2013 as PDCAAS's replacement, measures amino acid digestibility at the end of the small intestine specifically (via ileal sampling) rather than across the whole digestive tract, and — critically — has no upper cap at 1.0. This uncapped scoring revealed real quality differences among proteins that PDCAAS had flattened to the same "perfect" score: whey, for instance, scores meaningfully above 1.0 under DIAAS (some methodologies put it near 1.09-1.25 depending on the specific whey type), while several plant proteins that scored respectably under PDCAAS score lower and more differentiated under DIAAS.
Why this matters practically
The quality score reflects how efficiently a protein source can be converted into usable amino acids for your body's tissue-building needs. A protein with a high gram count but a low quality score (like collagen, missing tryptophan entirely) contributes far less to muscle protein synthesis than the label's total protein number would suggest.
How this site uses it
Every food entry on this site lists a quality figure — DIAAS-based where a reliable published value exists, and a clearly flagged literature estimate where it doesn't. Use it as a tiebreaker between similar protein-per-calorie foods, not as the only factor — cost, practicality, and dietary fit still matter more for most real-world decisions.