Grain protein is limited by lysine and reaches a DIAAS of about 35 to 60. Oats do a little better, corn worst. Strong heating can cost additional lysine: in toasted bread, usable lysine was a third below the analyzed value. Dairy, eggs or meat make the protein of a grain-based meal complete; legumes help in larger amounts.
The values at a glance
Herreman et al. 2020 averaged measured data from pig studies for the main types of grain:
Line = 100, from here on fully usable. Averages across several data sets, young child pattern (0.5–3 years). Data: Herreman et al. 2020, Table 2.
Other studies arrive at similar values:
| Grain | DIAAS | Reference pattern | Source |
|---|---|---|---|
| Whole wheat | 45 / 54 | young children / adults | Mathai et al. 2017, pigs |
| Wheat | 40 | young children | FAO 2013, example table |
| Rice, cooked | 60 | young children | Rutherfurd et al. 2015, rats |
| Rolled oats, cooked | 54 | young children | Rutherfurd et al. 2015, rats |
| Wheat bran | 41 | young children | Rutherfurd et al. 2015, rats |
The range is especially wide for wheat. According to Herreman et al., the individual data sets are up to 53 points apart. Depending on the variety and processing, wheat protein can score considerably better or worse.
Why grains lack lysine
All grains share the same weak spot: lysine. In the study by Mathai et al. 2017, lysine was the scarcest amino acid in wheat under every reference pattern. It was also digested less well than in all the other proteins tested, 77 % versus 93 to 98 %. The authors suspect the wheat had suffered heat damage during drying or milling.
An extreme case is wheat gluten, which seitan is made from. According to Reynaud et al. 2021, seitan contains only half as much lysine as whole wheat and reached only 28 for adults. More on this in the article Tofu, seitan, soy milk.
What baking and toasting do to the protein
None of the studies reviewed directly compared dough and finished bread. But there are clear signs that heat during baking, and especially toasting, costs lysine. In the Maillard reaction, which creates crust and roasted flavor, lysine reacts with sugar and becomes unusable for the body.
Moughan & Lim 2024 compare two ways of measuring digestible lysine: the standard analysis and the lysine that is actually usable (reactive):
Percentage by which reactive lysine falls below the value from standard analysis. Measured in pigs or rats. The bar shows a loss, not a DIAAS. Data: Moughan & Lim 2024, Table 2.
So in toasted wheat bread, about a third of the measured lysine was no longer usable, in whole-grain bread a sixth. A comparison with the unbaked dough is missing, though. The article Cornflakes and breakfast cereals shows how extreme this can get.
For the DIAAS, this means: many published values for grains are based on standard analysis and probably overestimate baked or toasted products. Herreman et al. themselves name this as a weakness of their analysis. Reynaud et al. 2021 cite a study that found a DIAAS of only 20 % for bread. We cannot check the conditions of that measurement, because the study is not in our source base.
A surprising observation in humans
The FAO 2013 reports on trials in humans in which wheat protein was used better than its lysine content would suggest. Postprandial utilization was given as 1.00 for milk protein and 0.68 for wheat protein with several small meals, and 0.93 and 0.61 with one large meal. However, the FAO notes that such trials can assess poorly digestible proteins only to a limited extent. So this is no free pass, but it does show that the DIAAS remains an approximation.
How to make a grain-based meal complete
Grains are limited by lysine but have relatively a lot of methionine. Partners rich in lysine make up for this. Calculated with the proteinscore database (adults):
| Meal | DIAAS | |
|---|---|---|
| 100 g whole-grain bread | 45 | open |
| 100 g whole-grain bread + 100 g low-fat quark | 113 | open |
| 60 g toast + 1 egg | 105 | open |
| 250 g pasta + 25 g Parmesan | 94 | open |
| 120 g whole-grain bread + 60 g hummus | 66 | open |
| 100 g whole-grain bread + 30 g peanut butter | 48 | open |
Dairy and eggs have the strongest effect. Hummus helps a little. Peanut butter hardly helps, because peanuts themselves are low in lysine. The FAO 2013 works through a similar example in its report: 400 g wheat with 100 g peas and 35 g whole milk powder comes to 82 for adults.
That this matters in everyday life is shown by the analysis of vegan food diaries by Soh et al. 2026: meals dominated by grains had the lowest protein quality. Lysine was the scarcest amino acid in 57 % of all meals. More on this in the article Combining proteins.
Limitations
- Almost all values come from pig or rat studies. For wheat there is data on protein digestibility and utilization measured directly in humans, but no DIAAS.
- None of the studies reviewed directly measured whether, and how much, normal baking lowers the DIAAS.
- Most values apply to the young child reference pattern. For adults they are higher, for wheat about 54 instead of 45.
Frequently asked questions
How good is the protein in bread?
Depending on the study and reference pattern, wheat protein reaches a DIAAS of about 40 to 54. It is limited by lysine. So with bread alone, the scarcest amino acid, lysine, covers only about half of the requirement.
Does bread lose protein when you toast it?
The amount stays the same, but part of the lysine becomes unusable. In one analysis, usable lysine in toasted wheat bread was about a third below the value from standard analysis.
Which grain has the best protein?
In a large analysis, oats came out on top with a DIAAS of 57, ahead of wheat (48), rice (47) and corn (36), each calculated for young children. Animal proteins, by contrast, are mostly above 100.
How does your meal score?
Go to 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
- Rutherfurd, S. M., Fanning, A. C., Miller, B. J. & Moughan, P. J. (2015): Protein digestibility-corrected amino acid scores and digestible indispensable amino acid scores differentially describe protein quality in growing male rats. Journal of Nutrition 145, 372–379. doi:10.3945/jn.114.195438
- 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
- Soh, B. X. P., Vignes, M., Smith, N. W., von Hurst, P. R. & McNabb, W. C. (2026): Beyond daily totals: meal-level digestible indispensable amino acid score reveals how food groups shape protein quality in vegan diets. Frontiers in Nutrition 13, 1752697. doi:10.3389/fnut.2026.1752697
- 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
Values for individual foods and where they come from are listed under Data & sources. How proteinscore calculates is explained under Method.