Measured in mini pigs, soy milk reaches a DIAAS of 117, tofu 97 and seitan only 28 (adults). All three are digested very well, at 92 to 97 %. The difference comes from their composition: seitan lacks lysine. Tofu scores below soy milk because its methionine was digested less well. The authors suspect coagulation is responsible.
The study
Reynaud et al. 2021 fed store-bought seitan, tofu and UHT soy milk, plus an emulsion made from pea protein isolate, to six mini pigs. Each test meal contained 30 g of protein. They measured how much of each amino acid is absorbed in the small intestine. To do this, the researchers determined what was still left at the end of the small intestine and subtracted it from the amount eaten. According to the authors, it is the first study to determine this for seitan, tofu and soy milk.
The results
Line = 100, fully usable from this point. DIAAS for adults (older children, adolescents, adults). Data: Reynaud et al. 2021, Table 6.
| Reference pattern | Soy milk | Tofu | Pea | Seitan |
|---|---|---|---|---|
| Infants (0–6 months) | 78 | 68 | 42 | 19 |
| Young children (0.5–3 years) | 99 | 83 | 51 | 24 |
| Older children and adults | 117 | 97 | 60 | 28 |
| Digestibility (all amino acids) | 92 % | 95 % | 94 % | 97.5 % |
Digestibility was high for all products and, according to the authors, comparable to meat (91 to 98 %) and milk (94 to 95 %). The ranking therefore comes almost entirely from composition: soy ahead of pea ahead of seitan.
Seitan: lots of protein, little lysine
Seitan is made from wheat gluten. In the study, lysine made up only 1.4 % of its protein; in tofu and soy milk it was around 5.6 %, in the pea product 7.0 %. The authors write that seitan has only half as much lysine as whole wheat. That puts its DIAAS at 28. For whole wheat, the authors cite values of 30 to 60, depending on the study.
That doesn't mean seitan is useless. It is digested almost completely, and the other amino acids are plentiful. What it lacks is a partner with plenty of lysine. Calculated with the proteinscore database (adults):
| Meal | DIAAS | Protein / of which usable | |
|---|---|---|---|
| 120 g seitan, ready to eat | 26 | 28.8 g / 7.5 g | open |
| + 200 g chickpeas | 64 | 46.6 g / 29.8 g | open |
| + 200 g lentils | 70 | 46.8 g / 32.9 g | open |
Even with plenty of legumes, lysine stays scarce. Seitan therefore works better as an addition alongside a lysine-rich protein source than as the only main source.
Tofu vs soy milk: what happens during coagulation
Tofu is made from soy milk. Even so, its DIAAS was 20 points lower. The reason: methionine from tofu was only 85.6 % digested, from soy milk 99.3 %. Cysteine showed a similar trend (83.8 versus 104.6 %), but it was not statistically significant. Because methionine and cysteine are the scarcest amino acids in soy, the overall score drops.
The authors suspect that these amino acids get trapped inside the gel during coagulation and are harder for digestive enzymes to reach. The heating during tofu production, on the other hand, probably broke down digestion inhibitors.
Why the calculator shows slightly different numbers
In the proteinscore calculator, tofu gets 89 instead of 97 and soy milk 108 instead of 117. The reason is the reference basis. The study relates the amino acids to the sum of all measured amino acids (Table 5 of the study). Like the FAO 2013, we relate them to the crude protein content, which is a bit higher. That gives somewhat more conservative values. The ranking stays the same.
What the study doesn't show
- One product each was tested in six animals. Other brands or production methods may differ.
- Tofu and seitan were ground and freeze-dried before feeding.
- For some numbers, the study text differs slightly from the tables. We use the table values throughout.
- The foods were provided by a company that co-funded the study.
For comparison: in the review by Herreman et al. 2020, soy as a protein ingredient averages 91, wheat 48 (both young child pattern). That fits the ranking in this study. More on plant protein in general in the article Protein on a vegan diet.
Frequently asked questions
Is tofu a complete protein?
For adults, almost. In the study by Reynaud et al. 2021, tofu reached a DIAAS of 97, limited by methionine and cysteine. The authors write that this is close to milk and meat.
Why does seitan score so poorly?
Seitan is digested almost completely, but it contains very little lysine, only 1.4 % of its protein. That results in a DIAAS of 28 for adults. Lysine-rich partners like lentils or chickpeas can partly make up for this.
Is soy milk better than tofu?
In the measurement, yes: 117 versus 97. When tofu is made, the protein is coagulated. The authors suspect that this makes methionine and cysteine harder to access.
How does your meal score?
Open the calculatorSources
- 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
- 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
- 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.