Not all protein is created equal. The FAO's official scoring systems - PDCAAS and DIAAS - quantify exactly how much of the protein you eat actually gets used by your body.
DIAAS is the FAO's recommended replacement for PDCAAS. It measures protein quality more accurately by scoring individual amino acids at the ileal level. Scores above 100 are "Excellent," 75–99 are "Good," and below 75 cannot carry a protein quality claim.
Protein quality refers to two factors: the amino acid profile (does the protein contain all essential amino acids in sufficient proportions?) and digestibility (how much of the protein is actually absorbed and available at the tissue level?). A protein source can be high in total grams but low in quality if it is missing key amino acids or if a large fraction passes through the gut undigested.
When nutrition labels list "25g protein per serving," they tell you how much protein is in the food - but nothing about how much of that protein your body can actually use.
This distinction matters because muscle protein synthesis requires all nine essential amino acids simultaneously. If even one is in short supply - the "limiting amino acid" - the rate of MPS is constrained regardless of how much total protein you consume. This is why 30g of whey protein and 30g of wheat protein produce very different anabolic responses: whey delivers all essential amino acids in abundance, while wheat is severely limited in lysine.
To quantify these differences, the Food and Agriculture Organization of the United Nations (FAO) has developed two scoring methods. PDCAAS (Protein Digestibility-Corrected Amino Acid Score) was adopted in 1991 and became the global standard used by regulatory agencies for protein quality claims on food labels. DIAAS (Digestible Indispensable Amino Acid Score) was recommended as the replacement method in 2013, addressing several key limitations of PDCAAS. Both systems attempt to answer the same question: for a given protein source, what percentage of its amino acids are both present and digestible?
Calculate your personalized daily protein target →PDCAAS multiplies two factors: the amino acid score (the ratio of the first limiting amino acid in the test protein compared to a reference pattern) and fecal true digestibility (the proportion of protein that does not appear in feces after consumption). The formula is straightforward:
PDCAAS = (mg of limiting amino acid per g of test protein ÷ mg of same amino acid per g of reference protein) × fecal true digestibility
The reference pattern is based on the amino acid requirements of preschool-age children (1–2 years), representing the most demanding life stage for protein quality. If a protein meets or exceeds the reference for every amino acid and is highly digestible, it receives a PDCAAS of 1.0 - the maximum possible score.
The most significant limitation of PDCAAS is its hard cap at 1.0. Any protein that exceeds the reference requirement for all amino acids receives the same score of 1.0, regardless of how far it exceeds it. This means whole egg, whey protein, and milk - which have very different amino acid profiles and digestibility characteristics - all receive identical PDCAAS scores of 1.0. The cap obscures meaningful differences between high-quality protein sources that matter for practical nutrition decisions, particularly for athletes and individuals optimizing for muscle growth.
The second limitation is the use of fecal digestibility. Measuring protein in feces reflects total-tract digestibility, but significant microbial fermentation of protein occurs in the large intestine. Amino acids metabolized by gut bacteria in the colon are not available for human protein synthesis, yet they disappear from fecal samples and are counted as "digested." This overestimates the true digestibility of proteins that are poorly absorbed in the small intestine but fermented in the colon - a particular issue for some plant-based protein sources.
| Food Source | PDCAAS Score | Limiting Amino Acid |
|---|---|---|
| Whole Egg | 1.00 | None (all exceed reference) |
| Cow's Milk | 1.00 | None |
| Whey Protein Isolate | 1.00 | None |
| Casein | 1.00 | None |
| Soy Protein Isolate | 1.00 | None (marginal methionine) |
| Chicken Breast | 1.00 | None |
| Beef | 0.92 | Methionine + Cysteine |
| Chickpeas | 0.78 | Methionine + Cysteine |
| Black Beans | 0.75 | Methionine + Cysteine |
| Peanuts | 0.52 | Lysine |
| Oats | 0.57 | Lysine |
| Rice (white) | 0.50 | Lysine |
| Wheat (whole) | 0.42 | Lysine |
Data from FAO/WHO (1991) and Schaafsma (2000). PDCAAS capped at 1.00.
