Protein Quality Explained: PDCAAS, DIAAS and Why Collagen Scores Zero
Two foods can both say "20 g protein" and still be very different. Protein quality is about how much of that protein your body can actually use to build and repair tissue.
What makes protein "high quality"
Proteins are built from amino acids. Nine of them are essential: your body can't make them, so food must supply them. A high-quality protein contains all nine in useful amounts and is easy to digest. If one essential amino acid is missing or low, it limits how much of the rest your body can use, like a recipe that stops when you run out of one ingredient.
PDCAAS: the established score
The protein digestibility-corrected amino acid score compares a protein's essential amino acids with human needs, then adjusts for how well it's digested. Scores run from 0 to 1, and anything above 1 is rounded down to 1. Typical values:
- 1.0: whey, casein and milk protein, egg
- About 0.9: soy protein isolate, beef
- About 0.8: pea protein
- About 0.4 to 0.6: rice protein, wheat, oats, peanuts
- 0: collagen and gelatin
Values vary between studies and with processing, so treat them as typical figures rather than exact constants.
DIAAS: the newer method
In 2013, an FAO expert group recommended a newer score, DIAAS (digestible indispensable amino acid score). It measures digestibility of each amino acid at the end of the small intestine, which is more precise, and it isn't capped at 1, so it can show that some proteins are better than others at the top end. The catch is that DIAAS values exist for far fewer foods, so PDCAAS remains the practical choice for everyday comparisons.
Why collagen scores zero
Collagen has no tryptophan, one of the nine essential amino acids. Because the score is limited by the scarcest essential amino acid, collagen scores 0 on its own. That doesn't make collagen useless, but it does mean a collagen bar with "18 g protein" contributes very little complete protein toward your daily needs.
Plant proteins and blends
Many plant proteins are low in one amino acid: grains tend to be low in lysine, legumes lower in methionine. Combine them, as in pea and rice blends or rice and beans, and they cover each other's gaps. Across a varied diet over a day, this mostly takes care of itself.
Check a product
Use the protein quality calculator with the calories, protein and main protein source from any label. It shows usable protein per 100 calories and, if you add the price, what 20 g of usable protein actually costs.
