What is Protein quality?

What is Protein quality?

Protein quality is a measure of how useful a given protein is to your body, judged on two things at once: whether it contains all nine essential amino acids in the proportions humans need, and how much of that protein you can actually digest and absorb. A food can be high in total grams of protein yet still be a low-quality source if it is short on one essential amino acid or if much of it passes through undigested.

That is why 20 grams of whey does more for muscle repair than 20 grams of protein from wheat. The concept exists because not all protein is interchangeable. Your body cannot make the nine essential amino acids (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine), so it must get them from food, and it can only build new tissue as fast as its scarcest essential amino acid allows.

Protein quality captures both the amino acid profile and the digestibility in a single score. Animal proteins such as egg, whey, casein, milk, and lean meat sit at the top because their amino acid pattern closely matches human needs and they digest almost completely. Most single plant proteins score lower because they run short on one amino acid, usually lysine in grains or methionine in legumes, and because fibre and anti-nutrients lower their digestibility. Understanding protein quality tells you not just how much protein to eat, but which sources actually deliver the building blocks, and how to combine plant foods so the total still covers everything you need.

How it works

Protein quality works by comparing the essential amino acid content of a food, after accounting for how much is truly absorbed, against a reference pattern of human amino acid requirements. Your body assembles muscle, enzymes, and other proteins from a pool of the twenty amino acids, but the nine essential ones must come from the diet.

Protein synthesis is rate-limited by the essential amino acid that is in shortest supply relative to need, called the limiting amino acid. If a food delivers plenty of every essential amino acid except lysine, then lysine caps how much of that protein your body can use, and the surplus of the others is oxidised for energy rather than built into tissue.

Scoring systems formalise this. The chemical score and the older biological value gave a first estimate, but the two systems used today, PDCAAS and DIAAS, combine the amino acid profile with a digestibility correction. PDCAAS multiplies the amino acid score of the limiting amino acid by faecal digestibility and caps the result at 1.0. DIAAS, endorsed by the FAO in 2013 as the preferred method, improves on this by measuring the true ileal digestibility of each individual amino acid at the end of the small intestine, using the growing pig as the model, and it is not truncated at 100 percent.

That change matters: measuring absorption at the ileum rather than in the faeces avoids crediting protein that gut bacteria degraded in the large intestine but that you never absorbed. The practical result is a number that reflects how many usable, absorbable essential amino acids a food actually delivers, which is why leucine content, total essential amino acid content, and bioavailability together decide where a protein lands on the quality spectrum.

How to apply it

• Anchor meals on high-quality proteins: Egg, whey, casein, milk, fish, and lean meat all score at or near the top on both PDCAAS and DIAAS. Building each meal around one of these guarantees a complete, absorbable essential amino acid profile without any combining maths.
• Complement plant proteins: Grains are low in lysine and legumes are low in methionine, so pairing them, such as rice with beans or hummus with pita, covers both gaps. The combined meal reaches a far higher effective quality than either food alone.
• Blend plant protein powders: A pea and rice protein blend is the classic example: pea supplies the lysine rice lacks, and rice supplies the methionine pea lacks. The blend approaches the DIAAS of whey, which single-source vegan powders rarely manage on their own.
• Hit the leucine threshold: Leucine is the amino acid that switches on muscle protein synthesis. Aim for roughly 2.5 to 3 grams of leucine per meal, which about 20 to 40 grams of a high-quality protein delivers, to maximise the anabolic response per feeding.
• Eat more total protein from lower-quality sources: If your diet leans heavily on plants, a larger total intake compensates for lower per-gram quality. Vegans and vegetarians are often advised to eat modestly more protein to offset reduced digestibility and limiting amino acids.
• Mind processing and digestibility: Cooking, soaking, and sprouting reduce anti-nutrients like trypsin inhibitors and phytates in legumes and grains, raising how much protein you actually absorb. Isolates and concentrates also strip fibre that would otherwise lower digestibility.

Types

Worked example

Here is how common foods rank by DIAAS, using the FAO reference pattern for young children and the grading bands of 100 or more for excellent, 75 to 99 for good, and under 75 for no claim. The scores show why animal sources dominate and where single plant proteins fall short.

Values are representative means from DIAAS reviews and vary with the food matrix and processing. Notice that grains and legumes have opposite limiting amino acids, which is exactly why combining them, or blending pea with rice protein, pushes the effective quality far above either food on its own.

Protein quality: animal vs plant proteins

Animal proteins win per gram on quality, but a well-planned plant-based diet closes the gap by combining complementary foods, choosing higher-scoring sources like soy, and eating slightly more total protein.

By goal

• Muscle building: Prioritise high-quality, leucine-rich proteins around training. Aim for 20 to 40 grams of a complete source such as whey, egg, or a pea-rice blend per meal so each feeding crosses the roughly 2.5 to 3 gram leucine threshold that drives muscle protein synthesis.
• Plant-based eaters: Lean on soy and combine complementary staples like rice with beans or lentils with grains across the day. Consider a total intake nearer 1.6 to 2.0 grams per kilogram of bodyweight to offset the lower per-gram quality of most plant foods.
• General health and older adults: Spread quality protein across meals rather than loading it at dinner. Older adults face anabolic resistance and benefit from complete, easily digested sources such as dairy, eggs, and fish at each meal to protect muscle mass with age.