In 2013, the FAO Expert Consultation on Protein Quality Evaluation recommended DIAAS as the replacement for PDCAAS, citing three critical improvements:
DIAAS measures amino acid absorption at the end of the small intestine rather than in feces. This eliminates overestimation caused by colonic bacterial fermentation and gives a truer picture of amino acids actually available for human metabolism.
DIAAS scores each essential amino acid individually rather than reporting a single composite score. The overall DIAAS is determined by the lowest-scoring amino acid, but all nine scores are available - providing a complete quality profile.
Proteins that exceed the reference pattern can score above 100, differentiating between high-quality proteins that PDCAAS lumps together at 1.0. Whey scores 109 on DIAAS versus a flat 1.0 on PDCAAS.
The DIAAS methodology calculates a digestibility score for each of the nine indispensable (essential) amino acids: histidine, isoleucine, leucine, lysine, methionine + cysteine (sulfur amino acids), phenylalanine + tyrosine (aromatic amino acids), threonine, tryptophan, and valine. For each amino acid, the score is:
DIAAS for amino acid X = (mg of digestible amino acid X per g of protein ÷ mg of amino acid X in the reference pattern) × 100
The overall DIAAS for the food is the lowest individual amino acid score. This means the quality of a protein source is only as strong as its weakest link. For wheat protein, the overall DIAAS of 40 is determined entirely by its lysine score - even though its scores for most other amino acids are above 80. This amino-acid-level resolution makes DIAAS far more useful for identifying exactly which amino acids need supplementation in a mixed diet.
| Food Source | DIAAS Score | Quality Tier | Limiting Amino Acid |
|---|---|---|---|
| Whole Milk | 143 | Excellent | None |
| Whole Egg | 113 | Excellent | None |
| Chicken Breast | 108 | Excellent | None |
| Whey Protein Isolate | 109 | Excellent | None |
| Beef | 112 | Excellent | None |
| Fish (cod) | 107 | Excellent | None |
| Casein | 117 | Excellent | None |
| Soy Protein Isolate | 90 | Good | Methionine + Cysteine |
| Chickpeas (cooked) | 83 | Good | Methionine + Cysteine |
| Pea Protein Isolate | 82 | Good | Methionine + Cysteine |
| Tofu | 78 | Good | Methionine + Cysteine |
| Peanuts | 43 | Low | Lysine |
| Kidney Beans (cooked) | 74 | Low | Methionine + Cysteine |
| Lentils (cooked) | 63 | Low | Methionine + Cysteine |
| Oats | 54 | Low | Lysine |
| Rice (white, cooked) | 59 | Low | Lysine |
| Corn | 42 | Low | Lysine |
| Wheat (whole) | 40 | Low | Lysine |
| Rice Protein Concentrate | 37 | Low | Lysine |
Data compiled from FAO (2013), Mathai et al. (2017), and Rutherfurd et al. (2015). Quality tiers: Excellent (≥100), Good (75–99), Low (<75).
The differences between the two scoring systems are most visible when comparing the same foods across both methods. Notice how PDCAAS compresses meaningful distinctions between high-quality proteins into an undifferentiated 1.0, and overestimates the quality of some plant sources due to colonic fermentation counting as "digestion."
| Food Source | PDCAAS | DIAAS | Key Difference |
|---|---|---|---|
| Whole Milk | 1.00 | 143 | DIAAS reveals milk as highest-quality protein |
| Whey Protein Isolate | 1.00 | 109 | PDCAAS cap hides 34-point gap vs milk |
| Whole Egg | 1.00 | 113 | Egg scores higher than whey on DIAAS |
| Soy Protein Isolate | 1.00 | 90 | PDCAAS overestimates; DIAAS shows sulfur AA limit |
| Chicken Breast | 1.00 | 108 | Similar - both systems rate highly |
| Beef | 0.92 | 112 | PDCAAS penalizes beef; DIAAS ranks it excellent |
| Chickpeas | 0.78 | 83 | Similar - both show sulfur AA limitation |
| Wheat (whole) | 0.42 | 40 | Similar - lysine deficiency dominates both |
| Rice (white) | 0.50 | 59 | DIAAS slightly higher - fecal method undervalued |
Sources: FAO (2013), Mathai et al. (2017), Schaafsma (2000).
