The investigators reviewed various lines of evidence and arrived at the conclusions discussed below regarding protein quality.

Essential amino acids are the building blocks of protein that the body cannot synthesize and must obtain from the diet. The limiting essential amino acid is the one with the lowest levels in the body, and it can limit protein function if there is not enough of it.

Protein quality is influenced by multiple factors, including the following:

Protein composition: The protein source’s amino acid profile and whether any essential amino acids are present in lower amounts affect quality. For example, grains tend to be low in lysine.
Protein bioavailability: The fraction of protein actually digested and absorbed after eating affects quality. Bioavailability is influenced by factors like food processing, cooking, and food particle size and structure.

Protein quality can be assessed using several chemical scoring metrics, from the the simplistic amino acid score (AAS) to the expensive and technical dual isotope tracer method.^[1] The AAS represents the amino acid content of food relative to dietary reference intakes (DRIs). Newer tools, like the meal protein quality score, attempt to account for complimentary proteins and habitual diet in relation to individual needs.

The most common methods are the protein digestibility-corrected amino acid score (PDCAAS) and the digestible indispensable amino acid score (DIAAS). Both correct the AAS for digestibility, but the PDCAAS uses an end-of-gut average estimated from the amount of protein found in feces, whereas the DIAAS uses the levels of individual essential amino acids estimated from the levels of amino acids collected at the end of the small intestine, or ileum, where dietary amino acid absorption ends.

Protein sources, scored using the DIAAS

Reviewing protein quality from essential amino acids to digestibility Original paper