Protein quality is the

digestibility Digestion is the breakdown of large insoluble food molecules into small water-soluble food molecules so that they can be absorbed into the watery blood plasma. In certain organisms, these smaller substances are absorbed through the small intest ...

and quantity of

essential amino acid An essential amino acid, or indispensable amino acid, is an amino acid that cannot be synthesized from scratch by the organism fast enough to supply its demand, and must therefore come from the diet. Of the 21 amino acids common to all life form ...

s for providing the proteins in correct ratios for human consumption. There are various methods that rank the quality of different types of

protein Proteins are large biomolecules and macromolecules that comprise one or more long chains of amino acid residues. Proteins perform a vast array of functions within organisms, including catalysing metabolic reactions, DNA replication, respo ...

, some of which are outdated and no longer in use, or not considered as useful as they once were thought to be. The

Protein Digestibility Corrected Amino Acid Score Protein digestibility-corrected amino acid score (PDCAAS) is a method of evaluating the quality of a protein based on both the amino acid requirements of humans and their ability to digest it. The PDCAAS rating was adopted by the US FDA and the Fo ...

(PDCAAS), which was recommended by the

Food and Agriculture Organization of the United Nations The Food and Agriculture Organization of the United Nations (FAO)french: link=no, Organisation des Nations unies pour l'alimentation et l'agriculture; it, Organizzazione delle Nazioni Unite per l'Alimentazione e l'Agricoltura is an intern ...

(FAO), became the industry standard in 1993. FAO has recently recommended the newer

Digestible Indispensable Amino Acid Score Digestible Indispensable Amino Acid Score (DIAAS) is a protein quality method proposed in March 2013 by the Food and Agriculture Organization to replace the current protein ranking standard, the Protein Digestibility Corrected Amino Acid Score (PDC ...

(DIAAS) to supersede PDCAAS. The dairy industry is in favor of this, because while PDCAAS truncates all protein types that exceed the

(EAA) requirements to 1.0, DIAAS allows a higher than 1.0 ranking: while for example both soy protein isolate and whey isolate are ranked 1.0 according to PDCAAS, in the DIAAS system, whey has a higher score than soy.

PDCAAS versus DIAAS

The main limitations of PDCAAS is that it does not take into account anti-nutrient factors like phytic acid and

trypsin inhibitor A trypsin inhibitor (TI) is a protein and a type of serine protease inhibitor (serpin) that reduces the biological activity of trypsin by controlling the activation and catalytic reactions of proteins. Trypsin is an enzyme involved in the breakdown ...

s, which limit the absorption of protein among other nutrients, and its use of

fecal Feces ( or faeces), known colloquially and in slang as poo and poop, are the solid or semi-solid remains of food that was not digested in the small intestine, and has been broken down by bacteria in the large intestine. Feces contain a relati ...

digestibility, whereas in the DIAAS system,

ileal The ileum () is the final section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms posterior intestine or distal intestine ...

digestibility of the essential amino acids is emphasized as a more accurate measure of protein absorption. For this reason, DIAAS is promoted as the superior method and preferable over the PDCAAS. Due to not factoring in anti-nutritional content into the digestibility equation, the PDCAAS has consequently been criticized for overestimating protein quality. Other older methods like BV, PER, NPU and

nitrogen balance Nitrogen balance is a measure of nitrogen input minus nitrogen output. Nitrogen Balance = Nitrogen intake - Nitrogen loss Sources of nitrogen intake include meat, fish, dairy foods, eggs, nuts and legumes, and grains and cereals. Examples of nitr ...

may not reveal much about the amino acid profile and digestibility of the protein source in question, but can still be considered useful in that they determine other aspects of protein quality not taken into account by PDCAAS and DIAAS. Due to the difficulties of measuring amino acid digestion through the ileum, a minimally invasive dual-tracer method has been developed for the DIAAS method.

Protein sources

Amino acid profile

The amino acid score is based on the prevalence of the essential amino acids and depends on if they reach sufficient quantity. PDCAAS scores do not take into account the quantity of the non-essential amino acids. Despite the insufficient essential amino acid profiles of most plant-based proteins, it is possible to combine low lysine with low methionine plant-based proteins, which would yield a more complete protein. Below follows a table that compares the complete amino acid profiles of various proteins. *Semi-essential, under certain conditions
**Branched-chain amino acid ( BCAA)

Different essential amino acid requirements based on age

While the amino acid scores for PDCAAS and DIAAS are based on toddler requirements (1–3 year olds), the essential amino acid requirements differ for adults and infants. The most demanding essential amino acid requirements are for infants, and as children get older and transition into adulthood, the less proportions of essential amino acids they will need. This also means that many of the vegan protein sources that are limited in one or more essential amino acids, are actually less deficient in essential amino acids for adults, perhaps not deficient at all. The essential amino acid requirements for infants is based on the essential amino acid proportions in human breast milk.

References

{{Reflist Proteins as nutrients Nutrition