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 There are various methods that rank the quality of different types of protein, some of which are outdated and no longer in use, or not considered as useful as they once were thought to be.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.

Considerations

Further information: Protein combining

Amino Acid Profile

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. 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.^[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.

First limiting amino acid   Second limiting amino acid, especially depending on certain age requirements and demanding conditions for EAAs   Amino acid is technically complete, but might turn out incomplete anyway after digestibility has been factored in

Amino acid profiles and requirements

Essential Amino Acids	Required (DRI)[3][4][5][6]	Human breast milk[7]	Quinoa[8]	Corn[9]	Oat[10]	Hemp seeds (shelled)[11]	Green peas[12]	Soybeans (Edamame)[13]	Spirulina[14]	Chlorella[15]	Whey[16]	Casein[17]	Egg white[18]
Histidine	1.8 g	2.230 g	3.455 g	2.710 g	2.414 g	2.821 g	2.495 g	2.756 g	1.888 g	3.3 g	1.974 g	3.2 g	2.660 g
Isoleucine**	2.5 g	5.673 g	4.279 g	3.928 g	4.137 g	3.744 g	4.547 g	4.514 g	5.584 g	3.5 g	5.001 g	5.4 g	6.064 g
Leucine**	5.5 g	9.623 g	7.132 g	10.597 g	7.654 g	6.296 g	7.532 g	7.334 g	8.608 g	6.1 g	9.475 g	9.5 g	9.321 g
Lysine	5.1 g	6.888 g	6.503 g	4.172 g	4.179 g	3.714 g	7.392 g	6.138 g	5.264 g	10.2 g	8.554 g	8.5 g	7.394 g
Meth + Cyst	2.5 g	4.052 g	4.346 g	2.832 g	4.292 g	4.672 g	2.658 g	2.178 g	3.151 g	1.6 g	3.684 g	3.5 g	6.293 g
Phen + Tyr	4.7 g	10.029 g	7.302 g	8.132 g	8.751 g	7.889 g	7.332 g	8.316 g	9.328 g	5.6 g	5.790 g	11.1 g	10.486 g
Threonine	2.7 g	4.660 g	3.574 g	3.928 g	3.428 g	3.694 g	4.734 g	4.087 g	5.168 g	2.9 g	5.001 g	4.2 g	4.119 g
Tryptophan	0.7 g	1.722 g	1.418 g	0.700 g	1.395 g	1.074 g	0.863 g	1.243 g	1.616 g	2.1 g	2.106 g	1.4 g	1.147 g
Valine**	3.2 g	6.382 g	5.043 g	5.633 g	5.585 g	5.173 g	5.480 g	4.562 g	6.111 g	5.5 g	5.001 g	6.3 g	7.422 g
Total EAAs	28.7 g	51.259 g	43.052 g	42.632 g	46.014 g	39.077 g	43.033 g	41.128 g	46.718 g	40.8 g	46.586 g	53.1 g	54.906 g
Individual Essential Amino Acids	Requirement	Human breast milk	Quinoa	Corn	Oat	Hemp seeds (shelled)	Green peas	Soybeans (Edamame)	Spirulina	Chlorella	Whey	Casein	Egg white
Meth	To be filled	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A
Phen	To be filled	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A
Non-Essential Amino Acids	Required?	Human breast milk	Quinoa	Corn	Oat	Hemp seeds (shelled)	Green peas	Soybeans (Edamame)	Spirulina	Chlorella	Whey	Casein	Egg white
Alanine	Varies	3.647 g	4.992 g	8.983 g	5.252 g	4.448 g	5.597 g	4.609 g	7.856 g	7.7 g	4.343 g	N/A	6.458 g
Arginine*		4.356	9.263 g	3.989 g	7.106 g	13.245 g	9.981 g	8.253 g	7.216 g	15.8 g	2.764 g	3.7 g	5.945 g
Asparagine*		N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A
Aspartic acid		8.307 g	9.628 g	7.430 g	8.632 g	10.660 g	11.567 g	11.943 g	10.080 g	6.4 g	9.738 g	N/A	11.192 g
Cysteine*		N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A
Glutamic acid		17.018 g	15.834 g	19.366 g	22.127 g	18.249 g	17.280 g	19.269 g	14.592 g	7.8 g	17.898 g	N/A	14.220 g
Glutamine*		N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A
Glycine*		2.634 g	5.892 g	3.867 g	5.013 g	4.690 g	4.291 g	4.269 g	5.392 g	6.2 g	1.842 g	N/A	3.789 g
Proline*		8.307 g	6.563 g	8.891 g	5.568 g	4.649 g	4.034 g	4.807 g	4.145	7.2 g	5.922 g	N/A	3.991 g
Selenocysteine		N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A
Serine*		4.356 g	4.814 g	4.659 g	4.471 g	4.987 g	4.221 g	5.710 g	5.217	3.3 g	4.606 g	N/A	7.321 g
Tyrosine*		5.369 g	2.267 g	3.745 g	3.409 g	3.677 g	2.658 g	3.675 g	4.496 g	2.8 g	2.500 g	N/A	4.193 g
Total non-EAAs		53.994 g	58.257 g	60.93 g	61.578 g	64.605 g	59.629 g	62.535 g	58.994 g	57.2 g	49.613 g	N/A	57.109 g
22nd Amino Acid	Required?	Human breast milk	Quinoa	Corn	Oat	Hemp seeds (shelled)	Green peas	Soybeans (Edamame)	Spirulina	Chlorella	Whey	Casein	Egg white
Pyrrolysine	Not used by humans	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A

