# DIAAS vs PDCAAS: what protein quality scores measure, and what they miss

October 15, 2026 · Nutrition & Macros · 8 min read · https://burnweek.fit/blog/protein-quality-diaas-explained/

> The same milk powder scores 69 or 143 depending on whose needs you compare it with. What protein quality scores can tell you, and what they can't.

**Key takeaways**

- FAO recommended DIAAS to replace PDCAAS in its 2013 report: it measures ileal digestion per amino acid and stops capping single foods at 100.
- In FAO's worked example, whole milk powder scores 69, 122 or 143 depending on the infant, young-child or adult reference pattern.
- Scores don't average: a wheat, pea and milk mix scored 68, above the ~57 you'd get by protein-weighting the three foods' scores.
- Protein quantity and amino-acid adequacy both matter; FAO: quality cannot be substituted for quantity.

## Both scores ask one question: how well the most limiting amino acid is covered

What do DIAAS and PDCAAS measure? Both protein quality scores answer the same narrow question: if you eat a gram of this protein, how well does it cover your need for its limiting essential amino acid, the one with the lowest ratio to the reference requirement, after accounting for digestion? PDCAAS (Protein Digestibility-Corrected Amino Acid Score) was the standard from 1991. DIAAS (Digestible Indispensable Amino Acid Score) was recommended to replace it by a Food and Agriculture Organization expert consultation whose report was published in 2013, mainly because it measures digestion more precisely and stops capping good proteins at 100 ([FAO, 2013](https://www.fao.org/ag/humannutrition/35978-02317b979a686a57aa4593304ffc17f06.pdf)).

An individual food's score does not describe your whole diet. Each food's score is graded against a reference pattern that depends on age, and neither score reflects how much protein you eat in total. That is why a food with a mediocre score can sit inside a diet that scores well, and why the score is the wrong tool for predicting muscle gain. Rankings of individual foods live in [best protein sources](/blog/best-protein-sources); this article is about what the numbers mean and where they stop meaning anything.

## How the two scores are built, and why FAO recommended DIAAS

Both scores compare the protein's content of each essential amino acid with a reference pattern of what people need; the limiting amino acid is the one with the lowest ratio (usually lysine in cereals, the sulfur amino acids in legumes). PDCAAS applies one protein-wide digestibility correction to that score, while DIAAS corrects each amino acid for its own digestibility **before** selecting the lowest ratio. The differences in detail:

| | PDCAAS (1991) | DIAAS (2013 recommendation) |
| --- | --- | --- |
| Where digestion is measured | Whole gut, from what is left in feces | End of the small intestine (ileal), where amino acids are actually absorbed |
| Digestibility applied to | Crude protein as a whole | Each essential amino acid separately |
| Scores above 100 | Truncated to 100 | Kept for single foods; mixed diets truncated at 100 |
| Heat-damaged lysine | Not accounted for | Uses "reactive" lysine where processing may have damaged it |

Fecal measurements can overestimate amino-acid absorption because colonic bacteria alter the nitrogen remaining in feces. The FAO report listed the resulting problems with PDCAAS directly: it does not credit extra nutritional value to high-quality proteins, it overestimates the quality of products containing antinutritional factors, and it does not adequately account for amino acid bioavailability ([FAO, 2013](https://www.fao.org/ag/humannutrition/35978-02317b979a686a57aa4593304ffc17f06.pdf)).

Truncation was the most visible problem. Capping at 100 made milk and soy look identical even though milk carries more of some amino acids that may be limiting elsewhere in a diet; the FAO consultation described the practice as having "created considerable confusion" ([FAO, 2013](https://www.fao.org/ag/humannutrition/35978-02317b979a686a57aa4593304ffc17f06.pdf)).

