In a rat study, a corn-based breakfast cereal scored a DIAAS of 1, the lowest value of 14 protein sources. Only 13 % of its lysine was usable. The authors blame the Maillard reaction during manufacturing. Not all cereals are affected equally, but processed grain products often lose a lot of lysine. With milk, the protein of the meal is still good, because milk supplies the missing lysine.
A DIAAS of 1
Rutherfurd et al. 2015 tested 14 protein sources in rats, in the form people eat them: milk and soy proteins, cooked peas, beans, rice and rolled oats, wheat bran, peanuts and a corn-based breakfast cereal from the supermarket. The study does not name a brand.
Line = 100, from here on fully usable. DIAAS, young child pattern (0.5–3 years), measured in rats. Of two soy protein isolates, the better one is shown (the other: 89.8). Data: Rutherfurd et al. 2015, Table 5.
Overall, 67 % of the cereal's protein was still digested. For lysine, though, it was only 13 %. Because lysine is scarce in corn to begin with, the DIAAS fell to 0.012, rounded to 1. The authors write that this shows how strongly processing can impair the protein quality of some foods.
What happens to the lysine
The authors consider Maillard reactions during manufacturing the most likely cause. In this reaction, sugars and amino acids bond under heat. It gives cereal flakes, bread crust and cookies their golden-brown color and roasted flavor. Lysine is especially prone to it. Moughan & Lim 2024 call it the amino acid most easily damaged during processing.
The tricky part is that standard lab analyses partly miss the damage. During analysis, the protein is broken down with strong acid. In the process, some early Maillard products turn back into lysine, which does not happen in the gut. An example from an FAO report (FAO sub-committee 2012) with a heated mixture of casein and sugar:
| Measurement method | Lysine g/100 g |
|---|---|
| Standard analysis with acid | 2.60 |
| Unaltered ("reactive") lysine | 1.98 |
| Of which actually absorbed | 1.40 |
That is why the FAO 2013 recommends measuring reactive lysine in processed foods. According to Moughan and Lim, though, this step has so far been largely overlooked.
How big are the losses in other products?
Moughan & Lim 2024 show how far usable lysine falls below the value from standard analysis:
Percentage by which usable (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.
Grain-based breakfast cereals and puffed rice lost more than half of their lysine. However, the values vary a lot from product to product. In the FAO sub-committee's rat data, reactive lysine from flakes and puffed products was 53 to 100 % usable depending on the product, and 66 % from corn flakes. So the 13 % from the study by Rutherfurd et al. is an extreme value, not an average.
Why milk solves the problem
Rutherfurd et al. also calculated what happens when the cereal is mixed with milk protein, at a ratio of 60 : 40 based on protein. According to the authors, this roughly matches a bowl of cereal with milk. They used milk protein concentrate for the calculation. Result: a DIAAS of 1.07. Milk protein contains so much lysine that it fills the gap completely.
Calculated with the proteinscore database (adults):
| Breakfast | Protein | DIAAS | |
|---|---|---|---|
| 40 g cornflakes, dry | 2.8 g | 1 | open |
| 40 g cornflakes + 200 ml milk | 9.4 g | 117 | open |
| 40 g cornflakes + 200 ml soy milk | 9.4 g | 76 | open |
| 60 g rolled oats + 250 ml milk | 16.2 g | 116 | open |
The protein in the meal then comes almost entirely from the milk. If you want more protein at breakfast, rolled oats are a better choice. They have more protein, and in the study 84 % of their lysine was available.
Limitations of the study
- The value of 1 comes from a single, unnamed product, tested in rats. It cannot be applied to all cornflakes.
- That the Maillard reaction was the cause is the authors' conclusion; it was not measured.
- The mix with milk was calculated, not measured.
- The study was funded by a New Zealand government funding program. Two of the four authors are employed by a dairy company that also supplied the milk proteins.
Frequently asked questions
Do cornflakes have protein?
A little, and hardly any of it is usable. One corn-based breakfast cereal that was tested scored a DIAAS of 1, because only 13 % of its lysine was available. Other cereals did better in other measurements.
What is the Maillard reaction?
A reaction between sugars and amino acids during heating that creates browning and roasted flavor. It mainly alters lysine, which the body can then no longer use.
Are cornflakes with milk okay?
For the protein, yes. Milk protein contains so much lysine that a mix like a typical bowl with milk works out to a DIAAS above 100. But then almost all the protein comes from the milk.
How does your meal score?
Go to the calculatorSources
- 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
- 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.
- FAO (2013): Dietary protein quality evaluation in human nutrition. Report of an FAO Expert Consultation. FAO Food and Nutrition Paper 92, Rome. PDF
Values for individual foods and where they come from are listed under Data & sources. How proteinscore calculates is explained under Method.