Common misconceptions

• "More grams of protein always means more usable protein." Quantity and quality are different. Twenty grams of wheat protein delivers fewer usable essential amino acids than twenty grams of whey, because wheat is limited in lysine and digests less completely. Total grams matter, but the amino acid profile and digestibility decide how much your body can build with.
• "Plant proteins are incomplete and cannot build muscle." Almost every plant food contains all nine essential amino acids, just in less ideal ratios. Soy scores as a high-quality protein on its own, and combining grains with legumes, or eating slightly more total protein, lets a plant-based diet fully support muscle growth.
• "You must combine complementary proteins in the same meal." The old protein-combining rule was overstated. Your body maintains a free amino acid pool for hours, so eating varied protein sources across the day covers your needs. Combining within one meal helps but is not strictly required for healthy adults.
• "PDCAAS and DIAAS give the same answer." They often disagree. PDCAAS caps scores at 1.0 and measures faecal digestibility, which flattens differences between top proteins and can overstate plant quality. DIAAS measures each amino acid at the small intestine and is uncapped, so it usually rates animal proteins higher and separates the best sources more clearly.

Related terms

Protein quality FAQ

What is protein quality in simple terms?

Protein quality is how useful a protein is to your body. It rates whether a food has all nine essential amino acids in the right amounts and how much you actually digest and absorb. Egg, whey, and milk are high quality; most single plant proteins are lower.

How is protein quality measured?

Protein quality is scored by comparing a food's essential amino acid content, corrected for digestibility, against human requirements. The main systems are DIAAS, the FAO-preferred method that measures absorption at the small intestine, and PDCAAS, the older method still used on food labels and capped at 1.0.

What is the difference between PDCAAS and DIAAS?

PDCAAS multiplies the limiting amino acid score by faecal digestibility and caps the result at 1.0. DIAAS measures the true digestibility of each amino acid at the end of the small intestine and is not capped, so it separates top proteins better and usually rates animal sources higher.

What is the highest quality protein?

Egg, whey, casein, milk, and lean meats score highest on both PDCAAS and DIAAS. Egg protein is often the reference standard with a biological value near 100, while casein, pork, and egg exceed a DIAAS of 100, the FAO threshold for an excellent-quality protein.

Are plant proteins lower quality than animal proteins?

Yes, per gram, because most single plant proteins are short on one essential amino acid and digest less completely. Grains lack lysine and legumes lack methionine. Soy is an exception that scores as high quality, and combining plant foods raises the overall quality substantially.

What makes a protein high quality?

Three things: a full set of essential amino acids in proportions matching human needs, a high leucine content to trigger muscle protein synthesis, and high digestibility so the amino acids are actually absorbed. When all three are present, as in egg or whey, the protein scores near the top.

What is a limiting amino acid?

A limiting amino acid is the essential amino acid present in the smallest amount relative to what your body needs. It caps how much of a protein you can use, because synthesis stops when the scarcest amino acid runs out. Lysine limits grains; methionine limits legumes.

Can you improve the quality of plant protein?

Yes. Combine complementary sources so their limiting amino acids cover each other, such as rice with beans or a pea and rice protein blend. Soaking, sprouting, and cooking cut anti-nutrients and raise digestibility, and eating slightly more total protein offsets the lower per-gram quality.

Does protein quality matter for building muscle?

It does. Higher-quality proteins deliver more usable essential amino acids and more leucine per gram, producing a stronger muscle-building response per meal. That is why 20 to 40 grams of whey or egg outperforms the same weight of a low-quality plant protein for muscle repair.

Do I need to combine proteins at every meal?

No. Healthy adults keep a free amino acid pool for several hours, so eating a variety of protein sources across the day covers your essential amino acid needs. Combining complementary proteins within one meal helps on a strict plant-based diet but is not a strict rule.

References

1. Protein quality. Wikipedia
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 Sci Nutr, 2020. PMC7590266
3. Mathai JK, Liu Y, Stein HH. Values for digestible indispensable amino acid scores (DIAAS) for some dairy and plant proteins may better describe protein quality than values calculated using PDCAAS. Br J Nutr, 2017. PubMed 28382889
4. Protein quality, nutrition and health. Front Nutr, 2024
5. Jäger R, et al. International Society of Sports Nutrition Position Stand: protein and exercise. J Int Soc Sports Nutr, 2017. PMC5568273
6. Marinangeli CPF, House JD. Potential impact of the digestible indispensable amino acid score as a measure of protein quality on dietary regulations and health. Nutr Rev, 2017. PMC5914309
7. Biochemistry, Essential Amino Acids. StatPearls, NCBI Bookshelf