Protein Quality: Complete vs Incomplete Proteins, DIAAS Score and What Actually Matters

Dietary protein delivers not just nitrogen but specific amino acids that the body uses to build and repair tissues, synthesise enzymes and hormones, support immune function and maintain every structural protein from collagen to haemoglobin. There are twenty amino acids required for these functions; nine are essential — meaning the body cannot synthesise them in adequate amounts and must obtain them from food: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine.

Protein quality refers to two distinct but related properties: amino acid composition (does the protein provide all essential amino acids in proportions that match human requirements?) and digestibility (what fraction of the protein is actually absorbed and made available for use?). A protein can be high in one essential amino acid but deficient in another — the deficient amino acid is called the 'limiting amino acid' and it constrains the body's ability to utilise the entire protein source for synthesis.

This matters for muscle building, immune function and metabolic health. According to a 2017 review by Wolfe et al. published in Advances in Nutrition (PMID: 28615064), optimising protein intake requires attention not just to total quantity but to the quality and distribution of protein across meals, with specific emphasis on leucine content — the primary trigger of muscle protein synthesis through the mTOR pathway. A meal below the leucine threshold (approximately 2–3 g) may fail to maximally stimulate muscle protein synthesis regardless of its total protein content.

The classification of proteins as 'complete' or 'incomplete' is a simplification that is more confusing than helpful. A complete protein contains all nine essential amino acids in quantities meeting or exceeding human requirements. Animal sources — meat, fish, eggs, dairy — are complete proteins. Soy is the notable plant-based complete protein. Quinoa and buckwheat are often cited as complete, though at lower overall concentrations.

An incomplete protein is simply one that is deficient in one or more essential amino acids relative to human needs. Most plant proteins fall into this category — legumes are typically low in methionine, while grains are typically low in lysine. However, calling a plant protein 'incomplete' does not mean it is nutritionally useless. At typical dietary quantities — particularly when two or more plant sources are eaten across the day — the amino acid profiles complement each other effectively.

The longstanding advice to 'combine proteins at each meal' (rice and beans together, for example) has been largely abandoned by nutrition scientists. Research indicates that the body maintains an amino acid pool over a period of hours, so complementary plant proteins eaten across the same day — not necessarily at the same meal — achieve a complete amino acid profile. For most healthy adults eating a varied plant-based diet with adequate total protein intake, incomplete protein status is rarely a practical limiting factor. The concern becomes more relevant at low total protein intakes or for highly active individuals with elevated requirements.

The Digestible Indispensable Amino Acid Score (DIAAS) was introduced by the Food and Agriculture Organisation (FAO) in its 2013 Food and Nutrition Paper 92 to replace the older Protein Digestibility-Corrected Amino Acid Score (PDCAAS). The DIAAS is considered the more accurate measure for three key reasons.

First, DIAAS uses ileal digestibility — measuring amino acid absorption at the end of the small intestine — rather than faecal digestibility, which overcounts digestibility by including bacterial amino acid synthesis in the large intestine. Second, DIAAS assesses the digestibility of each individual amino acid separately rather than applying a single correction factor to the whole protein, which provides a more precise picture of the limiting amino acid. Third, DIAAS is not truncated at 1.0 the way PDCAAS was — proteins can score above 1.0, providing a clearer hierarchy.

On the DIAAS scale, scores above 1.0 are excellent, 0.75–1.0 are acceptable, and below 0.75 are inadequate. Whole milk scores approximately 1.18; eggs score around 1.13; beef around 1.00. Soy protein isolate scores approximately 0.90–1.00. Pea protein isolate scores 0.82. Wheat protein scores around 0.45 — substantially lower. The practical implication is that to achieve equivalent anabolic stimulus from plant proteins as from animal proteins, a higher total quantity of plant protein is generally required. A 2015 study by van Vliet et al. in the Journal of Nutrition (PMID: 26224750) confirmed that the anabolic response to whole food plant protein sources is lower than for animal proteins, attributable primarily to lower digestibility and a less favourable essential amino acid profile.

The question of whether plant proteins support muscle building as effectively as animal proteins is one of the most actively studied areas in nutrition science. The short answer from current evidence is: animal proteins elicit a somewhat greater acute anabolic response at equivalent doses, but the long-term muscle outcomes for well-designed plant-based diets appear to be broadly equivalent when total protein intake is adequate.

The 2015 van Vliet et al. review in the Journal of Nutrition found that animal-derived proteins consistently outperformed plant proteins in stimulating muscle protein synthesis when compared at equal gram doses, primarily because animal proteins are more leucine-dense and more rapidly digestible. However, a 2018 study by Messina et al. in the International Journal of Sport Nutrition found no significant difference in lean mass or strength gains between soy protein and animal protein supplementation over 12 weeks of resistance training when doses were matched for total leucine content.

A landmark 2018 analysis by Gorissen et al. in Amino Acids (PMID: 29725989) assessed the amino acid composition of commercially available plant-based protein isolates. They found substantial variation — pea, soy and rice protein isolates had relatively complete amino acid profiles at high doses, while hemp and wheat proteins had significant deficiencies. Leucine content ranged from 6.0–8.0 g per 100 g in soy and pea isolates — lower than whey (10–11 g per 100 g) but sufficient at higher intakes. The consensus recommendation for plant-based athletes is to consume approximately 20–25% more total protein than omnivore equivalents and to prioritise leucine-rich plant proteins such as soy, pea and legumes.

