Most plant proteins reach a lower DIAAS than animal proteins. The main reason is their amino acid pattern: grains have too little lysine, legumes too little methionine and cysteine. In terms of digestibility, protein-rich plant products are often almost as good as meat. Soy and potato are exceptions with high scores. If you eat vegan, you can close the gaps by combining sources or by eating more protein.
How good is plant protein? The values at a glance
A comprehensive compilation comes from Herreman et al. 2020. The authors analyzed data from pig studies for 12 plant and 5 animal protein sources and calculated an average DIAAS for each. The values apply to the young child reference pattern (0.5 to 3 years), which the authors use as their main value. For adults, all values come out higher.
Line = 100, fully usable from this point. Average DIAAS across several datasets, young child pattern (0.5–3 years). Data: Herreman et al. 2020, Table 2.
From 100, a protein counts as fully usable; from 75, as a good source. By these thresholds, only potato (100) and soy (91) qualify among the plants. Pea, lupin, oat, wheat, rice and corn fall below. Corn, rice, wheat, faba bean, oat and hemp don't even reach 60 on average.
Why whey only gets 85 here: The authors themselves write that whey is actually considered an excellent protein with a DIAAS above 100. The datasets they averaged, however, only gave 78 to 88. They suspect that purity and processing of the products play a big role. The reference pattern adds to this: young children need more histidine (20 instead of 16 mg per g protein), and histidine is the limiting amino acid in whey. In the single study by Mathai et al. 2017, whey isolate scored 100 for young children and 125 for adults. More on this in the protein powder comparison.
By the way, the thresholds of 75 and 100 are not laws of nature. Mathai et al. 2017 point out that they were chosen arbitrarily. They help with classification, nothing more.
The weak spot: usually a single amino acid is missing
DIAAS is always set by the scarcest essential amino acid. In plants, the pattern is surprisingly clear. Herreman et al. 2020 describe it like this: grains are limited by lysine, legumes such as faba bean, pea, lupin and soy by the sulfur amino acids methionine and cysteine. For potato, it is histidine for adults. Hertzler et al. 2020 and Mathai et al. 2017 reach the same result.
Seitan, which is wheat gluten, shows how big the differences can be. In the study by Reynaud et al. 2021, lysine made up only 1.4 % of its protein. In tofu, soy milk and a pea product it was 5.6 to 7.0 %. That means seitan had only half as much lysine as whole wheat and reached a DIAAS of 28 for adults.
This also explains why combining works so well: grains have relatively a lot of methionine, legumes relatively a lot of lysine. More on this in the article Combining proteins.
Is plant protein harder to digest?
You often hear that plant protein is digested less well. The studies paint a more nuanced picture.
- Raw ingredients do differ. In the pig study by Mathai et al. 2017, the average digestibility of the essential amino acids was between 92 and 98 % for milk proteins, 96 % for soy protein isolate, 93 % for pea protein concentrate and soybean meal, and 85 % for wheat. Lysine from wheat was only 77 % digested.
- For finished foods, the gap is small. Reynaud et al. 2021 tested seitan, tofu, soy milk and a pea protein emulsion in mini pigs. Digestibility was 92 to 97 %. The authors compare this with literature values for meat (91 to 98 %) and milk (94 to 95 %).
- Composition is what matters. Herreman et al. 2020 write that the amino acid pattern has the biggest influence on DIAAS. Reynaud et al. 2021 reach the same conclusion for seitan and pea: they are digested almost completely, but their pattern is unbalanced.
The studies also name what can slow down digestibility in plants: so-called antinutritional factors such as trypsin inhibitors, phytate, tannins and saponins, plus fiber and the structure of the proteins themselves. Processing often helps. According to Hertzler et al. 2020, soy protein isolate is 96 % or more digested, soy flour only 84 %. In Mathai et al. 2017, soybean meal contained about three times as many trypsin inhibitors as the isolate.
Soy and potato: the exceptions
Soy reaches good values in all the studies reviewed. In Mathai et al. 2017, soy protein isolate scored 98 for adults, soybean meal 105. Reynaud et al. 2021 measured 117 for soy milk and 97 for tofu. The authors write that these values are close to those of milk and meat. More on this in the article Tofu, seitan, soy milk.
Potato protein averages 100 in Herreman et al. 2020. In the review by Hertzler et al. 2020 it is 85, limited by histidine. Good to know: the authors of Herreman et al. work for Avebe, a producer of potato starch and potato protein. For regular potatoes, the value matters less anyway, because they contain only about 2 g of protein per 100 g.
