Protein Quality and Bioavailability: From DIAAS to Practice

What “Protein Quality” Means

The term “protein quality” refers to a protein’s ability to meet human amino acid requirements once digestion and absorption are accounted for. Two proteins providing the same number of grams can differ substantially in the quantity of essential amino acids that actually appear in the systemic circulation in usable form. Quality reflects three factors: amino acid composition (especially essential amino acids and the indispensable amino acids most likely to be limiting), digestibility (the fraction of ingested protein that crosses the intestinal wall), and the presence of anti-nutritional factors that interfere with digestion.

PDCAAS and Its Limitations

The Protein Digestibility-Corrected Amino Acid Score (PDCAAS) was adopted by the FAO and WHO in 1991 as the standard quality metric. PDCAAS expresses protein quality as the lowest-scoring essential amino acid relative to a reference pattern, multiplied by overall protein digestibility. By design, PDCAAS scores are truncated at 1.0 — meaning that any protein meeting or exceeding the reference for all essential amino acids receives a score of 1.0, regardless of how much it exceeds.

This truncation produced a known artifact: high-quality animal proteins (whey, casein, egg, beef) all scored 1.0 even though their actual amino acid surplus differed substantially. The truncation also obscured meaningful quality differences between high-scoring and lower-scoring plant proteins.

DIAAS: The Updated Framework

In 2013, the FAO recommended replacing PDCAAS with the Digestible Indispensable Amino Acid Score (DIAAS). DIAAS differs from PDCAAS in three important ways:

1. No truncation. DIAAS scores can exceed 1.0, allowing meaningful differentiation among high-quality proteins.
2. Ileal rather than fecal digestibility. DIAAS measures amino acid absorption at the end of the small intestine, which more accurately reflects what reaches the systemic circulation; fecal digestibility (used in PDCAAS) overestimates true absorption because of microbial activity in the colon.
3. Per-amino-acid digestibility. DIAAS uses the digestibility of each individual amino acid rather than overall protein digestibility, capturing the fact that different amino acids in the same protein may be absorbed at different rates.

Approximate DIAAS scores for common protein sources:

• Milk protein concentrate: ~1.18
• Whey protein isolate: ~1.09
• Egg (whole): ~1.13
• Beef: ~1.10
• Soy protein isolate: ~0.90
• Pea protein isolate: ~0.82
• Wheat protein: ~0.40

The numbers communicate two things: animal proteins generally exceed reference patterns substantially (DIAAS > 1.0), and plant proteins vary widely, with soy and some legumes approaching but not matching animal sources, while cereals score notably lower due to lysine limitation.

The Leucine Question

Within the broader quality discussion, leucine deserves separate attention. Leucine is the principal driver of acute mTORC1 activation and the per-meal MPS response. Animal proteins generally contain 8 to 11 percent leucine by mass; plant proteins typically 6 to 8 percent. This difference, combined with lower overall digestibility, means that a per-meal serving of plant protein generally clears the leucine threshold (~2.5 to 3 g per meal) at a higher total protein dose than an animal-protein serving.

Per the framework reviewed in Helms et al. (2023), this translates into a practical recommendation: per-meal doses of 0.4 to 0.55 g/kg for animal protein, with somewhat higher doses (perhaps 0.5 to 0.65 g/kg) recommended for plant protein meals to ensure the leucine threshold is met.

Practical Implications for Athletes

For most lifters consuming a mixed diet with adequate total protein in the range identified by Morton et al. (2018) — 1.6 to 2.2 g/kg/day — protein quality concerns are largely automatic. A typical mixed diet built around dairy, eggs, meat, fish, and complementary plant sources easily meets the essential amino acid requirements, and the leucine threshold is met by every reasonable per-meal dose.

Quality concerns become more relevant in three contexts:

Predominantly plant-based diets. Lifters relying primarily on plant proteins should diversify across legumes, grains, soy, nuts, and seeds across the day to ensure complete amino acid coverage. Single-source plant protein meals (e.g., a meal built around rice or wheat alone) may underdeliver lysine. Soy protein is the closest plant analog to animal protein in DIAAS terms and is a useful primary source for plant-based athletes.

Older adults. Anabolic resistance shifts the per-meal dose-response curve to the right, and plant-protein meals — already requiring higher total dose to meet the leucine threshold — are particularly affected. Older adults building lean mass should generally favor higher-quality protein sources or use larger per-meal doses of plant protein.

Caloric restriction. When total intake is reduced, the marginal cost of low-quality protein (in terms of unmet amino acid requirements) is amplified. Quality matters more in deficit than in maintenance.

Bioavailability Beyond DIAAS

Several factors not fully captured by DIAAS scoring affect real-world bioavailability:

• Food matrix effects. Whole-food proteins are absorbed more slowly than isolated protein powders, which can be advantageous for sustained amino acid availability or disadvantageous for hitting a rapid post-training MPS response.
• Cooking and processing. Excessive heat treatment can reduce lysine availability through Maillard reactions, particularly in protein-and-sugar matrices.
• Anti-nutritional factors. Phytates, tannins, and protease inhibitors in some plant sources reduce digestibility; traditional preparation methods (soaking, fermentation, cooking) typically mitigate these effects.

Synthesis

Protein quality, captured in the modern DIAAS framework, refines but does not overturn the basic recommendations on total protein intake. For mixed-diet omnivores meeting the 1.6 to 2.2 g/kg daily target, quality concerns are usually moot. For plant-based lifters, older adults, and athletes in caloric restriction, attention to source diversity, per-meal dose, and the leucine threshold becomes more important. The DIAAS framework provides a useful tool for evaluating individual protein sources, with values above 1.0 indicating animal-quality protein and values around 0.8 to 0.9 indicating high-quality plant protein.