Understanding DIAAS scores and amino acid profiles is useful academically, but most people think in terms of food rather than protein fractions. Here is how protein quality maps onto everyday eating choices.

Among animal proteins, eggs are the reference against which other proteins are often compared — they have an exceptionally balanced amino acid profile, a DIAAS of approximately 1.13, and very high digestibility. Whey protein (from dairy) is the most studied supplement: it is rapidly digested, extremely leucine-rich and consistently shown to maximally stimulate muscle protein synthesis. Chicken breast, beef and fish score similarly well on DIAAS. Greek yoghurt and cottage cheese are excellent dairy protein sources with high digestibility.

Among plant proteins, soy is the standout: it is the only plant protein with a DIAAS consistently above 0.90 and is the most studied plant protein for muscle outcomes. Pea protein isolate performs well — a DIAAS of approximately 0.82 and good leucine content. Lentils and chickpeas provide solid amino acid profiles at higher serving sizes. Quinoa is complete but must be eaten in large quantities to deliver meaningful protein doses. Hemp seed provides a reasonable profile but at lower digestibility than isolates.

The most practical conclusion: a diverse diet that includes multiple protein sources across the day — whether omnivore, vegetarian or vegan — will meet protein quality requirements for most people. Protein quality only becomes a limiting concern at very low intakes, for highly trained athletes with peak muscle synthesis goals, or for populations with elevated needs such as the elderly (who require more leucine to overcome anabolic resistance).

Protein requirements are not static. They shift with age, activity level, physiological state and health status in ways that protein quality considerations amplify. The general recommended dietary allowance (RDA) of 0.8 g per kg of bodyweight is often cited but represents a minimum to prevent deficiency, not an optimal intake for health and function.

For active adults engaged in resistance exercise, the evidence base — reviewed extensively by Wolfe et al. in their 2017 Advances in Nutrition paper — supports intakes of 1.2–2.2 g per kg depending on training intensity and goals. For older adults (65+), increasing evidence suggests 1.2–1.6 g per kg is needed to counter the anabolic resistance that develops with age — a blunted sensitivity to dietary protein that means the leucine threshold for stimulating muscle protein synthesis rises. This has specific quality implications: older adults on plant-based diets may need carefully planned, leucine-rich meals to maintain muscle mass. Pregnancy requires approximately 1.1 g per kg through the second and third trimesters to support fetal development. Children have higher relative protein requirements than adults on a per-bodyweight basis. Endurance athletes have different protein priorities than strength athletes — they need adequate intake to prevent muscle protein breakdown rather than maximise synthesis.

The global protein supplement market exceeds £15 billion annually, and the marketing claims made for protein powders routinely exceed what the scientific evidence supports. A more grounded view of the evidence is useful. Whey protein is the most thoroughly studied protein supplement and has a well-established body of evidence supporting its use for muscle protein synthesis, particularly in the post-exercise window. It is not superior to high-quality whole food protein sources (eggs, chicken, fish) if those are consumed in adequate quantities and timing. Its primary practical advantage is convenience and speed of preparation.

Casein protein — the slow-digesting dairy protein — has evidence supporting its utility as a pre-sleep protein source; a 2012 Maastricht University study by Res et al. found 40 g of casein before sleep significantly increased overnight muscle protein synthesis rates. Plant-based protein blends (combining pea and rice proteins, for example) can achieve amino acid profiles approaching those of whey at slightly higher doses. Collagen peptide supplements are a distinct category — collagen protein has a highly incomplete amino acid profile and is not a useful source for muscle synthesis, though it has some evidence for connective tissue health at specific doses. Protein bars vary enormously in quality; many are nutritionally similar to confectionery with protein added. Whole food sources should always be the foundation, with supplements filling genuine dietary gaps rather than replacing quality eating.

Translating protein quality science into a daily eating approach involves a small number of consistent principles. First, aim for 25–40 g of protein at each main meal rather than front-loading or back-loading intake. Research consistently shows that muscle protein synthesis is maximised when protein is distributed evenly across 3–4 eating occasions rather than consumed in one or two large doses. The body can only utilise approximately 40–50 g per sitting for acute muscle protein synthesis; excess is oxidised.

Second, include at least one leucine-rich protein source at each main meal — eggs, meat, fish, dairy, tofu, edamame or a quality pea/soy protein are all practical options. The leucine threshold of 2–3 g per meal is easily achieved with a 100 g chicken breast, 2 eggs or 150 g Greek yoghurt. Third, for plant-based eaters: prioritise soy, lentils, pea protein and quinoa; diversify protein sources across the day; and consider consuming slightly higher total protein (around 1.6–1.8 g per kg) to compensate for lower average DIAAS scores. Fourth, time protein intake around exercise: consuming 20–40 g of protein within 2 hours of resistance training reliably supports muscle protein synthesis based on meta-analysis data. Fifth, do not obsess over protein quality at the expense of dietary variety — the evidence strongly supports that dietary pattern, not protein source alone, determines long-term health outcomes.