Complete vs Incomplete Protein

A complete protein contains all nine essential amino acids in adequate proportions; an incomplete protein is limiting in one or more of them. "Incomplete" does not mean low-quality or useless — most plant foods are incomplete on their own yet still contribute valuable protein, and eating a variety across the day supplies the full set.

The complete/incomplete label describes only a food's amino acid profile — whether it carries all nine essential amino acids in the right proportions. It says nothing about how much protein a food has, how many calories come with it, or how healthy it is overall. Nutrition scientists have largely moved past the simple binary in favor of graded protein quality scores, which account for both the amino acid profile and how well the protein is digested and absorbed. This page explains what makes a protein complete, what a limiting amino acid is, and how quality is actually measured.

Complete vs Incomplete: The Basics

Protein is built from twenty amino acids, nine of which are essential — the body cannot make them, so they must come from food. A complete protein supplies all nine in amounts that meet human requirements. A incomplete protein contains all the amino acids too, but at least one is present in too small an amount to fully support protein synthesis on its own.

Nearly all animal proteins — meat, poultry, fish, eggs, and dairy — are complete. Among plant foods, soy (tofu, tempeh, edamame), quinoa, and buckwheat are complete, while most grains, legumes, nuts, and seeds are incomplete individually. The practical fix is variety rather than perfection at every meal: because the body keeps a short-term pool of recently absorbed amino acids, combining complementary plant proteins across the day — not necessarily in the same dish — covers the gaps. For the full list of the nine amino acids and the day-not-meal rule, see our guide to the essential amino acids.

The Limiting Amino Acid

The limiting amino acid is the essential amino acid present in the smallest amount relative to what the body needs. It matters because protein synthesis works to the level of whatever runs out first: like a barrel with one short stave, the amount of usable protein is capped by the amino acid in shortest supply, no matter how abundant the others are. This is exactly what makes a protein "incomplete."

Different food groups tend to run short on different amino acids, which is why they complement one another:

• Cereal grains (wheat, rice, corn, oats) are typically limited in lysine.
• Legumes (beans, lentils, peas) are typically limited in the sulfur amino acid methionine; some are also relatively low in tryptophan.
• Nuts and seeds are generally low in lysine as well.

Because grains and legumes have opposite shortfalls, eating both across the day supplies the full set — the grain covers the legume's methionine gap and the legume covers the grain's lysine gap. This is the mechanism behind classic pairings like rice and beans or hummus and bread, though, as above, the pairing does not have to happen in a single meal.

How Protein Quality Is Scored

Because "complete vs incomplete" is a blunt yes/no, nutrition science grades protein on a scale instead. Protein quality reflects two things together: whether the essential amino acid profile matches human requirements, and how much of the protein is actually digested and absorbed. Three measures are worth knowing, in roughly the order they were developed.

Biological Value (BV)

Biological value is one of the oldest measures. It estimates the proportion of absorbed protein nitrogen the body retains and uses, with whole egg historically set near the top of the scale as a reference. BV captures how efficiently a protein is used once absorbed, but it reflects nitrogen retention under specific test conditions rather than the amino acid profile directly, and results vary with intake level — so it has largely been superseded for food labeling and comparison.

PDCAAS

The Protein Digestibility-Corrected Amino Acid Score (PDCAAS) was adopted by the FAO/WHO in 1991 and became the standard for decades. It compares a protein's limiting amino acid against a reference requirement pattern and corrects that score for digestibility. Its best-known limitation is that it is truncated at 1.0: any protein that exceeds the reference pattern is capped at a maximum score of 1.00. As a result, PDCAAS cannot distinguish a merely adequate protein from an outstanding one — whey, egg, and milk all pin to 1.00 even though they differ. PDCAAS also relies on fecal (whole-gut) digestibility, which can overstate how much of a given amino acid was truly absorbed in the small intestine.

DIAAS

The Digestible Indispensable Amino Acid Score (DIAAS) was recommended by an FAO expert consultation in 2013 to address those shortcomings and is now the preferred method. It differs from PDCAAS in two important ways. First, it uses ileal digestibility — measured at the end of the small intestine — for each individual amino acid, which better reflects what the body actually absorbs. Second, it is not truncated at 1.0, so genuinely high-quality proteins can score above 100%, allowing finer comparison. DIAAS is expressed as a decimal or an equivalent percentage. Following the FAO's classification, a protein scoring 1.0 (100%) or above can be described as an excellent-quality protein source, one scoring 0.75 to 0.99 (75–99%) as a high-quality source, and one below 0.75 cannot carry a protein-quality claim.