What this means for a vegan diet
A low DIAAS does not mean a food is worthless. It means that part of its protein can't be used without a suitable partner. The studies name two ways to make up for this.
Eat more protein. Herreman et al. 2020 calculate that you would need about 2.8 times as much corn protein, or 1.45 times as much pea protein, to reach a DIAAS of 100. They point out, though, that such large amounts are very filling. Hertzler et al. 2020 sum up training studies like this: larger amounts of plant protein, around 40 g a day or more, can bring results similar to whey.
Combine. "Any protein can reach a higher DIAAS when combined with a suitable complement," write Herreman et al. 2020. In their calculations, a mix of pea and rice reaches 84, soy with wheat 90. But not every mix helps: faba bean with corn only reaches 64.
A calculation by Moughan & Lim 2024 shows that this matters in practice: according to a study they cite, vegans in the UK eat an average of 0.99 g of protein per kg of body weight. At a utilization of 70 %, typical for a plant-based diet, 0.69 g of that would remain. That would be below requirements.
Try it: pasta alone gets a DIAAS of 54 in the calculator. With lentils it is 91.
How reliable are the numbers?
The values come from animal studies with pigs or rats. Much of the data applies to raw ingredients from animal feed research, not to prepared food, which the authors themselves name as a weakness. On top of that, the same food scores differently depending on the study:
| Source | Herreman 2020 | Mathai 2017 | Hertzler 2020 | Reynaud 2021 |
|---|---|---|---|---|
| Pea | 70 | 62 (concentrate) | 66 | 51 (emulsion) |
| Soy | 91 | 84 (isolate), 89 (meal) | 92 | 83 (tofu), 99 (soy milk) |
| Whey | 85 | 100 (isolate), 107 (concentrate) | 90 | – |
Several of the studies were co-funded by industry or written by company employees: the dairy industry for Mathai et al., a potato protein producer for Herreman et al., a maker of nutrition products for Hertzler et al. The core finding, that grains are limited by lysine and legumes by methionine, is the same in all of them.
Frequently asked questions
Is plant protein worse than animal protein?
On average, plant proteins reach lower DIAAS values. The main reason is their amino acid makeup, not their digestibility. Soy and potato come close to animal proteins, while grains score clearly lower.
Which amino acid do vegans most often run short of?
In grains it is lysine, in legumes methionine and cysteine. In an analysis of more than 3,700 vegan meals, lysine was the limiting amino acid in 57 % of meals.
Do I need to eat more protein as a vegan?
If you mostly eat proteins with a low DIAAS, the math says you need more of them to get the same amount of usable protein. Another option is to combine sources with different weak spots.
How does your meal score?
Open the calculatorSources
- Herreman, L., Nommensen, P., Pennings, B. & Laus, M. C. (2020): Comprehensive overview of the quality of plant- and animal-sourced proteins based on the digestible indispensable amino acid score. Food Science & Nutrition 8, 5379–5391. doi:10.1002/fsn3.1809
- Mathai, J. K., Liu, Y. & Stein, H. H. (2017): Values for digestible indispensable amino acid scores (DIAAS) for some dairy and plant proteins may better describe protein quality than values calculated using the concept for protein digestibility-corrected amino acid scores (PDCAAS). British Journal of Nutrition 117, 490–499. doi:10.1017/S0007114517000125
- Hertzler, S. R., Lieblein-Boff, J. C., Weiler, M. & Allgeier, C. (2020): Plant proteins: assessing their nutritional quality and effects on health and physical function. Nutrients 12, 3704. doi:10.3390/nu12123704
- Reynaud, Y., Buffière, C., Cohade, B. et al. (2021): True ileal amino acid digestibility and digestible indispensable amino acid scores (DIAASs) of plant-based protein foods. Food Chemistry 338, 128020. doi:10.1016/j.foodchem.2020.128020
- Moughan, P. J. & Lim, W. X. J. (2024): Digestible indispensable amino acid score (DIAAS): 10 years on. Frontiers in Nutrition 11, 1389719. doi:10.3389/fnut.2024.1389719
- FAO (2013): Dietary protein quality evaluation in human nutrition. Report of an FAO Expert Consultation. FAO Food and Nutrition Paper 92, Rome. PDF
You can find values for individual foods and where they come from under Data & sources. How proteinscore calculates is explained under Method.