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 There are various methods that rank the quality of different types of protein, some of which are outdated and no longer in use, or not considered as useful as they once were thought to be.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. The dairy industry is in favor of this, because while PDCAAS truncates all protein types that exceed the essential amino acid (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 inhibitors, which limit the absorption of protein among other nutrients, and its use of fecal digestibility, whereas in the DIAAS system, ileal digestibility of the essential amino acids is emphasized as a more accurate measure of protein absorption.[1][2][3] For this reason, DIAAS is promoted as the superior method and preferable over the PDCAAS.[2][4] Due to not factoring in anti-nutritional content into the digestibility equation, the PDCAAS has consequently been criticized for overestimating protein quality.[5] Other older methods like BV, PER, NPU and nitrogen balance 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 ileuem, a minimally invasive dual-tracer method has been developed for the DIAAS method.[6]

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[1]	82%[1]	91[1]		3.5 g/h[7]		1.0[1]		None	Yes
Whey	3.2[1]	92%[1]	104[1]		8–10 g/h[7]		1.0[1]	0.973–1.09[2][4]	Histidine[8]	Yes
Casein	2.5[1]	76%[1]	77[1]	97.6%[9]	6.1 g/h[7]	1.19[10]	1.0[1]	0.93[11]	None	Yes
Egg	3.9[1]	94%[1]	100[1]	97–98%[12]	1.3–2.8 g/h[7]	1.19[10]	1.0[1]		None	Yes
Beef	2.9[1]	73%[1]	80[1]	94–98%[12]		0.94[10]	0.92[1]		Tryptophan	No
Oat				72,[13] 91%[10]		0.63[10]	0.57[9]		Lysine	No
Wheat	0.8[1]	67%[1]	64[1]	96–99%[12]		0.26,[12] 0.44[10]	0.25–0.51[1][8]	0.45[8]	Lysine[8]	No
Maize	1.23[14]			85%,[13] 89%[15]			0.67[15]		Lysine[15]	No
Rice	2.2[14]						0.42[2]	0.37[2]	Lysine	No
Quinoa		75.7%[16]	82.6[16]	91.7%[16]			0.667[17]		None	Yes
Soy	2.2[1]	61%[1]	74[1]	95–98%[12]	3.9 g/h[7]		0.91–1.0[4][1][2]	0.90–0.91[2]	Meth+Cyst[8]	Yes[4][1][2]
Black bean							0.75[1]	0.53-0.65[11]	Meth+Cyst	No
Pea				88%[10][13]	2.4–3.4 g/h[7]		0.89[2]	0.82[2]	Meth+Cyst	Yes
Peanut	1.8[1]			94[13]			0.52[1]		Lysine	No
Hemp			87	94.9%[9]		0.64[9]	0.61[9]		Lysine[9]	No
Mycoprotein				86%[18]			0.996[18]		Meth+Cyst	No
Spirulina	1.8-2.6[14]	53–92%[14]	68[19]	83–90%					Lysine	Yes
Chlorella									Meth+Cyst	No

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.[20][1]

Below follows a table that compares the complete amino acid profiles of various proteins.

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 per 100 g of protein DRI[21][22][23][24]	Human breast milk[25]	Quinoa[26]	Corn[27]	Oat[28]	Hemp seeds (shelled)[29]	Green peas[30]	Soybeans (Edamame)[31]	Spirulina[32]	Chlorella[33]	Whey[34]	Casein[35]	Egg white[36]
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
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)

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),[21] the essential amino acid requirements differ for adults and infants.[21] 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.[21]

Essential amino acid requirements

Amino Acid required mg per g of protein	Infants[21]	1–3 year olds[21]	Adults (18+ y)[21]
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	7	6
Valine	56	32	29
Total mg of Essential Amino Acids per g of protein	496	287	262

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