DIAAS: The Best Way to Measure Protein Quality
- Orsolya Szathmari
- Sep 8
- 5 min read
Introduction
Protein has become one of the most talked-about nutrients in health, fitness, and nutrition. The selection of protein bars, protein shakes, and high-protein versions of everything from cereal to pasta is endless. But here’s the catch: the number of grams of protein on a label doesn’t necessarily tell you how much of that protein your body can actually use.
Not all proteins are equal. Different foods vary in how digestible their proteins are and whether they provide all the essential amino acids our bodies need. For years, scientists relied on PDCAAS (Protein Digestibility-Corrected Amino Acid Score) to measure protein quality. PDCAAS assesses protein value by comparing its amino acid content against human requirements, then adjusting for digestibility. It was an improvement over just counting grams of protein, but it has limitations—it often overestimates plant protein quality and masks the fact that some proteins are superior to others.
This is where DIAAS (Digestible Indispensable Amino Acid Score) comes in. Recommended by the Food and Agriculture Organization (FAO), DIAAS is now considered the gold standard for measuring protein quality. It gives us a much clearer picture of how effectively different foods provide usable protein for the human body.
What Is DIAAS?
DIAAS (Digestible Indispensable Amino Acid Score) evaluates protein quality by:
Measuring true ileal digestibility of each essential amino acid, rather than total protein digestibility.
Identifying the limiting amino acid—the essential amino acid present in the lowest proportion relative to human needs.
Not limiting scores at 100%, so very high-quality proteins are accurately recognized.
In other words, DIAAS tells us how much of the protein in a food can actually be absorbed and used by the body. DIAAS measure protein quality
DIAAS Data: How Foods Stack Up
Here’s a comparison of representative foods with both DIAAS values and protein per typical portion:
* Note on Limiting Amino Acids:
For animal proteins, the limiting amino acid (e.g., tryptophan in meat) is mostly a technical detail. These foods are complete, high-quality proteins, and typical portions provide more than enough of every essential amino acid.
For plant proteins, the limiting amino acid (e.g., methionine in legumes, lysine in grains) can significantly reduce protein quality, so combining different plant foods is often necessary to achieve a complete amino acid profile.
DIAAS vs. PDCAAS
PDCAAS caps scores at 100%, hiding differences between very high-quality proteins.
PDCAAS measures digestibility over the whole digestive tract, including microbial breakdown in the large intestine—amino acids from there aren’t actually absorbed.
It can overestimate plant protein quality, particularly for soy and wheat.
DIAAS avoids these limitations by focusing on ileal digestibility of individual amino acids and not capping scores, giving a more precise picture of protein quality for single foods.
A Note on Soy
Soy is frequently touted as the highest-quality plant protein. Indeed, soy protein isolate scores around 0.90 DIAAS, making it useful in powders and processed foods. Whole soy foods like tofu score slightly lower (~0.97), mainly due to fiber and anti-nutrients reducing digestibility. Soy also contains:
Protease inhibitors, which can reduce protein digestibility—though cooking and processing largely inactivate them.
Phytoestrogens (isoflavones), plant compounds that can modulate hormone receptors. Effects vary and remain debated; however, DIAAS doesn’t measure hormonal effects—only amino acid availability.
On Anti-Nutrients and Digestibility
Protease inhibitors: Naturally high in soy and many legumes; studies in animal models show they can reduce protein and amino acid digestibility by up to 50%, and protein quality by up to 100%. Processing (heat, fermentation) significantly reduces protease inhibitor content
Phytates predominantly impair the absorption of minerals (e.g., calcium, iron) rather than proteins. Their levels can be significantly reduced through food processing techniques such as fermentation and sprouting.
Other factors: Plant cell walls and fiber can hinder digestion; processing methods (soaking, cooking, fermenting, milling) help improve digestibility.
Animal vs. Plant Proteins in Practice
Animal proteins (milk, eggs, meat, whey) consistently have DIAAS >1 (>100%), all essential amino acids, high digestibility, and no anti-nutrient concerns—making them reliable and efficient protein sources.
Plant proteins vary more. Some (like soy isolate) come close—but most (e.g., legumes, grains) have lower DIAAS and may require larger portions or complementary combinations to meet needs. Anti-nutrients and lower digestibility further complicate their use as sole protein sources in diets. Some foods, like potatoes, have technically high DIAAS as isolated protein but contain very little protein per typical serving (~4 g per medium potato), making them insufficient as a primary protein source.
Key takeaway: High DIAAS does not automatically make a food a practical protein source. Both quality and quantity matter for real-world nutrition.
Why DIAAS Matters
Goes beyond counting grams of protein—focuses on usable amino acids.
Helps dietitians, athletes, and health professionals optimize meals for muscle synthesis, recovery, and overall health.
Guides public health nutrition strategies, particularly in regions where protein deficiency is a concern.
Final Thoughts
DIAAS offers a significantly improved framework for evaluating protein quality. It highlights that animal proteins are not just convenient—they supply the most efficiently digestible, complete proteins. While soy and other plant proteins can contribute with proper preparation and combinations for those who don’t want to consume meat, eggs and dairy, animal proteins remain unmatched in reliability and purity (i.e., no anti-nutrients or phytoestrogens that may affect digestion or physiology).
Bottom line: DIAAS gives a more accurate picture of protein quality, helping people make smarter, evidence-based dietary choices.
References
FAO Expert Consultation. Dietary protein quality evaluation in human nutrition: Report of an FAO Expert Consultation. FAO Food and Nutrition Paper 92. Rome: FAO; 2013.
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(4), 490–499.
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Cervantes-Pahm, S.K., Liu, Y., Stein, H.H. (2014). Digestible indispensable amino acid score and digestible amino acids in eight cereal grains. Br J Nutr, 111(9), 1663–1672.
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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. J Nutr, 145(2), 372–379.
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Kumar, V., Sinha, A.K., Makkar, H.P.S., Becker, K. (2010). Dietary roles of phytate and phytase in human nutrition: A review. Food Chem, 120(4), 945–959.
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