The FAO explicitly recommended DIAAS as the preferred method for protein quality evaluation. Ileal digestibility is physiologically more accurate, individual amino acid scoring enables food complementation strategies, and the uncapped scale differentiates high-quality sources - essential for populations with limited protein diversity.
Despite this recommendation, PDCAAS remains the regulatory standard in many countries including the United States, where the FDA still requires PDCAAS for protein quality claims on food labels. This regulatory lag means you will encounter PDCAAS more frequently on product packaging, while DIAAS appears primarily in research literature and international nutrition guidance. For practical purposes, both systems agree on which proteins are high versus low quality - DIAAS simply provides finer resolution at the top end.
Consistently score in the Excellent tier (DIAAS ≥ 100). A 30g serving of chicken breast delivers more usable essential amino acids than 30g of protein from lentils, even though both contain identical total protein.
Most individual plant sources score in the Good or Low tiers. Not "bad" - just less amino acid-dense per gram. At 1.6+ g/kg/day from mixed sources, the quality differences are largely compensated by volume.
The practical impact scales with context. For someone with lower protein intake, restricted calories, or heavy reliance on a single plant source, quality scores become more relevant. This is especially true for older adults, where anabolic resistance makes the amino acid profile of each meal more important. If you are building a high-protein diet, understanding these scores helps you make more efficient choices.
Legumes are limited in sulfur amino acids but rich in lysine. Grains are limited in lysine but adequate in sulfur amino acids. Combining these two groups - rice and beans, bread with hummus, tortillas with black beans - produces a combined DIAAS of 95–100, approaching the Excellent tier. Research shows these complementary proteins do not need to be consumed in the same meal - daily amino acid intake is what matters.
For a comprehensive guide to optimizing plant-based protein sources, including leucine considerations and combining strategies, see our dedicated article.
If your diet is mixed (animal + plant sources) at 1.6+ g/kg/day, you do not need to worry about DIAAS at all. Quality scores matter most when intake is low, sources are limited, or you are in a caloric deficit where every gram counts.
If you rely primarily on high-DIAAS sources (meat, dairy, eggs, whey), a daily target of 1.6 g/kg bodyweight provides more than enough essential amino acids for muscle maintenance and growth. If your diet is predominantly plant-based, aiming for the higher end of the range (2.0–2.2 g/kg) compensates for lower average digestibility and amino acid completeness. This adjustment is supported by Phillips et al. (2016), who recommended that individuals consuming primarily plant proteins increase total intake by approximately 20% to match the anabolic equivalence of animal-based diets.
When choosing a protein supplement, DIAAS scores provide an objective comparison. Whey protein (DIAAS 109) and casein (DIAAS 117) lead the supplement category, while pea protein isolate (DIAAS 82) and soy protein isolate (DIAAS 90) are the strongest plant options. Rice protein concentrate alone scores only 37, but blended with pea protein reaches competitive territory. Beyond quality scores, these supplements also differ dramatically in how quickly each protein delivers amino acids to your muscles.
For athletes and fitness-focused individuals, supplement protein quality is often more relevant than whole food scores because supplements are consumed specifically for their protein content. Here is how the most common protein powders compare on both scoring systems:
| Supplement | PDCAAS | DIAAS | Limiting Amino Acid | Leucine per 25g Serving |
|---|---|---|---|---|
| Whey Protein Isolate | 1.00 | 109 | None | 2.7 g |
| Whey Protein Concentrate | 1.00 | 107 | None | 2.5 g |
| Micellar Casein | 1.00 | 117 | None | 2.1 g |
| Milk Protein Isolate | 1.00 | 120 | None | 2.3 g |
| Egg White Protein | 1.00 | 113 | None | 2.2 g |
| Soy Protein Isolate | 1.00 | 90 | Methionine + Cysteine | 2.0 g |
| Pea Protein Isolate | 0.89 | 82 | Methionine + Cysteine | 2.0 g |
| Rice Protein Concentrate | 0.47 | 37 | Lysine | 2.1 g |
| Rice + Pea Blend (50/50) | 0.93 | ~97 | None (complementary) | 2.0 g |
| Hemp Protein | 0.63 | ~50 | Lysine | 1.4 g |
DIAAS data from Mathai et al. (2017) and Rutherfurd et al. (2015). Leucine values from USDA FoodData Central. ~approximate where direct DIAAS measurement not available.
