FAO 2013 + Rutherfurd 2015 + Mathai 2017 DIAAS data

Vegan vs Vegetarian Protein Quality: PDCAAS and DIAAS Scores

Protein quality is the second half of the protein question. Total grams matters, but so does the amino acid balance and the digestibility of the source. The current scoring systems (PDCAAS and the newer DIAAS) put soy near the top of the plant pile, with quinoa close behind. This page walks through what the scores mean, where plant proteins land on them, and what that means for a vegan or vegetarian planning their week.

The short answer. The complete plant proteins are soy, quinoa, buckwheat, amaranth, hemp, chia, and pistachios. Vegans should target around 1.0 g protein per kg body weight per day (about 10 to 15% above the omnivore RDA) to compensate for slightly lower DIAAS-adjusted digestibility. Combine grains with legumes across the day; you do not need to combine them within each meal. Soy and pea-rice blends are the closest plant equivalents to whey.

PDCAAS, DIAAS, and why both scores matter

The FAO and WHO adopted the Protein Digestibility Corrected Amino Acid Score (PDCAAS) in 1991 as the international standard for evaluating protein quality. PDCAAS multiplies the amino acid score (the limiting essential amino acid expressed as a ratio of human requirement) by the true fecal protein digestibility, with a cap at 1.0. The cap matters because it prevents proteins like whey or casein from scoring above 1.0 even though their amino acid composition objectively exceeds the reference pattern.

The 2013 FAO Expert Consultation on protein quality recommended replacing PDCAAS with the Digestible Indispensable Amino Acid Score (DIAAS). DIAAS uses ileal (small intestine) digestibility rather than fecal digestibility, which excludes the contribution of colonic bacterial protein synthesis to fecal nitrogen and gives more biologically accurate numbers. DIAAS also measures each indispensable amino acid digestibility individually rather than relying on whole-protein digestibility. And DIAAS is not capped at 1.0, so whey scores 1.09 and milk scores 1.18.

The practical effect is that DIAAS values for plant proteins are slightly lower than PDCAAS for the same protein, because plant proteins have lower ileal digestibility than fecal digestibility (some plant protein is fermented by colonic bacteria and recovered into the body). The score order across plant proteins is largely preserved: soy and quinoa near the top, peas in the middle, wheat and rice at the bottom.

The actual numbers

Protein source	PDCAAS	DIAAS	Limiting amino acid
Whey protein isolate	1.00 (capped)	1.09	None
Milk (cow)	1.00 (capped)	1.18	None
Egg (whole)	1.00 (capped)	1.13	None
Soy protein isolate	1.00 (capped)	0.91 to 0.99	Methionine borderline
Tofu (firm)	0.91	0.87	Methionine
Tempeh	0.86	0.79	Methionine
Quinoa	0.83	0.78	Lysine (borderline)
Hemp seed	0.66	0.66	Lysine
Chickpeas	0.78	0.71	Sulfur amino acids
Lentils	0.63	0.59	Sulfur amino acids
Pea protein isolate	0.93	0.62 to 0.82	Sulfur amino acids
Black beans	0.76	0.65	Sulfur amino acids
Brown rice	0.51	0.42	Lysine
Wheat protein (gluten)	0.42	0.40 to 0.45	Lysine
Almonds	0.43	0.40	Lysine
Peanut butter	0.52	0.43	Lysine
Pea + rice blend	0.93	0.90+	None (complementary)
Beans + grains combo	~0.95	~0.88	None (complementary)

Values pulled primarily from Rutherfurd SM et al. (J Nutr 2015; 145: 372-379) and Mathai JK et al. (Br J Nutr 2017; 117: 490-499). Where ranges are given, different processing and cooking methods produce different results.

The combining myth, gone but lingering

The 1970s advice that vegetarians and vegans had to combine complementary proteins at each meal (rice with beans, peanut butter with bread) came from Frances Moore Lappe's 1971 Diet for a Small Planet and influenced a generation of plant-based eaters. Lappe revised her own position in the 1981 edition, acknowledging that the meal-by-meal combining advice was unnecessary. The Academy of Nutrition and Dietetics has stated since 1988 (and reiterated in every position paper since, most recently 2016) that combining across the day, not within each meal, is sufficient.

