Protein Quality for Athletes: PDCAAS and DIAAS Scoring Systems Explained

Published: Nutrition Science & Protein Guide

Is Your Protein Actually Building Muscle?

You're eating 30g of protein from chicken breast versus 30g from wheat bread. Both show "30g protein" on the label. But here's the problem: your muscles don't care what the label says—they care about usable amino acids. That chicken delivers nearly twice the muscle-building protein as the bread, despite identical nutrition labels.

This is where protein quality scores come in. PDCAAS and DIAAS aren't just alphabet soup—they're scientific systems that rate how well your body can actually use different protein sources. Understanding these scores helps athletes choose proteins strategically, ensuring every gram you consume actually builds muscle instead of passing through unused.

Why Protein Quality Matters for Athletes

For athletes training hard and pushing their bodies, protein quality directly impacts recovery, muscle growth, and performance adaptation.

Impact on Training Performance

• Strength training: High-quality complete proteins provide all essential amino acids needed for muscle protein synthesis, leading to better recovery and faster strength gains
• Endurance training: Quality protein preserves lean mass during high-volume training and supports mitochondrial adaptations critical for endurance performance
• Recovery: Complete amino acid profiles from high-DIAAS proteins accelerate tissue repair and reduce recovery time between sessions

Research from the International Society of Sports Nutrition and Australian Institute of Sport demonstrates that athletes consuming primarily high-quality proteins (DIAAS >100) experience superior muscle protein synthesis responses compared to those relying on lower-quality sources, even when total protein intake is matched.

Two Critical Factors

Protein quality depends on two things:

Real-World Impact

Example: 30g protein from chicken vs 30g from wheat bread

• Chicken: ~27g actually absorbed, complete amino acid profile → ~27g usable protein
• Wheat bread: ~24g absorbed (lower digestibility), lysine-deficient → ~12-15g usable protein

The chicken provides nearly twice as much usable protein despite identical nutrition label values. This is why protein quality metrics exist.

What is PDCAAS?

The Old Standard (1989-2013)

PDCAAS = Protein Digestibility-Corrected Amino Acid Score

Developed by the FDA and WHO in 1989, PDCAAS was the gold standard for rating protein quality for over 20 years. It scores proteins on a 0-100 scale (capped at 100) based on:

1. Amino acid profile: Compares the protein's essential amino acids to a reference pattern (based on human requirements)
2. Limiting amino acid: Identifies which essential amino acid is lowest relative to needs
3. Fecal digestibility: Measures how much protein is absorbed (based on fecal nitrogen excretion)
4. Final score: Limiting amino acid score × digestibility (capped at 100)

PDCAAS Scores of Common Proteins

PDCAAS Limitations

While revolutionary for its time, PDCAAS has significant flaws:

• 100-point cap: Many proteins score 100, but some are actually better than others (no differentiation)
• Fecal digestibility: Measures protein reaching colon, not actually absorbed in small intestine
• Overestimates plant proteins: Some amino acids measured in feces were never absorbed
• Ignores individual amino acids: Only considers the lowest one, not the full profile
• No consideration of processing: Heat, preparation methods can affect digestibility

What is DIAAS?

The New Gold Standard (2013-Present)

DIAAS = Digestible Indispensable Amino Acid Score

Recommended by the FAO in 2013, DIAAS is a more accurate protein quality metric. Key improvements:

1. Ileal digestibility: Measures amino acids absorbed in the small intestine (where absorption actually happens), not feces
2. Individual amino acid tracking: Assesses digestibility of each essential amino acid separately
3. No 100-point cap: Scores can exceed 100, allowing differentiation between high-quality proteins
4. More accurate for plants: Better reflects true bioavailability of plant proteins
5. Age-specific scoring: Different reference patterns for infants, children, adults

DIAAS Score Categories

DIAAS Scores of Common Proteins

Key Differences: PDCAAS vs DIAAS

Practical Applications

For Omnivores

• Easy to hit quality targets: Most animal proteins score excellent on DIAAS
• Protein targets: 0.7-1.0g per lb bodyweight (1.6-2.2g per kg)
• Variety is fine: Mix animal proteins, eggs, dairy for complete nutrition
• Don't overthink it: If eating diverse animal proteins, quality is covered

