The DIAAS Scoring System — Not All Protein Is Created Equal

The international standard for protein quality tells you what the label can’t.

The Digestible Indispensable Amino Acid Score (DIAAS) replaced the older PDCAAS (Protein Digestibility Corrected Amino Acid Score) as the recommended method for evaluating protein quality, per the FAO’s 2013 report on dietary protein quality.

DIAAS improves on PDCAAS in two key ways: it measures ileal (small intestine) digestibility rather than fecal digestibility (which overestimates absorption), and it does not truncate scores at 1.0 — meaning high-quality proteins can score above 100%, reflecting their genuine superiority.

DIAAS is calculated based on the ratio of each digestible EAA in the food to the reference pattern, with the lowest ratio (the “limiting amino acid”) determining the score. This means a protein’s quality is determined by its weakest EAA link.

Representative DIAAS scores: whole milk powder 1.14, whey protein concentrate 1.09, eggs 1.13, chicken breast 1.08, soy protein isolate 0.90, pea protein 0.82, wheat 0.40, rice 0.37, and collagen ~0 (due to absent tryptophan).

Free-form EAAs, by definition, have no limiting amino acid — every EAA is present in the intended ratio and requires no digestion. They represent the theoretical maximum for amino acid delivery efficiency.

References

1. FAO. Dietary protein quality evaluation in human nutrition. FAO Food and Nutrition Paper 92. 2013. FAO
2. Rutherfurd SM, Fanning AC, Miller BJ, Moughan PJ. Protein digestibility-corrected amino acid scores and digestible indispensable amino acid scores differentially describe protein quality in growing male rats. J Nutr. 2015;145(2):372-379. PubMed