The physiology behind the change in advice: free amino acids absorbed from one meal enter a body pool that turns over with body protein synthesis and breakdown over hours to days. The pool is buffered; you do not need to deliver a perfectly balanced amino acid mix in each meal to support net protein synthesis. As long as the day's intake covers all essential amino acids at adequate amounts, the body assembles what it needs. Combining still helps in the sense that varied diets are easier to balance than monotonous ones, but the strict meal-by-meal rule is wrong.

Per-day protein targets by population

Population	Omnivore target (g/kg/day)	Vegan target (g/kg/day)	70 kg vegan example
Sedentary adult	0.8	0.9 to 1.0	63 to 70 g
Endurance athlete	1.2 to 1.4	1.4 to 1.6	98 to 112 g
Strength athlete	1.6 to 1.8	1.8 to 2.0	126 to 140 g
Pregnant (2nd to 3rd trimester)	1.1 (plus 25 g)	1.2 to 1.3 (plus 25 g)	~110 g
Lactation	1.1 (plus 25 g)	1.2 to 1.3 (plus 25 g)	~110 g
Older adult (over 65)	1.0 to 1.2	1.2 to 1.4	84 to 98 g
Recovering from illness	1.2 to 1.5	1.4 to 1.6	98 to 112 g

Vegan targets are modestly higher than the omnivore RDA, reflecting the ~10 to 15% downward adjustment for DIAAS-corrected digestibility. The differences are real but small; vegans hitting the omnivore RDA are not deficient, but the upward-adjusted target gives a comfortable margin.

The leucine question for muscle protein synthesis

Muscle protein synthesis (MPS) is triggered when intramuscular leucine concentration crosses a threshold of approximately 2.5 to 3 g per meal in adults, with the threshold rising in older adults due to anabolic resistance. Whey protein hits this threshold from about 20 g protein per dose; soy protein from about 25 g per dose; pea protein from about 30 g; wheat gluten (seitan) from about 35 g. The Tipton and colleagues work and the McMaster muscle protein synthesis lab studies converge on these numbers.

The practical implication for vegan strength athletes is to dose protein at meals rather than spread it too thinly. Four meals or snacks per day of 30 to 40 g protein each is more effective than six smaller doses. Soy products and pea-rice blends are the most leucine-dense plant options. Older vegan adults (over 65) should aim toward the higher end of these doses to compensate for anabolic resistance.

Athletes and the protein supplement market. A vegan athlete eating a varied whole-foods diet plus a 25 to 40 g protein dose at each main meal usually does not need protein powder. If you use one, soy isolate, pea-rice blend, or sprouted brown rice protein are the higher-quality plant options. Hemp protein is lower DIAAS but useful as part of a varied intake. See the athletes page.

Frequently asked questions about protein quality

What is the difference between PDCAAS and DIAAS?

PDCAAS (Protein Digestibility Corrected Amino Acid Score) has been the standard since the FAO and WHO adopted it in 1991. It scores protein quality on a 0 to 1 scale based on the limiting essential amino acid relative to needs, corrected for whole-food fecal digestibility, and capped at 1.0. DIAAS (Digestible Indispensable Amino Acid Score) is the 2013 FAO replacement that uses ileal rather than fecal digestibility (more biologically relevant), is not capped at 1.0, and measures each amino acid digestibility individually. DIAAS gives lower scores to most plant proteins than PDCAAS because it does not give a cap-credit and uses harsher digestibility numbers. The published comparisons (Rutherfurd 2015, Mathai 2017) show whey at 1.09 DIAAS, soy at 0.91 DIAAS, pea at 0.69 DIAAS, and wheat gluten at 0.42 DIAAS.

Which plant proteins are complete?

