How Protein Digestibility Scores Get Calculated

Not all dietary protein performs identically in the body. Two foods can share the same gram-for-gram protein content on a nutrition facts panel yet differ substantially in how much of that protein the human digestive tract can actually break down, absorb, and use. Protein digestibility scoring systems exist to quantify that difference in a single number, giving researchers, food technologists, and regulators a common language for comparing protein sources.

Two scoring systems dominate the field: the Protein Digestibility-Corrected Amino Acid Score (PDCAAS), which the Food and Agriculture Organization adopted in 1991 and which still appears on many labels, and the Digestible Indispensable Amino Acid Score (DIAAS), recommended by the FAO in 2013 as a more precise replacement. Both systems ask the same fundamental question — how well does a protein source supply the indispensable amino acids the human body cannot synthesize on its own — but they answer it through different experimental methods, and the differences matter.

How PDCAAS and DIAAS Are Actually Calculated, Step by Step

Both scoring systems begin with the amino acid profile of the protein being evaluated. A laboratory hydrolyzes a sample of the protein into its constituent amino acids and measures the concentration of each one. The result is compared against a reference pattern — a table of how much of each indispensable amino acid a human at a given life stage requires per gram of protein consumed. The FAO publishes these reference patterns, and the chosen reference population (infant, child, adult) affects the final score.

The ratio of the most limiting indispensable amino acid in the test protein to the same amino acid in the reference pattern is called the amino acid score. A protein that contains at least as much of every indispensable amino acid as the reference pattern scores 1.0 or higher before digestibility correction. Understanding which amino acids are "indispensable" in the first place is central to what a complete protein actually means biochemically — the body lacks the enzymatic pathways to synthesize them from scratch.

The critical difference between PDCAAS and DIAAS lies in how digestibility is measured. PDCAAS uses fecal digestibility: the fraction of protein nitrogen that disappears between intake and the end of the large intestine, measured in rats. This method overestimates true protein availability because it counts nitrogen absorbed anywhere along the gut, including microbial nitrogen in the colon that is never incorporated into human tissue. The PDCAAS value for a protein is then calculated by multiplying the amino acid score by the fecal digestibility coefficient, and any score above 1.0 is truncated to 1.0 — a ceiling that obscures real differences among high-quality proteins.

DIAAS replaces fecal digestibility with true ileal digestibility: the fraction of each individual amino acid absorbed specifically at the end of the small intestine (the ileum), before colonic bacteria can alter the nitrogen pool. This is measured either in pigs (whose digestive tract is physiologically similar to a human's) or, increasingly, in human ileostomy studies. Because DIAAS measures digestibility amino acid by amino acid rather than as a bulk nitrogen figure, it can detect that a protein may be highly digestible for most amino acids yet poorly digestible for one critical limiting amino acid. DIAAS also does not truncate scores at 1.0, so a protein that genuinely exceeds the reference pattern in every indispensable amino acid can score above 1.0.

The final DIAAS value is: (mg of digestible dietary indispensable amino acid in 1 g of the test protein ÷ mg of the same dietary indispensable amino acid in 1 g of the reference protein) × 100, taken at the most limiting amino acid. The most limiting amino acid — the one with the lowest ratio — sets the ceiling for the whole protein's score, reflecting the biochemical reality that protein synthesis stalls when any single indispensable amino acid is unavailable.

The Compounds, Organs, and Regulatory Bodies Involved in Scoring

Indispensable amino acids. Nine amino acids — histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine — cannot be synthesized by human metabolism in sufficient quantities. They are the only amino acids that appear in the FAO reference patterns, because they are the only ones that can become rate-limiting for protein synthesis. Conditionally indispensable amino acids (such as arginine or glutamine under physiological stress) are not factored into PDCAAS or DIAAS calculations.

The small intestine and ileum. The small intestine is where pancreatic proteases (trypsin, chymotrypsin, elastase) and brush-border peptidases complete the breakdown of dietary protein into free amino acids and small peptides. Absorption occurs primarily through enterocytes lining the jejunum and ileum. DIAAS specifically measures what has been absorbed by the end of the ileum, capturing the digestive tract's actual contribution before colonic interference.

The large intestine and gut microbiota. Unabsorbed protein nitrogen entering the colon is fermented by resident bacteria. Some bacterial amino acids are absorbed as ammonia and excreted as urea, not used for tissue protein synthesis. PDCAAS's fecal method cannot distinguish this bacterial nitrogen from genuinely absorbed dietary nitrogen, which is why it systematically overestimates digestibility for some protein sources.

The Food and Agriculture Organization / World Health Organization (FAO/WHO). These United Nations bodies publish the reference amino acid scoring patterns and the methodological standards for both PDCAAS and DIAAS. The FAO's 2013 expert consultation report formally recommended transitioning to DIAAS, though PDCAAS remains embedded in many national labeling regulations, including those administered by the U.S. FDA.

