Protein quality

The amino acid score is based on the proportion of amino acids in a food, compared to nutritional requirements. As such, only essential amino acids are considered in the two most common measurements of quality, the PDCAAS and the DIAAS. ^[1][2]

The following is a table of the amino acid profiles of some common protein sources, not accounting for digestibility. The requirement profile is the required amounts of an amino acid in every 100 g of protein in the Dietary Reference Intake. Each profile reflects the amount of an amino acid per 100 g of protein, not 100 g of the food source.

In considering protein quality, the sulfur amino acids (methionine + cystine) and the aromatic amino acids (phenylalanine + tyrosine) are grouped together. This is because while methionine and phenylalanine are essential amino acids, cystine and tyrosine are synthesized from methione and phenylalanine, respectively.^[3] Nevertheless, common protein analytical methods such as ISO 13903 can easily distinguish these pairs of amino acids.

Additionally, glutamic acid is easilty interconvertible with glutamine and aspartic acid is easily interconvertible with asparginine. easily interconvertible, via acid hydrolysis.^[3] Thus, common amino acid analysis methods such as ISO 13903 only measure glutamic acid and aspartic acid, not glutamine or asparginine, and these measured values may be treated as a sum of the two.^[4]

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.^[5]

*Semi-essential, under certain conditions
**Branched-chain amino acid (BCAA)

For many foods, the quantity of amino acids absorbed by the body may differ significantly from the quantities of amino acids originally present in the food, as a result of various digestive processes. The digestion of proteins begins in the stomach and is largely complete by the time food exits the small intestine.^[22][23] However, digestion may be reduced by antinutritional factors or the presence of other food components such as dietary fiber.^[24] Gut microbes may also impact protein digestion due to their own digestion of protein.^[25]

Digestibility may also differ between amino acids. While the fecal digestibility of the whole protein is likely 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, and tryptophan can be overestimated.^[26][27]

The cooking of protein sources, particularly animal protein, may significantly alter both the composition and digestibility of amino acids. Since this is not always reflected in nutritional data,^[28] changes during cooking may be factored into measurements of protein quality. For example, the browning of many meats causes the Maillard reaction, which may decrease the availability of lysine, the limiting amino acid for many foods.^[29] As a result, it is recommended to use lysine as the limiting amino acid for any food likely to have undergone the Maillard reaction.^[30]

While the amino acid scores for PDCAAS and DIAAS are based on toddler requirements (1–3 year olds),^[6] the essential amino acid requirements differ for adults and infants.^[6] 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. Old age and pregnancy also change amino acid requirements, because of the necessity of supporting a fetus or slowing the loss of muscle due to age.^[31][29][24] The essential amino acid requirements for infants are based on the essential amino acid proportions in human breast milk.^[6]

^{[e.g. methionine and cysteine have different empirical formulae and different molecular weights, so it is unclear how their quantities should be added together.]}

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.

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 the PDCAAS of 1989.^[26] 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.^[32] For the amino acid score, it considers the quantity of each essential amino acid in the food as a proportion of the food's protein content and compares this to nutritional recommendations for preschool-age kids. The amino acid score used is that of the lowest, or 'limiting', amino acid. The amino acid score is then multiplied with the fecal digestibility score to get an overall score between zero and one.^[26]

The PDCAAS has a tendency to over-estimate digestibility, because it measures feces instead of what remains at the end of the small intestine, where most protein absorption is believed to occur.^[33] Antinutrient factors like phytic acid and trypsin inhibitors may decrease absorption of protein, as does the absorption of protein by gut bacteria in the test subject's large intestine.^[22] In addition, older rats show lower PDCAAS-estimated fecal digestibility compared to young rats when the protein source contains anti-nutritional factors.^[32]

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 what is actually absorbed.^[2][34][35] It also considers age by using different reference patterns for infants, toddlers, and people over three.^[36]: 29 DIAAS is considered the superior method to PDCAAS.^[34][37]

DIAAS is more complicated to measure than PDCAAS partly because the contents of the ileum are 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.^[36] For measurement in humans, a minimally invasive dual-tracer method has been developed for the DIAAS method.^[27]

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).^[38]