A complete protein contains all nine essential amino acids in proportions adequate for human needs. Among plant foods, the unambiguously complete are soy products (tofu, tempeh, edamame, soy milk), quinoa, hemp seeds, buckwheat, amaranth, chia seeds, and pistachios. Spirulina is often listed but its bioavailability is questioned (and supplement form rarely justifies a meaningful protein contribution). Most other plant foods are limiting in one essential amino acid: grains are usually low in lysine, legumes are low in methionine and cysteine, and nuts and seeds are low in lysine. Combining grains with legumes across the day (not necessarily in the same meal) covers all essentials.

Do I really not need to combine proteins at each meal?

The amino acid pool in the body is replenished from dietary protein continuously and held in equilibrium with body protein turnover. The 1970s protein combining advice (popularised by Frances Moore Lappe in Diet for a Small Planet and later retracted in the 1981 revision) was based on a misunderstanding of how amino acid balancing works in practice. The Academy of Nutrition and Dietetics 2016 position paper and the FAO 2013 protein quality expert consultation both state explicitly that combining is not necessary at each meal as long as variety is maintained across the day. Eating beans and rice in the same meal works; so does beans at lunch and rice at dinner.

Do vegans need more protein than meat-eaters?

Modestly. The American College of Sports Medicine position stand and the Academy of Nutrition and Dietetics suggest that vegans target 10 to 15% higher protein intake than the standard 0.8 g/kg/day RDA, to account for slightly lower DIAAS-adjusted digestibility of plant proteins on average. A 70 kg vegan adult therefore targets roughly 65 g per day rather than 56 g per day. Athletes and older adults adjust upward further: 1.2 to 2.0 g/kg/day for athletes, 1.0 to 1.2 g/kg/day for over-65s to mitigate sarcopenia. Most actively eating vegans hit these targets without supplementation.

Is pea protein as good as whey?

Close but not identical, on the standard metrics. Whey scores PDCAAS 1.0 and DIAAS 1.09; pea protein isolate scores PDCAAS 0.93 and DIAAS 0.62 to 0.82 depending on the preparation. Pea is limiting in methionine and cysteine (the sulfur amino acids); whey has those covered. A pea-rice blend (Vega Sport and others) reaches DIAAS close to whey because the limiting amino acids of each are filled by the other. For muscle protein synthesis after training, whey hits a higher peak more quickly due to faster gastric emptying, but soy and pea-rice blends produce similar 24-hour muscle protein synthesis when matched for total essential amino acids.

Is wheat protein really that low quality?

Wheat protein is limiting in lysine, which gives it a low PDCAAS (0.42) and DIAAS (0.40 to 0.45) score. This sounds alarming until you remember that no one eats wheat as their sole protein source. Wheat eaten alongside legumes (a bean burrito, lentil soup with bread, hummus on toast) provides complementary amino acids and the combined PDCAAS approaches 0.9. Seitan, which is concentrated wheat gluten, is genuinely high in protein per gram (75 g protein per 100 g) but inherits the wheat lysine limitation. Use seitan as one protein among several rather than as a daily mainstay, and pair it with lysine-rich foods like soy, beans, or quinoa.

Sources cited. FAO Expert Consultation. Dietary protein quality evaluation in human nutrition, FAO Food and Nutrition Paper 92, 2013; Rutherfurd SM et al. Protein digestibility-corrected amino acid scores and digestible indispensable amino acid scores differentially describe protein quality, J Nutr 2015; 145: 372-379; Mathai JK et al. Values for digestible indispensable amino acid scores (DIAAS) for some dairy and plant proteins may better describe protein quality than values calculated using the concept for protein digestibility-corrected amino acid scores (PDCAAS), Br J Nutr 2017; 117: 490-499; Melina V, Craig W, Levin S. Position of the Academy of Nutrition and Dietetics: vegetarian diets, J Acad Nutr Diet 2016; 116: 1970-1980; Tipton KD et al. Protein dose and the regulation of muscle protein synthesis, Sports Med 2020; BDA Protein food fact sheet. All values as of May 2026.