What Is a Complete Protein? Plant Foods, Combining and DIAAS

A complete protein contains all nine essential amino acids in amounts close to what the body needs. It describes composition, not a benefit. Soya sits in the same DIAAS band as whey and matched milk protein across 17 trials; most pulses, grains, nuts and seeds are low in one amino acid, and a varied day's eating covers that without pairing foods at the same meal.^{1, 2, 13} This is the science page of the Wellgard guide to vegan protein sources.

What does "complete protein" mean?

Nine amino acids are essential because the body cannot make them: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine. A protein that supplies all nine in the proportions of the human reference pattern is called complete; one short in at least one is called incomplete, and the amino acid in shortest supply is its limiting amino acid. It is a description of composition, not a legal standard in the UK and not an authorised claim: the GB register's authorised wording is that protein contributes to the maintenance of muscle mass, and a food may carry it when at least 12% of its energy comes from protein, whatever its amino acid pattern.¹

Which plant foods are complete proteins?

Isolate (protein extracted for powders) and whole-food scores are different numbers, so each row says which it quotes. And "complete" tells you about a food's amino acids, not how much protein it carries: quinoa is often called complete, a label with no human DIAAS behind it, and gives about 8 g in a 60 g dry portion.⁶

Digestibility note: a whole food and an isolate of the same plant give different values; protein is per 100 g as eaten unless marked dry.⁶ Soya and pea protein have their own pages; how much protein each food carries per portion is on the high-protein foods page.

Do you have to combine proteins at the same meal?

No. The idea comes from the limiting-amino-acid arithmetic, and the arithmetic works across a day: amino acids from one meal are still available at the next. A 2019 review of protein in vegetarian and vegan diets concluded that traditional legumes, nuts and seeds are sufficient for full protein adequacy in adults and that the question of amino acid deficiency "has been substantially overstated".⁹ Dahl with rice, beans on toast and houmous with bread are complementary pairs that happen anyway.

What are DIAAS and PDCAAS, and what do the numbers mean?

Both score protein quality: each essential amino acid in a protein against the reference pattern, adjusted for how much is digested. PDCAAS, the older score, applies one digestibility figure to the whole protein, measured across the entire gut, and is capped at 1.0. DIAAS, which the FAO recommended in 2013 to replace it, uses the digestibility of each amino acid at the end of the small intestine and is not capped, so a protein that over-supplies the pattern scores above 100.¹⁰ Published compilations group scores as 100 or more (excellent), 75 to 99 (high) and below 75 (no quality claim).² In 14 foods fed to growing rats, DIAAS ran from 0.01 (a corn-based cereal) to 1.18 (milk protein concentrate), and uncapped PDCAAS generally scored higher.¹⁰ UK law uses neither: "source of protein" and "high protein" are label terms set on the share of a food's energy from protein, 12% and 20%.¹

Why do powders blend pea or fava bean with rice?

Because the gaps are in different places. In a 2018 analysis of commercial isolates, methionine and lysine averaged 1.0% and 3.6% of plant proteins against 2.5% and 7.0% of animal proteins, pulses short of methionine and cereals short of lysine; pea or fava bean protein brings the lysine that rice lacks, rice brings the methionine that pulses lack.⁴ The compilation's simulation found potato, soya and pea proteins complement a broad range of plant proteins at specific ratios.² The human evidence is acute: 24 young men who drank 30 g of a wheat, corn and pea blend raised muscle protein synthesis no differently from 30 g of milk protein (0.064 against 0.053% an hour, P=0.08), though their blood essential amino acids rose about half as much.¹¹ No trial has followed a pea-and-rice or fava-and-rice blend over weeks for muscle mass or strength: a gap, not a verdict. Reading a label is covered on is vegan protein powder healthy.

Soya, lupin, peanuts, tree nuts, sesame and cereals containing gluten are among the 14 allergens UK law requires on labels; peas, chickpeas, lentils and beans are not, yet pea protein isolates have caused anaphylaxis, including children who reacted to pea protein hidden in processed foods.^{7, 8} Anyone with a legume or nut allergy speaks to their allergy clinic before trying a new isolate.

Does leucine matter more than completeness?

For a single meal, often yes. Leucine is the amino acid that switches on muscle protein synthesis, and the dose scales with body weight: young men reached their ceiling at about 0.24 g of protein per kilogram in one meal, older men at about 0.40 g/kg.¹² The 2018 analysis worked out how much protein from each isolate supplies 2.7 g of leucine, the amount in 25 g of whey: 20 g of corn protein, 31 g of milk protein, 37 g of brown rice, 38 g of pea, 40 g of soya, 45 g of wheat and 54 g of hemp protein.⁴ Scaled to a 20 g serving (estimated; digestibility left out), pea and soya carry about 1.4 g of leucine, hemp about 1.0 g and milk protein about 1.7 g: hence the 25 to 30 g per plant meal rule on how much protein vegans need.

