Plant protein digestibility and DIAAS

The label score on your protein powder is not the same number as how much amino acid your body actually absorbs. Most supplement labels use PDCAAS — a 1993 scoring method that caps every protein at 1.0 and measures whole-gut digestibility of crude protein. The FAO replaced it in 2013 with DIAAS, which tracks true ileal absorption of each indispensable amino acid individually. The new scores show a real hierarchy: soy protein qualifies as high quality; pea falls short of that threshold; wheat, rice, and hemp fall further still (Herreman et al., 2020).

The tl;dr

Under the FAO 2013 DIAAS framework (0.5–3 yr reference pattern):

Protein source	DIAAS	Limiting AA	Quality tier
Egg	~101	—	Excellent
Potato protein	100	None	Excellent
Soy protein	91	Met + Cys	High quality
Pea protein	70	Met + Cys	No claim
Hemp protein	~54	Lys	No claim
Wheat protein	48	Lys	No claim
Rice protein	47	Lys	No claim

Source: Herreman et al., 2020

FAO quality thresholds (FAO, 2013): 100+ = excellent; 75–99 = high quality; 50–74 = good source; below 50 = no protein quality claim.

Why PDCAAS and DIAAS diverge

PDCAAS measures the digestibility of crude protein in feces — a blunt proxy that mixes bacteria, mucus, and undigested food — then caps every result at 1.0. That ceiling hides real differences: a score of 0.99 and a score of 1.21 look identical on a label even though the second source delivers substantially more absorbable amino acids per gram.

DIAAS uses standardised ileal digestibility: the proportion of each specific indispensable amino acid absorbed before the large intestine, where gut bacteria confound measurement. It scores each amino acid against an age-group reference pattern, then reports the ratio for the most-limiting amino acid — with no cap at 1.0. This is why potato protein can score 100 and soy can score 91: they genuinely deliver those amino acids at those rates (Mathai et al., 2017).

The overall digestibility gap is smaller than often assumed. True ileal digestibility for plant protein isolates and flours (soy, pea, wheat, lupine) runs at 89–92%, compared to 90–95% for animal sources — a gap of only a few percentage points, though one that can matter when intakes are already at the margin (Mariotti & Gardner, 2019).

Limiting amino acids: what is actually constrained

Each plant protein class has a predictable weak point:

• Cereals (wheat, rice, corn) and hemp are limited by lysine. They contain adequate sulfur amino acids but run short of lysine relative to the reference pattern.
• Legumes (soy, pea) are limited by methionine and cysteine — the sulfur amino acids. They contain plentiful lysine.

This complementarity is the basis of the classic cereal-plus-legume combination: rice and beans, bread and hummus, oats and soy milk. Eating across both categories throughout the day — not necessarily at the same meal — closes the amino acid gap for most healthy adults (Mariotti & Gardner, 2019).

The limiting-amino-acid concept also explains why hemp’s “complete protein” marketing is technically accurate but practically misleading. Hemp contains all indispensable amino acids, but lysine is present at low enough levels that the DIAAS score (~54) falls well below the 75 threshold needed even to claim “good source” status (Herreman et al., 2020).

How processing changes the score

A DIAAS value is not fixed for a given food — it shifts substantially depending on preparation.

Processes that raise scores: Cooking destroys trypsin inhibitors and other antinutritional factors that interfere with digestion. Soaking, sprouting, and fermentation have similar effects (Samtiya et al., 2020). Cooked Canadian pulses — yellow peas, lentils, chickpeas — score meaningfully higher than raw counterparts (Nosworthy et al., 2017). High-moisture extrusion can push pea protein DIAAS further — Herreman et al. (2020) report DIAAS 82 or 86 depending on extrusion temperature.

Processes that lower scores: Maillard reactions — the browning from dry-heat processing like baking and roasting — can destroy lysine bioavailability even when total lysine content looks intact on a label. Herreman et al. (2020) document a 30–40% decrease in reactive lysine in soybean meal and rapeseed meal after toasting — a reminder that processing losses are invisible on a standard label. Concentration and isolation increase protein percentage but do not fix amino acid imbalances: a wheat protein isolate is still limited by lysine regardless of its purity level.

What the numbers mean for real diets

For athletes tracking intake carefully, the quality gap is large enough to plan around. Applying DIAAS to actual dietary records, vegetarian athletes would need approximately 10 g more total protein per day than omnivores to reach equivalent effective protein quality at a 1.2 g/kg/day target (Lynch et al., 2019). That study is observational and cross-sectional — the 10 g figure is directional, not a universal prescription — but it is consistent with the digestibility data.

For general healthy adults eating varied plants, the practical implication is simpler: soy (tofu, tempeh, edamame, soy milk) delivers demonstrably higher per-gram quality than other common plant sources. Relying primarily on wheat or rice protein without complementary legumes leaves a real gap, not a theoretical one. For dose recommendations specific to muscle protein synthesis and athletic performance, see protein for vegan athletes.

Practical guidance

• Default to soy as a primary plant protein source. It is the only common plant protein that clears the FAO high-quality threshold without blending.
• Combine cereals and legumes across the day. Timing within a single meal is not required; daily totals matter.
• Cook your pulses. Standard cooking is enough to raise digestibility significantly compared to raw or undercooked legumes.
• Read isolate labels by source, not by protein percentage. Concentration raises the protein number but cannot fix amino acid imbalances inherent to the source.
• If you rely heavily on hemp or rice protein, pair them. A soy or pea addition produces a blend much closer to soy-level composite quality.
• Athletes: adjust total protein upward, not just toward plant sources. The evidence supports modestly higher intakes on plant-heavy diets.

Common misconceptions

• “The PDCAAS score on my label tells me how much protein I absorb.” PDCAAS measures whole-gut crude protein digestibility and caps at 1.0. DIAAS measures true ileal absorption of each amino acid individually. The gap is largest for cereals and hemp.

• “Pea protein and soy protein are basically the same quality.” Soy scores 91 on DIAAS (high quality); pea scores 70 (no quality claim). Both are useful, but soy consistently outperforms pea as a standalone source.

• “Hemp is a high-quality protein because it has all the amino acids.” Technically complete does not mean high quality. Hemp scores ~54 on DIAAS — limited by lysine — and falls below the threshold for any FAO protein quality claim.

• “Processing hurts protein quality.” It depends on the method. Cooking, soaking, and extrusion generally raise DIAAS by eliminating antinutritional factors. Dry-heat browning can cut bioavailable lysine even when the label shows the same total protein content.

• “DIAAS only matters for bodybuilders.” Protein quality matters whenever intake is close to the minimum — including in older adults, children, and anyone eating primarily cereal grains as their main protein base.

The punchline

PDCAAS built an overly optimistic picture of plant protein quality that persists on most labels today. DIAAS reveals a real hierarchy: soy is genuinely high quality; most other plant proteins are useful but need quantity or complementarity to compensate for what they lack per gram. That gap is manageable — most healthy adults eating varied plants close it without calculating anything — but it is real.

For the full picture on plant protein — how much to eat, which sources to prioritise, and how the evidence on muscle synthesis applies — see the protein pillar.