The leucine column is included because leucine is the primary amino acid trigger for muscle protein synthesis. Hitting the 2.5–3g leucine threshold per meal is a practical marker for MPS activation, regardless of DIAAS score - see our leucine content comparison table for 40+ foods ranked by leucine per serving. Notice that rice protein, despite its low DIAAS of 37, actually provides competitive leucine per serving - its problem is specifically a lysine deficiency, not a leucine one. This is why DIAAS and leucine content are complementary metrics: DIAAS tells you about overall amino acid completeness, while leucine content predicts the acute anabolic signal.
This is arguably the most important practical question, and the answer is nuanced. At higher total protein intakes (≥1.6 g/kg/day from mixed sources), the differences in quality between protein sources become less impactful because the sheer volume of amino acids consumed compensates for any individual-source deficiency. If you eat rice at lunch and beans at dinner, your daily lysine and methionine intakes are likely adequate even though neither meal was individually complete. A 2019 systematic review by Burd et al. in the Journal of Nutrition concluded that when total protein intake is sufficient, the source of protein has only a modest effect on lean mass outcomes.
However, quality becomes disproportionately important in three scenarios:
At only 0.8–1.0 g/kg/day (the RDA), every gram needs to count. Low-DIAAS sources at low intakes can result in functional essential amino acid deficiency.
A diet where 80% of protein comes from wheat or rice creates a genuine limiting amino acid problem that volume alone does not solve without extreme caloric surplus.
During a fat loss phase, choosing higher-DIAAS sources means more amino acid bang per caloric buck - critical for preserving muscle during a deficit.
A 2019 systematic review by Burd et al. concluded that when total protein intake is sufficient (≥1.6 g/kg/day from mixed sources), the source of protein has only a modest effect on lean mass outcomes. Focus on total intake first, quality second.
For most readers of this article - fitness-focused individuals eating 1.6+ g/kg/day from a mix of animal and plant sources - protein quality is a secondary optimization. Get your total daily intake right first, then use DIAAS as a tiebreaker for food and supplement selection. For beginners building their protein strategy, starting with high-DIAAS sources simplifies hitting your amino acid targets without overthinking combinations.
PDCAAS measures protein quality using fecal digestibility and caps scores at 1.0, while DIAAS uses ileal digestibility (more accurate) and scores individual amino acids without a cap. DIAAS can exceed 1.0 for high-quality proteins like whey (1.09) and distinguish between proteins that PDCAAS rates equally at 1.0.
The FAO classifies DIAAS scores into three tiers: Excellent (≥100) - whey, eggs, milk, chicken breast; Good (75–99) - soy protein isolate, chickpeas, pea protein; No quality claim allowed (<75) - wheat, rice, most grains alone. For muscle building, aim for protein sources scoring ≥75, ideally ≥100.
Yes, but less than most people think. Higher quality scores mean more efficient amino acid delivery per gram of protein. If you eat 2.0 g/kg/day from mixed sources, quality differences are largely compensated by volume. Quality matters most when protein intake is low, when relying on a single plant source, or when optimizing on a caloric budget.
Yes. Complementary plant proteins compensate for each other's limiting amino acids. Rice protein (low in lysine, high in methionine) combined with pea protein (high in lysine, low in methionine) produces a combined DIAAS comparable to animal proteins. The combination does not need to occur in the same meal - daily complementation is sufficient.
This guide is part of the Protein Guide Hub. Browse every related guide there, organized from beginner to advanced.