Protein Quality Explained: What is PDCAAS & Digestibility Really Mean

When it comes to building muscle, not all proteins are created equal. You’ve probably heard that whey is “high-quality” or that plant proteins need to be combined, but what does that really mean?

Protein quality isn’t about marketing terms, it’s a measurable, scientific concept that describes how efficiently your body can digest and use amino acids for growth and repair. The key metric used to evaluate this is the Protein Digestibility-Corrected Amino Acid Score (PDCAAS), and more recently, DIAAS.

Understanding these scores helps you choose protein sources that give you the best anabolic return per gram, whether you’re drinking a shake or eating whole foods.

What Is Protein Quality?

Protein quality measures how well a protein source supports human needs for essential amino acids (EAAs) and how digestible it is.

Two main factors define it:

1. Amino acid composition: Does it provide all nine essential amino acids in the right amounts?
   
2. Digestibility: How efficiently can your body absorb and use those amino acids?
   

A high-quality protein delivers both, a complete amino acid profile and high digestibility. This is what makes whey, eggs, and milk proteins top performers for athletes. To understand how complete proteins compare with amino acid supplements, read our guide to BCAAs, EAAs and complete proteins.

PDCAAS – The Gold Standard for Protein Quality

Definition

PDCAAS (Protein Digestibility-Corrected Amino Acid Score) is the most widely used method for evaluating protein quality.
It was developed by the FAO/WHO (1991) to reflect how effectively dietary protein meets human amino acid requirements¹.

The PDCAAS value ranges from 0.0 to 1.0, where:

• 1.0 = excellent quality protein (contains all essential amino acids and is highly digestible)
  
• <1.0 = incomplete or less digestible protein
  

In simple terms, PDCAAS tells you how efficiently your body can use a given protein source for growth and repair.

How PDCAAS Is Calculated

The PDCAAS formula combines two variables:

PDCAAS = (Amino acid score) × (True digestibility)

• Amino acid score: Compares the limiting essential amino acid in a protein to the amino acid requirements of humans.
  
• True digestibility: The percentage of protein that is absorbed during digestion (measured using nitrogen balance studies).
  

Example:

If a protein source has an amino acid score of 1.0 and 95% digestibility, its PDCAAS is 0.95.

However, PDCAAS scores are truncated at 1.0, meaning anything that exceeds this value (like whey or egg) is rounded down to 1.0, even though it may technically be superior.

Common PDCAAS Scores (Approximate Values)²

Protein Source	PDCAAS Score	Notes
Whey Protein Isolate	1.00	Complete amino acid profile, very high digestibility
Egg Protein	1.00	Excellent bioavailability and amino acid balance
Milk Protein (Casein)	1.00	Slow digestion, sustained amino acid release
Soy Protein	0.91-0.95	Complete protein, slightly lower lysine digestibility
Pea Protein	0.82-0.89	Good EAA content but limited in methionine
Rice Protein	0.50-0.60	Low lysine, benefits from blending with pea protein
Wheat Protein	0.40-0.45	Low in lysine, incomplete on its own
Collagen	0.00-0.10	Very low essential amino acid content (lacks tryptophan)

Sources: FAO/WHO (1991); Schaafsma (2000); Gorissen & Witard (2018)

The Limitation of PDCAAS

Although PDCAAS is widely accepted, it has several limitations:

1. It truncates scores at 1.0, masking differences between very high-quality proteins (e.g., whey vs. egg).
   
2. It uses fecal digestibility, not ileal (small intestine) digestibility, which can misrepresent absorption accuracy.
   
3. It doesn’t account for antinutritional factors (like phytates) that affect amino acid absorption in plant proteins.
   

Because of this, researchers developed a newer and more precise system: DIAAS.

DIAAS – The Next-Generation Protein Quality Score

What Is DIAAS?

DIAAS (Digestible Indispensable Amino Acid Score) is a modern method that measures protein quality more accurately than PDCAAS³.