The U.S. Food and Drug Administration. The FDA's nutrition labeling regulations for the U.S. market reference PDCAAS as the basis for the "% Daily Value" for protein on supplement and food labels. A product's protein quality claim on a label is tied to this regulatory framework, not to DIAAS, even though DIAAS is now considered scientifically superior. The FDA does not pre-approve dietary supplement formulas before sale; it sets labeling standards and takes enforcement action after the fact.

Animal models used in digestibility assays. Pigs are the preferred animal model for DIAAS ileal digestibility measurements because porcine digestive physiology — stomach acidity, pancreatic enzyme output, intestinal transit time — closely parallels human digestion. Rat-based measurements, standard under PDCAAS, produce digestibility coefficients that can differ meaningfully from human values for certain plant proteins.

Where Digestibility Scores Break Down or Produce Unexpected Results

The truncation problem in PDCAAS. Because PDCAAS caps scores at 1.0, a high-quality animal protein with a score of 1.24 and a moderately processed plant protein with a score of 1.02 appear identical on a label. This truncation was a deliberate policy choice — the FAO reasoned that a protein meeting 100% of requirements needs no further distinction — but it prevents any meaningful comparison among proteins that all clear the bar. DIAAS eliminates this ceiling, which is part of why it reveals wider gaps between protein sources than PDCAAS does.

Processing effects on amino acid availability. Heat, alkaline treatment, and oxidation during manufacturing can alter amino acid digestibility in ways that a raw amino acid profile does not capture. The Maillard reaction between lysine and reducing sugars during high-heat processing, for example, produces bound lysine that resists proteolytic digestion. A protein powder's DIAAS measured before processing may not reflect the DIAAS of the finished, heat-treated product. How whey protein gets processed from milk illustrates how filtration, drying temperature, and pH adjustments each alter the protein matrix in ways that can affect digestibility measurements.

Antinutritional factors in plant proteins. Many plant-based protein sources contain trypsin inhibitors, phytates, lectins, and tannins that reduce proteolytic enzyme activity or bind amino acids in forms that resist absorption. These factors partially explain why soy protein isolate scores higher than whole soybean flour under DIAAS — processing removes or denatures many antinutritional factors — and why DIAAS scores for unprocessed legumes tend to fall well below those for refined animal proteins.

Reference pattern selection changes outcomes. DIAAS scores are not fixed values; they shift depending on which FAO reference pattern is applied. A protein scored against the infant reference pattern (which demands higher concentrations of indispensable amino acids per gram) will score lower than the same protein scored against the adult reference pattern. A product label citing a DIAAS value without specifying the reference population is providing incomplete information.

Complementary proteins and meal-level scoring. PDCAAS and DIAAS are typically calculated for single protein sources in isolation. When two plant proteins with different limiting amino acids are consumed together — rice (limited by lysine) and legumes (limited by methionine) — the combined amino acid profile can approach or exceed the reference pattern even though neither source alone would. Neither scoring system, as standardized, automatically accounts for this complementarity at the meal level, which means single-ingredient DIAAS values can understate the protein quality of mixed diets.

What a Protein Label Actually Shows — and What It Does Not

The protein content listed in grams on a U.S. Nutrition Facts or Supplement Facts panel is derived from total nitrogen measurement (typically the Kjeldahl or Dumas method) multiplied by a nitrogen-to-protein conversion factor. This figure reflects crude protein — all nitrogen-containing compounds in the sample — not only intact, digestible protein. Creatine, free amino acids, and other nitrogen-containing compounds present in a supplement contribute to this number.

The "% Daily Value" for protein that may appear on a label is permitted only when the manufacturer has determined the protein's PDCAAS-corrected quality. If the PDCAAS-corrected value is below a threshold, the % DV for protein must be omitted or reduced. However, many protein supplement labels simply list grams of protein without a % DV, which means no quality-corrected figure is being communicated to the consumer at all.

Third-party certification seals (from organizations that test for label accuracy, contaminants, and banned substances) verify that the protein content stated on the label matches what is actually in the product. They do not verify the DIAAS or PDCAAS of the protein. A certified product with an accurate label may still use a protein source with a low digestibility score; the seal says nothing about amino acid quality or ileal digestibility. This is a parallel limitation to how standardization works in other supplement categories — for instance, how herbal extracts get standardized to a marker compound without that marker necessarily representing full biological activity.

No dietary supplement in the United States is FDA-approved before it reaches the market. The FDA's role under the Dietary Supplement Health and Education Act (DSHEA) is post-market: the agency can act against products that are adulterated, misbranded, or make illegal disease claims, but it does not evaluate or certify protein quality scores before a product is sold.

Protein digestibility scoring has moved from a single bulk-nitrogen correction toward amino-acid-by-amino-acid ileal measurement over three decades — a shift that makes the underlying biochemistry more visible but also makes the gap between what a label communicates and what the science can now measure more apparent.

Note: This explains how supplements work. This statement has not been evaluated by the Food and Drug Administration. This information is not intended to diagnose, treat, cure, or prevent any disease, and it is not a substitute for advice from a physician, pharmacist, or registered dietitian.