When researchers measured both kinds of value side by side, using growing pigs as the digestion model and the same 6-to-36-month reference pattern for both, the PDCAAS-like values came out higher than DIAAS for skimmed milk powder, pea protein concentrate, soy isolate, soy flour and whole-grain wheat, which led them to conclude PDCAAS-like values may overestimate the quality of those proteins ([Mathai et al., 2017](https://pubmed.ncbi.nlm.nih.gov/28382889/)). A decade-on review of the consultation's recommendations reported that the pig assay has been validated as a stand-in for human ileal digestion, and that a newer dual-isotope method estimates digestibility in humans, though broad independent validation remained incomplete. Its conclusion was careful: DIAAS is "currently the most accurate score for routinely assessing the protein quality rating of single source proteins" ([Moughan & Lim, 2024](https://pubmed.ncbi.nlm.nih.gov/39021594/)). The authors declared no conflicts of interest.

> DIAAS is the better ruler. An individual food's score still does not describe your whole diet, and it is set against one age group at a time.

## Same food, three scores: the reference pattern problem

The part that surprises most people is that a DIAAS is not a fixed property of a food. The score depends on whose amino acid needs you compare it with. The FAO report specifies three reference patterns: breast milk for infants, a young-child pattern for 6 months to 3 years, and the 3-to-10-year-old pattern for older children, adolescents and adults. For food labeling and regulation it recommends the young-child pattern for everything except infant formula ([FAO, 2013](https://www.fao.org/ag/humannutrition/35978-02317b979a686a57aa4593304ffc17f06.pdf)).

The report's own worked examples show how much this moves the number:

| Worked example (FAO 2013) | Infant pattern | Young-child pattern | Older child, adolescent, adult pattern |
| --- | --- | --- | --- |
| Whole milk powder | 69 | 122 | 143 |
| Mixed diet of wheat, peas and whole milk powder | 56 | 68 | 82 |

Whole milk powder scores 69 or 143 depending on the reference, which is a doubling from the choice of denominator alone. The practical consequence: many published DIAAS tables, and most label-style claims, use the young-child pattern, which is stricter than the one that describes adult needs. An adult reading a value computed against the child pattern is looking at a conservative number.

## What the scores miss

**Your diet is a mixture, and scores do not simply average.** In the same FAO worked example, a simplified illustration using four amino acids, wheat alone scored 40, peas 64 and whole milk powder 122 against the young-child pattern. A mixture of 400 g wheat, 100 g peas and 35 g milk powder scored 68, whereas weighting the three individual scores by the protein each supplies gives roughly 57 (our arithmetic). Moughan and Lim stress that DIAAS values "are not necessarily additive": to score a meal, you must add up the digestible amino acids from each food and score the total, not average the foods' scores ([Moughan & Lim, 2024](https://pmc.ncbi.nlm.nih.gov/articles/PMC11252030/)). How complementary proteins cover each other's gaps is covered in [complete vs incomplete proteins](/blog/complete-vs-incomplete-proteins).

**Quantity is not in the score.** A DIAAS is per gram of protein. The FAO report put it bluntly: "In assessing the quality of proteins, quality cannot be substituted for quantity" ([FAO, 2013](https://www.fao.org/ag/humannutrition/35978-02317b979a686a57aa4593304ffc17f06.pdf)). In its example labeling scheme, wheat and peas meet the quantity criterion but fail the proposed quality criterion for a protein claim, while whole milk powder meets both, because the FAO's suggested cut-off for any quality claim was a DIAAS of 75.

**Processing moves the number.** Heating and drying can damage lysine through Maillard reactions (the same browning chemistry that colors toast), lowering how much is usable. The 2024 review lists processing effects on digestibility and lysine availability among the things that still limit DIAAS accuracy ([Moughan & Lim, 2024](https://pubmed.ncbi.nlm.nih.gov/39021594/)). A score measured on one preparation may not describe another.

**Requirements are averages.** Reference patterns are population averages for defined ages, while real amino acid needs shift with age, illness and diet ([Moughan & Lim, 2024](https://pmc.ncbi.nlm.nih.gov/articles/PMC11252030/)).