For Vegetarians (Eggs/Dairy)

• Still excellent quality: Eggs, milk, whey, casein all score ≥100 DIAAS
• Protein targets: Same as omnivores (0.7-1.0g per lb bodyweight)
• Include some whole plants: Legumes, whole grains complement animal proteins
• No special concerns: Protein quality easily met with dairy/eggs

For Vegans (Plants Only)

• Higher protein targets recommended: 0.8-1.1g per lb bodyweight (10-20% more than omnivores)
• Prioritize high-DIAAS plants: Soy protein isolate (98), pea protein isolate (82)
• Combine complementary proteins: Rice + beans, wheat + legumes to balance amino acids
• Consider leucine: Plant proteins often lower in leucine; may need 30-40g protein per meal vs 25-30g
• Use variety: Diverse plant proteins throughout the day ensures all amino acids covered

Protein Complementation for Vegans

Combining proteins with different limiting amino acids creates a more complete amino acid profile:

Does Protein Quality Really Matter?

When It Matters Most

• Low total protein intake: Quality matters more when eating only 50-80g daily
• Vegan/vegetarian diets: Need to be more strategic about protein choices
• Cutting/calorie deficit: Every gram of usable protein counts for muscle retention
• Athletes with high demands: Maximize recovery and adaptation
• Elderly individuals: Reduced protein synthesis efficiency requires higher quality

When It Matters Less

• Already eating enough protein: 150g daily from diverse sources covers quality gaps
• Mixed omnivore diet: Variety naturally ensures all amino acids covered
• Maintenance/surplus calories: Less pressure on protein quality
• Young, healthy individuals: Better protein utilization efficiency

Common Questions About Protein Quality

Do I need to worry about PDCAAS vs DIAAS scores?

Focus on DIAAS when available—it's more accurate. For practical purposes, eat primarily animal proteins or high-quality plant proteins (soy isolate, pea isolate) and you'll hit excellent quality scores. The difference between DIAAS 108 (chicken) and 113 (eggs) is negligible in real-world muscle building.

How does protein quality affect my muscle gains?

Higher-quality proteins (DIAAS >100) provide complete amino acid profiles that maximize muscle protein synthesis. This is most critical post-workout and during calorie deficits. Athletes consuming excellent-quality proteins typically experience faster recovery, better strength progression, and superior muscle retention when cutting.

Can plant-based athletes build as much muscle as omnivores?

Yes, absolutely. Consume 10-20% more total protein (to account for lower average DIAAS scores), prioritize high-quality plant proteins (soy isolate, pea isolate), combine complementary proteins, and ensure adequate leucine per meal. Many elite plant-based athletes build exceptional muscle mass using these strategies.

How do I track protein quality in FitnessRec?

FitnessRec's nutrition database includes quality indicators for protein sources. The app identifies high-DIAAS proteins, suggests complementary protein pairings for plant-based diets, and adjusts protein targets based on your dietary preferences to account for quality differences automatically.

Quick Reference Guide

Excellent Quality Proteins (DIAAS ≥100)

• Eggs (whole, whites)
• Dairy (milk, Greek yogurt, cottage cheese)
• Whey and casein protein powders
• Chicken, turkey, other poultry
• Beef, pork, lamb
• Fish and seafood (salmon, tuna, shrimp, etc.)
• Game meats (bison, venison, elk)

Good Quality Proteins (DIAAS 75-99)

• Soy protein isolate
• Pea protein isolate
• Tempeh and tofu (whole soy)
• Hemp protein (with complementary source)
• Quinoa

Lower Quality Proteins (DIAAS <75) - Need Complementation

• Beans, lentils, chickpeas (alone)
• Rice, wheat, oats (alone)
• Nuts and seeds (almonds, peanuts, etc.)
• Most vegetables (broccoli, spinach, etc.)

Protein quality—measured by PDCAAS and DIAAS—determines how well your body can actually use the protein you eat. While animal proteins naturally score excellent, strategic plant protein choices and combinations can meet quality standards too. Use FitnessRec to track both quantity and quality of protein, ensuring you're not just hitting numbers but actually getting the usable protein your muscles need to grow and recover.