Heat works both ways. Cooking is what makes some proteins properly digestible: in a study with humans, cooked egg protein was 90 % digested, raw only 51 %. Too much or too direct heat does harm: fried ground beef lost quality, and in roasted pistachios the digestibility of lysine went down. Lysine is the most sensitive amino acid.
Why heat helps
When proteins are heated, they unfold. Digestive enzymes can then reach them more easily. In plants, heat also destroys compounds that slow down digestion, such as trypsin inhibitors. Moughan & Lim 2024 write that processing such as soaking, heating or extrusion often increases digestibility, especially in plant foods. That is why they call for foods to be tested the way people actually eat them, not raw.
Two examples from an FAO report (FAO sub-committee 2012):
| Food | raw | cooked | Measurement |
|---|---|---|---|
| Egg, protein digestibility | 51 % | 90 % | humans, small intestine |
| Soybean meal (defatted), protein digestibility | 37 % | 77 % | pigs, small intestine (apparent), autoclaved |
| Soybean meal, lysine digestibility | 41 % | 80 % | as above |
The egg result comes from one of the few studies that measured directly in humans. Raw egg protein was only about half digested there.
Meat: cook it, but don't overcook it
Bailey et al. 2020 studied in pigs how the way beef is prepared affects it. Beef roast (ribeye) was cooked in a smokehouse at 121 °C to three core temperatures: 56 °C (medium rare), 64 °C (medium) and 72 °C (well done). Ground beef was tested raw and fully cooked. For that, it was browned in a kettle until no pink was visible and it had reached over 72 °C.
Line = 100, from here on fully usable. DIAAS for adults, measured in pigs. Data: Bailey et al. 2020, Table 6.
- Doneness of the roast played a small role. All three roasts stayed above 100, but the differences between them were statistically significant. The authors write that the degree of doneness had little effect on digestibility and protein quality. They explain that 64 °C scored best mainly because that cut contained less fat and more essential amino acids.
- Well-done ground beef lost a lot. The DIAAS dropped from 121 to 99, the lowest value of all meat products tested. According to the authors, the measured contents of lysine and histidine went down, and several amino acids were digested less well.
The authors name oxidation as a possible cause: when heated, reactive groups can form in the protein that alter mainly lysine, histidine and arginine. Ground meat is more prone to this because grinding gives it more surface area in contact with oxygen. A whole roast is better protected. Their conclusion: meat that is ground or cooked with direct heat in a pan can lose quality, especially when it is overcooked. A meat thermometer could help.
For context: 99 is still a very good value. Other researchers found lower values for beef (raw 97, cooked 91). The authors attribute this to differences in fat, in the measurement method and in the unreported reference pattern.
Pistachios: roasting costs lysine
Bailey & Stein 2020 fed raw and roasted pistachios to pigs. They were roasted the way the industry usually does it: about 30 minutes in a roaster preheated to 176 °C, until the nuts reached 115 °C.
Standardized ileal digestibility in %, measured in pigs. Data: Bailey & Stein 2020, Table 4.
After roasting, almost all essential amino acids were digested less well. The authors suspect heat damage and clumping of the proteins, which can happen at temperatures above 100 °C and affects lysine, histidine and arginine in particular.
The DIAAS dropped from 86 to 83, limited by lysine both times. This difference was not statistically significant, and both versions count as a good protein source. One detail is interesting: the older score PDCAAS, calculated with a different reference pattern, actually went up after roasting, from 73 to 81. The authors conclude that DIAAS reflects heat damage better than PDCAAS. More on these scores in the article Biological value vs. DIAAS.
Why lysine in particular suffers
When heated, especially dry and in the presence of sugar, lysine reacts with other molecules. This is the Maillard reaction, which also creates browning and roasted flavors. The body can no longer use the altered lysine. Moughan & Lim 2024 write that lysine is the amino acid most easily damaged during processing. Arginine, methionine, cysteine, threonine and tryptophan can also be affected.
That is doubly unfortunate, because lysine is already the scarcest amino acid in grains. How much this matters for bread and breakfast cereals is covered in the articles Bread and grains and Cornflakes and breakfast cereals. Herreman et al. 2020 cite a study in which soybean meal and rapeseed meal lost 30 to 40 % of their usable lysine after toasting with sugar.
What you can take away
- Eggs, legumes and soy should be cooked. The heat makes their protein much more available.
- For meat, normal doneness hardly does any harm. Cooking it until very well done, especially ground meat, can cost some quality.
- Long, dry heating such as roasting and toasting mainly affects lysine.
Limitations of the studies
- Meat and pistachios were studied in pigs, the egg in humans. These studies have no data on grilling, boiling in water or microwaving.
- One product was tested per preparation method, with six to eight animals.
- The meat study was funded by a cattle producers' association, the pistachio study by a funding program of the US Department of Agriculture.
Frequently asked questions
Is cooked egg better than raw egg?
For the protein, yes. In a study with humans, cooked egg protein was about 90 % digested, raw egg protein only about 51 %.
Does meat lose protein quality when you cook it?
It depends on how it is cooked. A beef roast stayed excellent at core temperatures of 56, 64 and 72 °C. Well-done ground beef, on the other hand, dropped from a DIAAS of 121 to 99. The authors advise against overcooking meat.
Why is lysine so sensitive?
When heated, lysine reacts with sugars and other compounds, the so-called Maillard reaction. The body can no longer use the altered lysine. According to a review, lysine is the amino acid most easily damaged during processing.
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- Moughan, P. J., Gilani, S., Rutherfurd, S. M. & Tomé, D. (2012): Report of a Sub-Committee of the 2011 FAO Consultation on “Protein Quality Evaluation in Human Nutrition”, with appendix by Moughan & Rutherfurd: True ileal amino acid and protein digestibility for selected human foods. FAO, Rome.
- Bailey, H. M., Mathai, J. K., Berg, E. P. & Stein, H. H. (2020): Most meat products have digestible indispensable amino acid scores that are greater than 100, but processing may increase or reduce protein quality. British Journal of Nutrition 124, 14–22. doi:10.1017/S0007114520000641
- Bailey, H. M. & Stein, H. H. (2020): Raw and roasted pistachio nuts (Pistacia vera L.) are ‘good’ sources of protein based on their digestible indispensable amino acid score as determined in pigs. Journal of the Science of Food and Agriculture 100, 3878–3885. doi:10.1002/jsfa.10429
- 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
- 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
Values for individual foods and where they come from are listed under Data & sources. How proteinscore calculates is explained under Method.