The Complete Guide to Protein Quality Scores: PDCAAS and DIAAS

Why Protein Quality Scoring Exists

When you read that chicken breast contains 31 grams of protein per 100 grams and black beans contain 9 grams per 100 grams cooked, you have only part of the picture. What matters nutritionally is not just how much protein is present, but how well your body can use it — which depends on two factors: the amino acid composition and how completely the protein is digested and absorbed.

Protein quality scores attempt to capture both factors in a single number, making it possible to compare protein sources meaningfully.

Essential Amino Acids: The Foundation

Your body needs 20 amino acids to build proteins. Of these, 9 are "essential," meaning your body cannot synthesize them and must obtain them from food: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

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A protein source that contains all nine essential amino acids in proportions that meet human requirements is called a "complete" protein. Animal proteins (meat, poultry, fish, eggs, dairy) are uniformly complete. Most plant proteins are technically complete but deficient or low in one or more essential amino acids — lysine is the most common limiting amino acid in grains; methionine is limiting in legumes.

The practical consequence: if you rely on a single plant protein source, you may be undersupplying one or more essential amino acids. Combining complementary plant sources (grains with legumes, for example) addresses this.

PDCAAS: The Previous Standard

The Protein Digestibility-Corrected Amino Acid Score (PDCAAS) was developed by the FAO and WHO in 1991 and became the standard for evaluating protein quality in food regulations, product labeling, and nutrition research for two decades.

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How PDCAAS is calculated:

1. Determine the amino acid composition of the protein source.
2. Compare each essential amino acid to the amino acid requirements of a reference population (originally 2-5 year old children, since they have the highest requirements relative to body size).
3. Identify the most limiting amino acid — the one that falls furthest below the reference pattern. This ratio is the "amino acid score."
4. Multiply the amino acid score by the protein's true digestibility (typically measured in rats).
5. The result is the PDCAAS, capped at 1.0.

PDCAAS values for common protein sources:

The score is truncated at 1.0, meaning proteins with amino acid scores above 1.0 are simply reported as 1.0 — which obscures real differences between high-quality proteins.

DIAAS: The Current Standard

The FAO published a revised protein quality evaluation method in 2011, the Digestible Indispensable Amino Acid Score (DIAAS), which addresses several limitations of PDCAAS:

Key differences from PDCAAS:

1. Ileal digestibility instead of fecal digestibility: PDCAAS uses fecal digestibility, which overestimates how much protein the body actually absorbs because it cannot distinguish between unabsorbed dietary protein and protein from gut bacteria and shed intestinal cells. DIAAS measures digestibility at the end of the small intestine (ileal), which is more accurate.

2. Individual amino acid digestibility: Rather than applying a single digestibility value to the whole protein, DIAAS measures how digestible each essential amino acid is individually — because the digestibility of different amino acids within the same food can vary.

3. No truncation at 1.0: DIAAS can exceed 1.0, allowing meaningful differentiation between high-quality proteins rather than lumping them all at the ceiling.

4. Multiple reference patterns: Separate reference patterns for different age groups (infant, young child, older child, adolescent, adult) rather than one universal standard.

DIAAS values for common protein sources:

What These Scores Mean in Practice

For most people eating a varied diet with multiple protein sources, these scores matter less than they do for athletes or anyone relying on a limited number of protein sources.

Animal proteins uniformly score well on both systems because they contain all essential amino acids in favorable proportions and are highly digestible. Eggs, dairy, meat, and fish all provide excellent protein quality.

Plant proteins vary considerably. Soy is the highest-quality plant protein and scores near animal proteins on DIAAS. Pea protein scores moderately. Rice protein is low in lysine and scores poorly. Wheat gluten is severely deficient in lysine.

For plant-based eaters, these scores support the practice of combining protein sources. Rice and pea protein together (commonly combined in plant-based protein blends) complement each other's amino acid limitations, and their combined DIAAS is meaningfully higher than either alone.

For supplement selection, DIAAS provides a more accurate framework than marketing language. A "high-protein" plant supplement with a low DIAAS score may deliver fewer usable amino acids per gram than a lower-total-protein animal source with a high DIAAS.

Practical Takeaways

• Complete protein at adequate total intake is the key concern, not obsessing over individual scores for most healthy adults.
• Animal proteins reliably provide complete, highly digestible protein; this is nutritional fact, not bias.
• For plant-based eaters, combining legumes with grains or seeds across the day effectively compensates for individual amino acid limitations.
• When choosing plant protein supplements, pea protein or soy protein isolate outperform rice protein or wheat protein on quality metrics.
• DIAAS is the more accurate current standard; PDCAAS still appears on older literature and some product labeling.