Does being "complete" make a protein better for muscle?

Completeness on its own did not decide the trial results. A 2025 pooling of 30 randomised trials compared plant with animal protein at matched amounts: plant protein gave slightly less muscle mass overall (standardised mean difference -0.20), an effect seen in adults under 60 and not in those 60 and over (-0.05, not significant); soya matched milk protein across 17 trials; animal protein was ahead of single non-soya sources (rice, chia, oat and potato) across five trials and of plant-based diets across seven; there was no difference in strength across 14 trials or physical performance across five.¹³ The authors asked for trials of a wider range of plant proteins and diets. Total intake comes first and quality second: how much protein you need sets the day's figure.

Frequently asked questions

Is pea protein a complete protein?

Pea protein isolate supplies all nine essential amino acids, methionine the lowest; in a maker-co-authored study of 15 adults it scored a DIAAS of 1.00 against 1.45 for casein.^{3, 4} More on what pea protein is.

Is tofu a complete protein?

Yes, by composition: tofu is soya, and soya protein scores 75 or more on DIAAS.² How much protein it carries depends on the tofu: 8.1 g per 100 g steamed, 23.5 g fried.⁶

Are lentils, chickpeas or beans complete proteins?

No: pulses are low in methionine, and cooked chickpeas and peas delivered about 72 to 75% of their essential amino acids in the human study, so a grain at another meal rounds them out.^{4, 5} Soya mince and TVP are soya, so they are complete by composition.²

Is quinoa a complete protein?

It is usually listed as one, but no human DIAAS was found, so the label is reported as a label, not a score. It carries 13.8 g of protein per 100 g dry, about 8 g in a 60 g dry portion.⁶

What is an incomplete protein, and is it bad?

A protein low in at least one essential amino acid against the reference pattern: most grains (lysine) and most pulses (methionine).⁴ It is not bad: a varied plant-based diet eats both groups across the day without planning.⁹

Sources

1. Great Britain nutrition and health claims (NHC) register, spreadsheet of 19 May 2026: rows 169, 171 and 173 (protein) and the nutrition claim conditions. Read 11 October 2026. gov.uk.
2. Herreman L, et al. Comprehensive overview of the quality of plant- and animal-sourced proteins based on the digestible indispensable amino acid score. Food Science and Nutrition, 2020. Authors employed by a potato-protein producer. PubMed 33133540.
3. Guillin FM, et al. Real ileal amino acid digestibility of pea protein compared to casein in healthy humans: a randomized trial. American Journal of Clinical Nutrition, 2022. Co-authored by the pea protein's maker. PubMed 34665230.
4. Gorissen SHM, et al. Protein content and amino acid composition of commercially available plant-based protein isolates. Amino Acids, 2018. PubMed 30167963.
5. Kashyap S, et al. True ileal digestibility of legumes determined by dual-isotope tracer method in Indian adults. American Journal of Clinical Nutrition, 2019. PubMed 31374575.
6. Public Health England. McCance and Widdowson's Composition of Foods Integrated Dataset (CoFID), 2021. gov.uk.
7. Food Standards Agency. Allergen labelling for food manufacturers: the 14 allergens. Read 11 October 2026. gov.uk.
8. Lavine E, et al. Anaphylaxis to hidden pea protein: a Canadian pediatric case series. Journal of Allergy and Clinical Immunology: In Practice, 2019. PubMed 30794967.
9. Mariotti F, Gardner CD. Dietary protein and amino acids in vegetarian diets: a review. Nutrients, 2019. PubMed 31690027.
10. Rutherfurd SM, et al. Protein digestibility-corrected amino acid scores and digestible indispensable amino acid scores differentially describe protein quality in growing male rats. Journal of Nutrition, 2015. PubMed 25644361.
11. Pinckaers PJM, et al. The muscle protein synthetic response to the ingestion of a plant-derived protein blend does not differ from an equivalent amount of milk protein in healthy young males. Journal of Nutrition, 2023. PubMed 36170964.
12. Moore DR, et al. Protein ingestion to stimulate myofibrillar protein synthesis requires greater relative protein intakes in healthy older versus younger men. Journals of Gerontology Series A, 2015. PubMed 25056502.
13. Reid-McCann RJ, et al. Effect of plant versus animal protein on muscle mass, strength, physical performance, and sarcopenia: a systematic review and meta-analysis of randomized controlled trials. Nutrition Reviews, 2025. PubMed 39813010.