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DIAAS
**DIAAS** (Digestible Indispensable Amino Acid Score) is a protein-quality metric proposed by the Food and Agriculture Organization (FAO) in March 2013 to replace PDCAAS, scoring a protein by the digestibility of each indispensable amino acid relative to human requirements.
How DIAAS is measured
DIAAS is calculated per indispensable amino acid, taking the lowest digestible amino acid ratio as the score for the protein. The method uses true ileal digestibility — measured at the end of the small intestine — rather than the fecal digestibility used by PDCAAS, giving a more accurate estimate of how much of each amino acid is actually absorbed. The FAO formalized the method in FAO Food and Nutrition Paper 92 (2013) and, through its 2013 Expert Consultation on Protein Quality Evaluation, recommended its adoption over PDCAAS.
DIAAS versus PDCAAS
The central difference is truncation. PDCAAS, recommended by the FAO in 1993, caps scores at 1.00 (100%), so several high-quality proteins all register as 1.00 with no way to distinguish them. According to the FAO, DIAAS values above 100% are not truncated and can exceed 100%, preserving the surplus of digestible amino acids a protein supplies. The two methods also diverge in absolute value: for unprocessed soya products, DIAAS is 86% versus a PDCAAS of 92%, a practically significant difference. A 2024 review in Frontiers in Nutrition (PMID 39021594) examined such gaps a decade after the metric’s introduction.
Because it is uncapped, DIAAS can rank proteins above the maximum. In a controlled human study, pea protein isolate scored a DIAAS of 1.00 compared with 1.45 for casein (Am J Clin Nutr, 2022, PMID 34665230) — a separation PDCAAS would have hidden by truncating both. For readers comparing supplements, this distinction matters when reading labels; the protein powder buyer’s guide covers how quality scores factor into a purchase.
DIAAS and potato protein
The DIAAS for potato protein isolates has been reported as high as 100 (Food Science & Nutrition, Herreman et al., 2020, PMID 33133540). Soy and potato protein isolates both reach a DIAAS at or above 100% for children and adults, placing them in the same protein-quality range as whey isolates (DIAAS 94%–100%). Egg protein generally scores above 100, indicating a surplus of digestible essential amino acids relative to human requirements. These figures position select plant isolates closer to animal proteins than the older PDCAAS scale tended to suggest, though plant proteins in general still score lower than animal proteins on both metrics.
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