Understanding Protein Quality Metrics: PDCAAS vs DIAAS
Quick Summary
- PDCAAS (Protein Digestibility Corrected Amino Acid Score) is the older method, still used in FDA nutritional labeling; DIAAS (Digestible Indispensable Amino Acid Score) is the newer FAO/WHO-recommended method
- The key methodological difference: PDCAAS uses fecal digestibility data and caps scores at 1.0; DIAAS uses true ileal digestibility of individual amino acids and does not cap — enabling more precise discrimination between protein sources
- For YNP catalogue proteins: dairy proteins (WPC, WPI, casein, MPC) score highest; soy protein is strong; pea and rice protein are good but modestly lower — primarily due to limiting amino acids
- For label claims: confirm which metric applies in your target market. FDA regulations reference PDCAAS for Daily Value; EFSA references DIAAS; FSSAI follows its own framework
PDCAAS — The Legacy Method
PDCAAS was adopted by FAO/WHO in 1989 and incorporated into FDA nutritional labeling regulations. It is calculated as:
PDCAAS = Amino Acid Score × Fecal Digestibility Coefficient
The Amino Acid Score compares the first-limiting essential amino acid in the protein to a reference pattern. The fecal digestibility coefficient reflects the proportion of the protein that was apparently absorbed (measured at the level of fecal excretion). PDCAAS scores are capped at 1.0 — any protein that meets or exceeds all essential amino acid requirements scores 1.0, regardless of how much it exceeds them.
Limitation: Fecal digestibility overestimates true protein digestibility because it cannot distinguish between unabsorbed dietary protein and microbial protein in the colon. The cap at 1.0 also means a very high-quality dairy protein and a just-passing plant protein receive the same score, masking real differences.
DIAAS — The Current FAO/WHO Standard
DIAAS was recommended by FAO in 2013 to replace PDCAAS. It is calculated as:
DIAAS = True Ileal Digestible Indispensable Amino Acid Content / Reference Amino Acid Pattern
Key differences from PDCAAS: true ileal digestibility (measured at the end of the small intestine, before colonic fermentation) for each individual indispensable amino acid; no cap at 1.0 — scores above 1.0 are possible for high-quality proteins; identifies the specific limiting amino acid rather than just the overall score.
Comparative Scores for YNP Catalogue Proteins
| Protein | DIAAS (approximately, varies by study) | First-limiting amino acid |
|---|---|---|
| Whey Protein Concentrate/Isolate | >1.0 (typically 1.09–1.25) | None limiting |
| Micellar Casein Isolate | ~1.0–1.08 | None limiting |
| Milk Protein Concentrate | ~1.0 | None limiting |
| Soya Protein Isolate 90% | ~0.9–1.0 | Methionine |
| Pea Protein Isolate | ~0.82–0.93 | Methionine/SAA |
| Rice Protein 80% | ~0.5–0.7 | Lysine |
| BCAA 2:1:1 / individual BCAAs | Not applicable — incomplete protein source | N/A |
*(Table is horizontally scrollable on mobile; values are approximate ranges from published literature and may vary by study and grade)*
Practical Implications for Formulators
If making "complete protein" claims: DIAAS ≥0.75 is generally considered a "good" source; ≥1.0 is "excellent" per FAO guidance. Pea protein at ~0.82–0.93 is approaching the "good" threshold; rice protein below 0.75 may not support unqualified "complete protein" claims alone — the pea + rice blend improves the combined score.
For regulatory labeling: Confirm which scoring method applies in your target market before finalising claims. FDA nutritional labeling references PDCAAS; EFSA references DIAAS; FSSAI has its own framework.
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References
Industry Information
- FAO. *Dietary Protein Quality Evaluation in Human Nutrition.* FAO Food and Nutrition Paper 92, 2013 — introduces and recommends DIAAS to replace PDCAAS.
- Mathai JK, Liu Y, Stein HH. *Values for digestible indispensable amino acid scores (DIAAS) for some dairy and plant proteins.* Br J Nutr. 2017;117(4):490–499.
- FAO/WHO. *Protein Quality Evaluation — Report of the Joint FAO/WHO Expert Consultation.* FAO Food and Nutrition Paper 51, 1991 — the original PDCAAS paper.