DIAAS is the most rigorous of the three, but it is not the last word. Published values vary with the food source, processing, and cooking method; much of the underlying ileal-digestibility data comes from animal models; and a single score for one food does not capture the mixed-meal reality in which complementary foods raise each other's effective quality. Treat the numbers as a well-founded guide, not a precise ranking.

Per-Food Protein Quality (DIAAS)

The table below lists approximate DIAAS values for common protein sources, from highest to lowest. Animal proteins and soy sit at the top; most single plant foods score lower, largely because of their limiting amino acid and lower digestibility. These figures are approximate and vary by source, processing, and measurement method — use them to compare foods, not as exact values.

Notice that soy and quinoa are complete — all nine amino acids are present — yet still score below the top animal proteins, a reminder that "complete" and "high-scoring" are related but not identical. DIAAS is driven by the digestible amount of a protein's most limiting amino acid relative to human requirements: soy protein isolate, for instance, is highly digestible, so its score is held down chiefly by its marginal methionine rather than by poor digestibility, while for some less-digestible plant foods lower digestibility does contribute.

Published DIAAS figures for the same food often differ from one source to another, so the values above are best read as representative, not fixed constants. Several things move them: processing and cooking (protein isolates and cooked foods are usually digested more completely than raw or whole-food forms); whether a number describes a concentrated isolate or the whole food it comes from; the reference amino-acid pattern chosen for the comparison, which differs by age group; and the measurement method itself, since ileal digestibility is often derived from animal models rather than measured directly in humans. It is more useful to think of a protein's quality as a band than as a single decimal — small differences between similar foods usually fall within this normal variation.

For Plant-Based Eaters

None of this means plant proteins are inadequate — it means they reward a little planning. Most single plant foods are incomplete and several score lower on DIAAS, but that rarely matters for someone eating enough total protein from a range of sources. Two practical habits cover it.

Eat for variety across the day. Combining grains with legumes lets each cover the other's limiting amino acid, and staples like soy, quinoa, and buckwheat are already complete on their own. There is no need to engineer "complete" combinations at every meal — a varied day is enough.

Aim a little higher on total protein. Because plant proteins are, on average, somewhat less digestible and lower-scoring, plant-based eaters may need somewhat more total protein than the baseline to obtain the same usable amount. How much more depends on the specific foods and the overall mix, so the sensible approach is to prioritize higher-quality plant sources and hit a comfortably adequate total rather than chase a fixed number. For practical plant-protein strategies, see our vegan macro calculator and vegetarian macro guide; for overall targets, see the protein intake guide or set yours with the free macro calculator.

Frequently Asked Questions

What is the difference between complete and incomplete protein?

A complete protein contains all nine essential amino acids in adequate amounts — animal foods, plus soy and quinoa. An incomplete protein is low in one or more of them. Incomplete does not mean low quality: eating a variety of plant proteins across the day supplies the full set.

Is incomplete protein bad for you?

No. "Incomplete" describes a food's amino acid profile, not its nutritional worth. As long as you eat enough total protein from varied sources across the day, incomplete proteins together supply all nine essential amino acids — you do not need to combine them at every meal.

What is DIAAS and how is it different from PDCAAS?

DIAAS (Digestible Indispensable Amino Acid Score) rates protein quality using the ileal digestibility of each individual amino acid and is not capped at 1.0, so it can distinguish very high-quality proteins. PDCAAS, the older score, uses whole-gut digestibility and is truncated at 1.0. An FAO expert consultation recommended DIAAS in 2013.

What is a limiting amino acid?

The limiting amino acid is the essential amino acid present in the smallest amount relative to the body's needs. It caps how much of the protein can be used to build new protein. Lysine is typically limiting in cereal grains, while methionine is typically limiting in legumes.

References

• FAO. Dietary Protein Quality Evaluation in Human Nutrition: Report of an FAO Expert Consultation. FAO Food and Nutrition Paper 92. Rome, 2013. fao.org
• Institute of Medicine (now the National Academies of Sciences, Engineering, and Medicine). Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. National Academies Press, 2005. nap.nationalacademies.org
• U.S. National Library of Medicine, MedlinePlus. Amino acids. medlineplus.gov
• Harvard T.H. Chan School of Public Health. The Nutrition Source: Protein. nutritionsource.hsph.harvard.edu

This page is for general education and is not medical advice; individual needs vary.