Protein Quality

Protein quality is the digestibility and quantity of essential amino acids for providing the proteins in correct ratios for human consumption. There are various methods that rank the quality of different types of protein, some of which are outdated and no longer in use, or not considered as useful as they once were thought to be.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 (PDCAAS), which was recommended by the Food and Agriculture Organization of the United Nations (FAO), became the industry standard in 1993. FAO has recently recommended the newer Digestible Indispensable Amino Acid Score (DIAAS) to supersede PDCAAS.

Measures

Primitive measures of protein quality use relatively few measurements about the body, mainly mass measurements.
* Protein efficiency ratio (PER) is the ratio of weight gain to the amount of protein ingested. It is usually tested with rats.
* Biological value (BV) essentially estimates the proportion of food nitrogen kept in the body by subtracting out nitrogen found in urine and feces. Nitrogen is assumed to originate from protein. Net protein utilization (NPU) is similar, except it only subtracts out urine nitrogen. They are methods based on nitrogen balance.

PDCAAS versus DIAAS

Modern measurements analyze two separate aspects: protein digestibility and amino acid balance. The former is measured by comparing how much protein is found in the food before and after it goes through the digestive tract or a part of it. The latter is measured by taking the amino acid profile of a protein and comparing it to essential amino acid needs of an organism, typically humans.

The earlier "modern" measurement is PDCAAS of 1989.
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For protein digestibility it compares the amount of protein-nitrogen that goes into a rat and out of the rat through feces, with a correction for "metabolic fecal protein": the amount of protein that occurs in feces when the rat is on a protein-free diet. For the amino acid score it uses a system established in FAO/WHO 1990. The amino acid score is multiplied with the fecal digestibility score and capped to 100%. Because it deals with feces instead of the end of the small intestine where most protein absorption is believed to occur, it has a tendency to over-estimate the digestibility. Antinutrient factors like phytic acid and trypsin inhibitors may prevent the test animal's body from absorbing the protein like it does in humans, but they do not have much of an effect on gut bacteria, so their effects end up understated. In addition, older rats show lower PDCAAS-estimated fecal digestibility compared to young rats when the protein source contains antinutritional factors.

To address the problems of PDCAAS, DIAAS was introduced in 2013. It measures digestibility from the mouth to the end of the ileum (the final section of the small intestine), individually for every amino acid. The absorbed amount of each essential amino acid is compared with the reference pattern. In other words, it scores the amino acid profile of the actually-absorbed part. It also has an updated reference amino acid pattern, for three age-groups of humans. DIAAS is promoted as the superior method and preferable over the PDCAAS.

DIAAS is more complicated to measure than PDCAAS partly because the contents of the ileum is harder to obtain than simple collection of feces. Moreover, DIAAS prefers digestibilities measured in humans, though a growing pig or growing rat are acceptable alternatives. For measurement in humans, a minimally invasive dual-tracer method has been developed for the DIAAS method.

Protein sources

Notes:
* With scores greater or equal to 1.0 or 100%, the concept of "limiting amino acid" technically still applies as the amino acid with the lowest ratio compared to the amounts in the reference protein. It is hardly relevant, however, so such columns are parenthesized.
* PD is determined per PDCAAS ("true fecal") unless otherwise stated.
* AAS explicitly does not take into account digestibility. It compares the amino acid profile to a reference profile, which is the PDCAAS profile unless otherwise stated.
* Limiting AA may be determined by either PDCAAS (or a similar profile-only method) or the DIAAS (which gives the limiting ''absorbed'' AA). If unstated it is more likely to be PDCAAS.
* PDCAAS values are officially capped, but the limit can be removed by manually calculating PD × AAS. Examples of 5 uncapped values are seen in Schaafsma (2000).

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 and DIAAS 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.

A DIAAS-style amino acid profile of the ''actually digested part'' of a protein would be superior to the values presented herein, but such profiles are not common enough to be presented in a table such as the below. Such a profile is equivalent to multiplying the amount each amino acid amount with its digestibility. According to the PDCAAS publication, the fecal digestibility of the whole protein is a fair approximation of the digestibility of individual amino acids for non-legume (beans, peas, lentils) proteins with a maximum difference of 10%. With legume proteins, the digestibility of methionine, cystine, tryptophan can be overestimated.

Below follows a table that compares the complete amino acid profiles of various proteins. The requirement profile is the required amounts of an amino acid in every 100 g of protein in the Dietary Reference Intake. The profile for the amounts in each food is the amount of an amino acid per 100 g of protein from this food (not 100 g of the food itself).

Explanations for missing values:
* In the field of nutrition, the sulfur amino acids (methionine + cystine) and the aromatic amino acids (phenylalanine + tyrosine) are merged into the same entity for convenience. Technically speaking, methionine and phenylalanine are the actual essential amino acids; cystine and tyrosine are made from methione and phenylalanine in the body respectively. Nevertheless, common protein analytical methods such as ISO 13903 easily distinguish these pairs of amino acids. Individual values should be available from the source that provided the original profile, so expect these to be filled at a later date.
* Glutamic acid and glutamine are easily interconvertible. Common amino acid analysis methods such as ISO 13903 only measure glutamic acid, not glutamine. This is because the amide glutamine easily converts to glutamic acid during acid hydrolysis. A lone glutamic acid value should therefore be treated as a sum of the two.
* Similarly, asparagine easily converts to aspartic acid during acid treatment. A lone aspartic acid value should be treated as the sum of the two.
* Selenocysteine is usually not measured as part of amino acid analysis. It is usually analyzed directly as the amount of selenium, which mostly occurs as selenomethionine and selenocysteine in food.

*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; when children become adults, they need lower proportions of essential amino acids. 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 are based on the essential amino acid proportions in human breast milk.

Limitation

Focus on single proteins

The modern methods may also still be considered incomplete, since human diets, except in times of famine, almost never contain only one kind of protein. However, calculating the PDCAAS/DIAAS of a diet solely based on the PDCAAS/DIAAS of the individual constituents is impossible, because one food may provide an abundance of an amino acid that the other is missing, in which case the PDCAAS/DIAAS of the diet is higher than that of any one of the constituents. To arrive at the final result, all individual amino acids would have to be taken into account, though, so the PDCAAS of each constituent is largely useless.

For example, grain protein has a PDCAAS of about 0.4 to 0.5, limited by lysine. On the other hand, it contains more than enough methionine. White bean protein (and that of many other pulses) has a PDCAAS of 0.6 to 0.7, limited by methionine, and contains more than enough lysine. When both are eaten in roughly equal quantities in a diet, the PDCAAS of the combined constituent is 1.0, because each constituent's protein is complemented by the other.

A more extreme example would be the combination of gelatine (which contains virtually no tryptophan and thus has a PDCAAS of 0) with isolated tryptophan (which, lacking all other essential amino acids, also has a PDCAAS of 0). Despite individual scores of 0, the combination of both in adequate amounts has a positive PDCAAS, with the limiting amino acids isoleucine, threonine, and methionine. Further, according to a 2000 study by Gerjan Schaafsma, "The questions about the validity of the amino acid scoring pattern and the application of the true fecal rather than the true ileal digestibility correction, as well as the truncation of PDCAAS values warrant a critical evaluation of PDCAAS in its current form as a measure of protein quality in human diets."

References

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Proteins as nutrients
Nutrition