The key differences:

• Uses ileal digestibility, measuring amino acid absorption at the end of the small intestine (where most digestion occurs).
  
• No truncation at 1.0, so proteins can score higher than 100%.
  
• Evaluates each amino acid individually, not as a composite value.
  

DIAAS vs. PDCAAS Comparison

Feature	PDCAAS	DIAAS
Digestibility measurement	Fecal	Ileal (more accurate)
Max score cap	Truncated at 1.0	Not truncated
Amino acid evaluation	Based on lowest limiting amino acid	Based on individual digestible amino acids
Recognized by	FAO/WHO (1991)	FAO (2013)
Best use	General nutrition labeling	Advanced scientific analysis

In practice, DIAAS often rates animal-based proteins (like whey and milk) above 100%, showing their superior bioavailability, while plant proteins remain slightly lower but can improve when combined⁴.

Protein Digestibility and Bioavailability

Protein digestibility refers to how well the body breaks down protein into absorbable amino acids, while bioavailability describes how effectively those amino acids are used.

Factors Affecting Protein Digestibility:

• Processing: Overheating or overprocessing can reduce amino acid availability.
  
• Fiber and antinutrients: Plant-based sources may contain phytates or tannins that lower digestibility.
  
• Protein structure: Some proteins naturally resist enzymatic breakdown (e.g., gluten).
  
• Fermentation and sprouting: Can increase digestibility and amino acid availability in plant foods.
  

Why PDCAAS and Digestibility Matter for Strength Training

For anyone focused on muscle growth, repair, or recovery, protein quality is just as important as quantity⁵. Two people might eat the same total grams of protein, but if one consumes higher PDCAAS proteins, they’ll typically experience better outcomes in:

• Muscle protein synthesis (MPS)
  
• Recovery speed
  
• Lean mass retention during cutting phases
  
• Training performance
  

That’s why combining high-PDCAAS proteins (like whey, eggs, or soy) with complementary sources ensures your body receives all essential amino acids in optimal ratios. Quantity still matters, so if you need to calculate your intake target first, read our guide on how much protein you actually need for muscle growth.

Practical Takeaways

• Prioritize high-quality proteins such as whey, eggs, casein, or soy.
  
• Don’t rely solely on total grams, 150 g of low-quality protein is not equal to 150 g of complete, highly digestible protein.
  
• Use slow- and fast-digesting proteins strategically: whey post-workout, casein before bed. For a practical comparison, read Whey vs. Isolate vs. Casein.
  

Final Word – Quality Multiplies Results

Building strength isn’t just about eating “more protein”, it’s about eating better protein.
Understanding PDCAAS and digestibility helps you get the most from every gram you consume, turning your nutrition into a performance advantage.

For clean, high-PDCAAS protein formulas built on science, explore Ethica Nutrition, where quality and digestibility meet real results. If you need a structured buying framework, read how to choose a clean science-backed protein supplement, or see Ethica Isolate 100 Whey Isolate Protein.

1. FAO/WHO. (1991). Protein quality evaluation: Report of the Joint FAO/WHO Expert Consultation. Rome: Food and Agriculture Organization. ↩︎
2. Schaafsma, G. (2000). The Protein Digestibility-Corrected Amino Acid Score. J Nutr, 130(7):1865S–1867S. ↩︎
3. FAO. (2013). Dietary protein quality evaluation in human nutrition: Report of an FAO Expert Consultation. Rome: Food and Agriculture Organization.
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4. Gorissen, S.H.M., & Witard, O.C. (2018). Characterising the muscle anabolic potential of dairy, meat, and plant-based protein sources. Nutr Rev, 76(8):605–618.
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5. Phillips, S.M., & Van Loon, L.J.C. (2011). Dietary protein for athletes: from requirements to optimum adaptation. J Sports Sci, 29(S1):S29–S38.
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