Written by the Spacemilk team · Last updated October 2026

PDCAAS (Protein Digestibility Corrected Amino Acid Score) rates how well a protein covers your essential amino acid needs, adjusted for how well it is digested. It is calculated as the amino acid score multiplied by digestibility, and any result above 1.0 is cut to 1.0, so 1.0 is the highest possible score. Whey, casein, egg and yeast protein score 1.0; most plant proteins score lower.

What is PDCAAS?

PDCAAS is the protein quality score adopted by FAO/WHO after a 1989 expert consultation, and it is the method US food labels use today. It answers one question: if this were your only protein, how well would it cover your needs for the nine essential amino acids, the ones your body can't make?

The score has two parts (Schaafsma, 2000, The Journal of Nutrition):

• Amino acid score. The protein's essential amino acids are compared with a reference pattern based on the needs of a preschool-age child. The amino acid in shortest supply, called the limiting amino acid, sets the score.
• Digestibility. That score is then multiplied by the protein's true fecal digestibility, traditionally measured in rats.

How is PDCAAS calculated? A worked example

Take beef, using the published figures in Schaafsma's 2000 review.

1. Find the limiting amino acid. Compare each essential amino acid in beef protein with the reference pattern. The one that comes up shortest gives beef an amino acid score of 0.94. In other words, beef supplies 94% of the reference amount of its least plentiful essential amino acid.
2. Correct for digestibility. Beef protein is about 98% digestible.
3. Multiply. 0.94 × 0.98 = 0.92. Beef's PDCAAS is 0.92.

Wheat shows how a low score happens. Wheat protein is low in lysine, which gives it an amino acid score of 0.47. Even at 91% digestibility, its PDCAAS lands at 0.42 (Schaafsma, 2000). One short amino acid holds the whole score down.

Why is PDCAAS capped at 1.0?

Some proteins calculate above 1.0. In the same review, egg works out to 1.18 (1.21 × 0.98) and cow's milk to 1.21 (1.27 × 0.95). The method truncates these to 1.0, on the reasoning that essential amino acids beyond the reference pattern give no extra benefit when that protein is your only source (Schaafsma, 2000).

US labeling rules say the same thing: "If the corrected score is above 1.00, then it shall be set at 1.00" (21 CFR 101.9(c)(7)(ii)).

The cap has a side effect. Every protein that clears the bar looks identical at 1.0, so the score can't tell you which of them is richer in essential amino acids. Schaafsma argued that in mixed diets, truncated scores hide how well a high-quality protein can make up for a weaker one.

PDCAAS of common proteins

Protein	PDCAAS	Source
Whey protein isolate	1.00	Rutherfurd et al., 2015
Casein	1.00	Hoffman & Falvo, 2004
Egg	1.00	Schaafsma, 2000 (1.18 before the cap)
Yeast protein (Spacemilk)	1.00	Spacemilk protein quality testing; Cao et al., 2025
Soy protein isolate	0.98–1.00	Rutherfurd et al., 2015 (two isolates)
Beef	0.92	Schaafsma, 2000
Pea protein concentrate	0.89	Rutherfurd et al., 2015
Rice protein concentrate	0.42	Rutherfurd et al., 2015
Wheat gluten	0.25	Hoffman & Falvo, 2004

Scores vary with the product, processing and lab. Treat these as typical published values. Rutherfurd et al. measured 14 protein sources in growing rats; Hoffman & Falvo compiled values from earlier reference data.

A separate rat study comparing six proteins measured yeast protein isolate at a PDCAAS of 100% for people over 3 years old, ahead of soy, wheat and pea (Cao et al., 2025, Food Chemistry). For side-by-side detail, see Yeast protein vs whey and Yeast protein vs pea protein.

What does "complete protein" mean?

The nine essential amino acids are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine (MedlinePlus). A complete protein supplies all nine in the proportions your body needs. A PDCAAS of 1.0 is the usual proof: it means no essential amino acid falls short of the reference pattern once digestibility is counted.

A lower score doesn't mean a protein is missing an amino acid. Wheat, rice and pea contain all nine. One just runs low relative to the others. That's why pairing proteins works, and you don't need to do it at every meal. MedlinePlus notes that getting a balance over the whole day is what matters.

PDCAAS vs DIAAS: what's the difference?

In 2013, an FAO expert consultation recommended a newer score, DIAAS (Digestible Indispensable Amino Acid Score), to replace PDCAAS (FAO, 2013, Food and Nutrition Paper 92). The main changes:

	PDCAAS	DIAAS
Where digestibility is measured	Whole gut (fecal)	End of the small intestine (ileal)
Digestibility applied to	The protein as a whole	Each essential amino acid separately
Top of the scale	Capped at 1.0	Not capped for single foods
Preferred test subjects	Rats	Humans, then growing pigs, then rats
Used on US labels	Yes	No

Why DIAAS is considered more precise

• Amino acids that reach the colon are mostly lost to your body's protein building. The FAO report concluded that, on balance, measuring at the end of the small intestine better reflects what is actually absorbed.
• Each amino acid can be digested differently, so scoring them one by one is more accurate.
• Without a cap, DIAAS can rank high-quality proteins against each other and show how much one can make up for a weaker protein in a meal.

