Reference
Protein Quality
**Protein quality** is a measure of how well a dietary protein meets human essential amino acid (EAA) requirements. It is a function of two things: the protein's amino acid composition and how completely the body digests and absorbs it.
A protein can carry every essential amino acid and still rate poorly if it digests badly, and it can digest well yet fall short if it lacks enough of one EAA. Both factors are weighed together. The amino acid present in the lowest amount relative to requirement — the limiting amino acid — sets the ceiling on how much usable protein the food delivers.
How protein quality is measured
The two standard scoring methods are PDCAAS and DIAAS. The Protein Digestibility-Corrected Amino Acid Score (PDCAAS) is the FAO/WHO-recommended method, evaluating protein against human amino acid requirements and correcting for digestibility (Schaafsma, Journal of Nutrition, 2000, PMID:10867064). Scores are capped, so any protein meeting or exceeding the reference pattern is truncated to a maximum of 1.00 — several high-quality proteins, including whey, egg, and casein, reach this ceiling. By established FAO/WHO values, egg protein scores 1.00, while wheat gluten scores around 0.25.
The Digestible Indispensable Amino Acid Score (DIAAS) was proposed by the FAO in March 2013 to replace PDCAAS as the protein quality standard. Unlike PDCAAS, DIAAS is not truncated at a maximum, so it can distinguish among proteins that all reach the PDCAAS ceiling.
Why animal and plant proteins differ
Plant proteins generally score lower than animal proteins on protein quality metrics, reflecting both less favorable amino acid profiles and lower digestibility. Scores are not the whole story, however. Soy protein isolate and whey can differentially stimulate muscle protein synthesis despite sharing the same PDCAAS value, which illustrates why digestion speed and leucine content also matter beyond a single score.
Relevance to potato protein
Among vegetable proteins, potato protein is an exception to the lower-quality pattern. Its PDCAAS is among the highest of any vegetable protein source, and the DIAAS for potato protein isolate has been reported as high as 100 (Food Science & Nutrition, Herreman et al., 2020, PMID:33133540). For more on how the isolate is produced and used, see our guide to potato protein.
Related terms