**A score is not a muscle multiplier.** Protein with a 60 score does not build 60% as much muscle as protein with a 100 score. A review of the anabolic evidence found that soy and wheat protein produce a lower muscle protein synthesis response than several animal proteins in acute studies, The review proposed lower digestibility, greater amino-acid diversion to urea, and lower amounts of certain essential amino acids, including leucine, as possible explanations. It described larger servings and combining sources as proposed strategies whose efficacy still required testing ([van Vliet et al., 2015](https://pubmed.ncbi.nlm.nih.gov/26224750/)). Later trials comparing matched plant and animal diets over weeks are covered in [plant vs animal protein](/blog/plant-vs-animal-protein).

## When protein quality is worth your attention

For most people eating a varied diet with some animal foods or a range of plant proteins, quality scores add little to the decision. Total intake does most of the work, as set out in [how much protein per day](/blog/how-much-protein-per-day). The score starts to matter when the margin for error shrinks:

- **Plant-only diets built on one staple.** When most protein comes from a single cereal or legume, there is no second food to cover its limiting amino acid. Variety, and pairing cereals with legumes, is the fix; [protein for vegetarians and vegans](/blog/protein-for-vegetarians-and-vegans) goes into the practical side.
- **Low total intake.** Complementary proteins work by one food's surplus covering another's shortfall. If total protein sits near the minimum, the surplus is thin.
- **Older adults.** Older muscle needs a larger dose per meal to respond, as covered in [protein for older adults](/blog/protein-for-older-adults), so a lower-quality source leaves less room. Eating somewhat more protein, or leaning on higher-quality sources at key meals, is the usual answer.

In each case, protein quantity and amino-acid adequacy both matter.

## FAQ

### Is a DIAAS above 100 better than 100?

For a single food, yes, in a limited sense: a score above 100 means the food supplies more of its limiting amino acid than the reference requires, so it can help cover a weaker protein eaten alongside it. PDCAAS discarded that information by capping at 100; DIAAS keeps it for single foods but truncates scores for whole diets ([FAO, 2013](https://www.fao.org/ag/humannutrition/35978-02317b979a686a57aa4593304ffc17f06.pdf)).

### Why do DIAAS values for the same food differ between sources?

Mostly because of the reference pattern and the preparation. Whole milk powder scores 69 against the infant pattern, 122 against the young-child pattern and 143 against the adult pattern in the FAO's own example ([FAO, 2013](https://www.fao.org/ag/humannutrition/35978-02317b979a686a57aa4593304ffc17f06.pdf)), and processing can lower lysine availability.

### Can I average the DIAAS of foods to score my meal?

No. The scores are not additive; a meal has to be scored from the total digestible amino acids supplied by each food ([Moughan & Lim, 2024](https://pmc.ncbi.nlm.nih.gov/articles/PMC11252030/)). In the FAO example, a wheat, pea and milk mixture scored 68, above the roughly 57 you would get by protein-weighting the three foods' scores.

### Does a low-scoring protein count toward my daily protein target?

Yes. Its grams count; its quality means you may need more of it, or a complementary food alongside, to supply the same amount of the limiting amino acid. The FAO report's own line is that quality cannot be substituted for quantity.

## Sources

- [FAO. Dietary protein quality evaluation in human nutrition: Report of an FAO Expert Consultation. FAO Food and Nutrition Paper 92. Rome: FAO; 2013.](https://www.fao.org/ag/humannutrition/35978-02317b979a686a57aa4593304ffc17f06.pdf)
- [Mathai JK, Liu Y, Stein HH. 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). Br J Nutr. 2017;117(4):490-499.](https://pubmed.ncbi.nlm.nih.gov/28382889/)
- [Moughan PJ, Lim WXJ. Digestible indispensable amino acid score (DIAAS): 10 years on. Front Nutr. 2024;11:1389719.](https://pubmed.ncbi.nlm.nih.gov/39021594/)
- [van Vliet S, Burd NA, van Loon LJ. The Skeletal Muscle Anabolic Response to Plant- versus Animal-Based Protein Consumption. J Nutr. 2015;145(9):1981-1991.](https://pubmed.ncbi.nlm.nih.gov/26224750/)

Source: BurnWeek — "DIAAS vs PDCAAS: what protein quality scores measure, and what they miss", https://burnweek.fit/blog/protein-quality-diaas-explained/. Licensed CC BY 4.0: free to quote or reuse with a link to this page.