The two scores can disagree. In Rutherfurd's 14-protein study, PDCAAS scored higher than DIAAS for 12 of the sources, by 59% on average, and lower for milk protein concentrate and whey isolate (Rutherfurd et al., 2015).

Why DIAAS hasn't replaced PDCAAS yet

• Ileal digestibility is hard to measure in people. The FAO report noted only a limited amount of human data, so most values come from pigs or rats.
• Many foods don't have DIAAS values yet, and the FAO said its suggested claim cut-offs (such as 100 or more for "excellent/high" and 75 to 99 for "good") needed further consideration.
• US labeling rules still specify PDCAAS.

We use PDCAAS for Spacemilk because it is the score US labeling rules specify and the one our protein quality testing measures.

How does the FDA use PDCAAS on nutrition labels?

Under 21 CFR 101.9(c)(7), the grams of protein on a US label are the total protein in a serving. The percent Daily Value is different. It is based on a "corrected" amount: grams of protein multiplied by the PDCAAS, capped at 1.0. For adults and children 4 and older, that corrected amount is divided by a Daily Value of 50g (21 CFR 101.9).

Example serving	Protein	PDCAAS	Corrected protein	%DV
Protein scoring 1.0	20g	1.0	20g	40%
Protein scoring 0.5	20g	0.5	10g	20%

A %DV for protein is required when a label makes a protein claim, or for foods meant for children under 4. Otherwise it is optional. That's why two products with the same grams of protein can show different %DVs, and why some labels show no protein %DV at all. Spacemilk's label lists 20g of protein at 40% DV, which reflects its PDCAAS of 1.0.

What PDCAAS doesn't tell you

The score measures quality per gram. It's a useful filter, and it leaves out several things that matter.

• Dose. A perfect score on a small serving is still a small serving. The International Society of Sports Nutrition suggests 20 to 40g of high-quality protein per dose for active people, spread every 3 to 4 hours (Jäger et al., 2017, JISSN).
• Leucine. PDCAAS only looks at the amino acid in shortest supply. It says nothing about leucine, the amino acid most tied to muscle protein synthesis. The same ISSN position stand suggests 700 to 3,000mg of leucine per dose.
• Your total diet. You rarely eat one protein alone. A lower-scoring protein can be perfectly fine as part of a varied diet.
• The method's own limits. Its reference pattern comes from children's needs, and fecal digestibility can overstate how much protein is absorbed (Schaafsma, 2000).

How much you need overall matters too. Our guide to how much protein per day covers that.

Where Spacemilk fits

Spacemilk is protein isolated from non-GMO baker's yeast, grown through fermentation. Each serving of Spacemilk Complete Protein gives you 20g of complete protein with a PDCAAS of 1.0, the same protein quality score as whey and egg. It has about 1.8g of leucine per 20g of protein, 0g sugar, and it's free from all 9 major allergens. Read more in What is yeast protein?

FAQ

What is a good PDCAAS score?

1.0 is the highest possible score and means the protein fully meets the reference pattern. Scores in the 0.9s are close. Scores below about 0.5, like rice protein or wheat gluten, mean one essential amino acid is well short and is best balanced by other proteins in your diet.

Can a protein score higher than 1.0?

Its calculation can, but the reported PDCAAS can't. Egg calculates to about 1.18, for example, and is reported as 1.0.

Is PDCAAS or DIAAS better?

DIAAS is the more detailed method and is what the FAO recommends. PDCAAS has far more published data and is the score US labels use. Both measure quality per gram. Neither tells you how much protein you're eating.

Does a PDCAAS of 1.0 mean a protein builds more muscle?

Not on its own. Total protein, dose per meal, leucine and resistance training all matter. Protein helps build and repair muscle; the score tells you about the quality of each gram.

Sources

• Schaafsma G. The protein digestibility-corrected amino acid score. J Nutr. 2000;130(7):1865S–1867S. doi:10.1093/jn/130.7.1865S
• Rutherfurd SM, et al. Protein digestibility-corrected amino acid scores and digestible indispensable amino acid scores differentially describe protein quality in growing male rats. J Nutr. 2015;145(2):372–379. doi:10.3945/jn.114.195438
• Hoffman JR, Falvo MJ. Protein: which is best? J Sports Sci Med. 2004;3(3):118–130. PMC3905294
• Cao X, et al. Evaluation of the nutritional quality of yeast protein in comparison to animal and plant proteins using growing rats and INFOGEST model. Food Chem. 2025;463:141178. doi:10.1016/j.foodchem.2024.141178
• FAO. Dietary protein quality evaluation in human nutrition: report of an FAO Expert Consultation. FAO Food and Nutrition Paper 92. Rome; 2013. PDF
• Jäger R, et al. International Society of Sports Nutrition position stand: protein and exercise. J Int Soc Sports Nutr. 2017;14:20. doi:10.1186/s12970-017-0177-8
• 21 CFR 101.9(c)(7), Nutrition labeling of food: protein. eCFR
• MedlinePlus. Amino acids. medlineplus.gov