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

However, as 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, tryptophan can be overestimated.^[19]: 31

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 made from methione and phenylalanine in the body respectively.^[20] Nevertheless, common protein analytical methods such as ISO 13903 can easily distinguish these pairs of amino acids.

Additionally, glutamic acid and glutamine are easily interconvertible.^[20] 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.^[21] Similarly, asparagine easily converts to aspartic acid during acid treatment, so the aspartic acid value should be treated as the sum of the two.^[21]

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.^[22]

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),^[3] the essential amino acid requirements differ for adults and infants.^[3] 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.^[23] The essential amino acid requirements for infants are based on the essential amino acid proportions in human breast milk.^[3]

Essential amino acid requirements (mg per g of protein)

Amino Acid required	Infants[3]	1–3 year olds[3]	Adults (18+ y)[3]
Histidine	23	18	17
Isoleucine	57	25	23
Leucine	101	55	52
Lysine	69	51	47
Methionine + Cysteine	38	25	23
Phenylalanine + Tyrosine	87	47	41
Threonine	47	27	24
Tryptophan	18	07	06
Valine	56	32	29
Total Essential Amino Acids	496	287	262

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

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.

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.

PDCAAS versus DIAAS

The earlier "modern" measurement is the PDCAAS of 1989.^[19] 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.^[24] 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.^[19]

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.^[25] 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.^[26] In addition, older rats show lower PDCAAS-estimated fecal digestibility compared to young rats when the protein source contains anti-nutritional factors.^[24]

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][27][28] It also considers age by using different reference patterns for infants, toddlers, and people over three.^[29]: 29 DIAAS is considered the superior method to PDCAAS.^[27][30]

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.^[29] For measurement in humans, a minimally invasive dual-tracer method has been developed for the DIAAS method.^[31]

Protein sources

Protein ranking methods and standards

Protein type	PER	NPU	BV	Protein Digestibility (PD)	Protein absorption rate	Amino Acid Score (AAS)	PDCAAS	DIAAS	Limiting amino acid	Complete protein?
Cow's milk	2.5[2]	82%[2]	91[2]	95[32]	3.5 g/h[33]	127[32]	1.0[2]		(Met+Cys)	Yes
Whey	3.2[2]	92%[2]	104[2]		8–10 g/h[33]		1.0[2]	0.973–1.09[27][30]	His[34]	Yes
Casein	2.5[2]	71.2%[35]	77[2]	94.0%[35]	6.1 g/h[33]	1.19[36]	1.0[2]	1.45[35]	(Met+Cys)	Yes
Egg	3.9[2]	94%[2]	100[2]	97–98%[37]	1.3–2.8 g/h[33]	1.19[36]	1.0[2]	1.13[38]	(His)	Yes
Beef	2.9[2]	73%[2]	80[2]	94–98%[37]		0.94[36]	0.92[2]		Trp	Yes
Oat				72,[39] 91%[36]		0.63[36]	0.57[40]		Lys	No
Wheat	0.8[2]	67%[2]	64[2]	96–99%[37]		0.26,[37] 0.44[36]	0.25–0.51[2][34]	0.45[34]	Lys[34]	No
Maize	1.23[41]			85%,[39] 89%[42]			0.67[42]		Lys[42]	No
Rice	2.2[41]						0.42[27]	0.37[27]	Lys	No
Quinoa		75.7%[43]	82.6[43]	91.7%[43]		0.97[44]	0.667[45]		Leu	Yes
Soy	2.2[2]	61%[2]	74[2]	95–98%[37]	3.9 g/h[33]		0.91–1.0[30][2][27]	0.90–0.91[27]	Met+Cys[34]	Yes[30][2][27]
Black bean							0.75[2]	0.53-0.65[46]	Met+Cys	No
Pea				88%[36][39]			0.89[27]	0.82[27]–1.00[35]	Met+Cys	Yes
Peanut	1.8[2]			94[39]		0.75[47]	0.52[2]		Lys	No
Hemp			87	94.9%[40]		0.64[40]	0.61[40]		Lys[40]	No
Mycoprotein				86%[48]			0.996[48]		Met+Cys	No
Spirulina	1.8-2.6[41]	53–92%[41]	68[49]	83–90%		1.10[50]			Lys	Yes
Chlorella									Met+Cys	No

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).^[32]

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."^[32]