Introduction
When a protein powder label reads "25g protein per serving," you might think that is the final number. In reality, what matters is not only how much protein you consume, but also how well your body can digest and use it. This is exactly what protein digestibility scoring systems were designed to measure.
What Is PDCAAS?
PDCAAS (Protein Digestibility Corrected Amino Acid Score) is a scoring system used by the World Health Organization (WHO) since 1993 (FAO/WHO, 1991). It measures two things:
1. Amino acid profile -- does the protein source contain all essential amino acids in sufficient amounts?
2. Digestibility -- how much of the protein can your body actually absorb?
The PDCAAS scale runs from 0 to 1.0, where 1.0 is the maximum score.
PDCAAS scores for common protein sources:
- Whey protein: 1.0
- Casein: 1.0
- Egg: 1.0
- Soy: 1.0
- Pea protein: 0.89
- Rice protein: 0.50-0.62
- Hemp protein: 0.46-0.51
- Wheat gluten: 0.25
An important limitation: PDCAAS is capped at 1.0. This means a protein with an actual score of 1.2 still shows as 1.0. It does not differentiate among high-quality protein sources (Rutherfurd et al., 2015).
What Is DIAAS and Why Is It Better?
DIAAS (Digestible Indispensable Amino Acid Score) is a newer scoring system introduced by the WHO in 2013 (FAO, 2013). It improves on several PDCAAS shortcomings:
- No upper limit -- scores can exceed 100, distinguishing "good" from "excellent" proteins
- Measures each amino acid individually -- identifies specific limiting amino acids
- More accurate digestibility measurement -- measures absorption in the small intestine, not just from faeces
DIAAS quality categories:
- Above 100: Excellent (whey protein isolate, eggs, milk)
- 75-99: Good (pea protein, soy)
- Below 75: Should be combined with other protein sources
In practical terms: the higher the DIAAS score, the less protein you need to achieve the same result (Mathai et al., 2017).
Why Digestibility Matters
Digestibility directly affects how much protein your body actually uses. For example:
- If you consume 25g of protein with 95% digestibility (whey isolate), your body can use ~24g
- If you consume 25g of protein with 50% digestibility (wheat gluten), your body can use only ~12.5g
This means from a low-digestibility source, you need to consume nearly double to achieve the same result.
Factors that affect digestibility:
- Anti-nutrients -- phytates and tannins in plant proteins reduce digestibility
- Processing -- heating and hydrolysis improve digestibility
- Fibre -- high fibre content can slow protein digestion
- Amino acid profile -- the limiting amino acid determines the overall digestibility score
How to Choose Protein Based on Digestibility
If your goal is maximum protein synthesis efficiency:
Choose a protein source with a high PDCAAS/DIAAS score: whey isolate, casein, or egg.
If you prefer plant protein:
Pea protein is the best single plant option (PDCAAS 0.89). Combine pea protein with rice protein (70:30 ratio) for a near-perfect profile (Herreman et al., 2020).
If you are looking for the best value:
Whey concentrate offers excellent digestibility (PDCAAS 1.0) at a reasonable price. Protein contributes to the growth and maintenance of muscle mass.
With a varied diet:
If your diet is already diverse, every individual protein source does not need to be "perfect." Amino acids consumed throughout the day complement each other.
Frequently Asked Questions
Do I always need to check the PDCAAS score when choosing protein?
Not necessarily. If you eat a varied diet and consume enough protein (1.6-2.2g/kg body weight), the score of any single source is less critical. However, if you rely primarily on one protein source, a higher score is better.
Does a PDCAAS score of 1.0 mean the protein is perfect?
It means the protein meets the minimum essential amino acid requirement and is well digestible. But there can still be quality differences between two PDCAAS 1.0 proteins -- which is exactly why DIAAS is a better system.
Is plant protein significantly worse than animal protein?
No. Some plant proteins (soy, pea protein) are high quality. The key is choosing the right plant sources and combining them when needed.
Does heating damage protein digestibility?
Normal cooking (baking, boiling) actually tends to improve digestibility. Extreme heat can cause damage, but this is not a concern in everyday cooking.
What is a limiting amino acid?
It is the essential amino acid that a protein source contains in the lowest amount relative to requirements. For example, the limiting amino acid in rice protein is lysine.
References
1. FAO/WHO. (1991). Protein quality evaluation: Report of the Joint FAO/WHO Expert Consultation. FAO Food and Nutrition Paper, 51.
2. FAO. (2013). Dietary protein quality evaluation in human nutrition: Report of an FAO Expert Consultation. FAO Food and Nutrition Paper, 92.
3. Rutherfurd SM, Fanning AC, Miller BJ, Moughan PJ. (2015). Protein digestibility-corrected amino acid scores and digestible indispensable amino acid scores differentially describe protein quality in growing male rats. The Journal of Nutrition, 145(2), 372-379.
4. Mathai JK, Liu Y, Stein HH. (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.
5. Herreman L, Nommensen P, Pennings B, Laus MC. (2020). Comprehensive overview of the quality of plant- and animal-sourced proteins based on the digestible indispensable amino acid score. Food Science & Nutrition, 8(10), 5379-5391.
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See also:
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Disclaimer
A food supplement is not a substitute for a varied and balanced diet and a healthy lifestyle.
