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The quality of protein is evaluated based on the content of essential amino acids (EAA), leucine content, and digestibility/bioavailability. In practice, the following indices are used: BV (biological value based on nitrogen balance), AAS (amino acid score without digestibility), PDCAAS (AAS × digestibility, but capped at 1.0) and, today, most accurately, DIAAS (digestibility of amino acids in the small intestine, without capping).
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These indices evaluate isolated sources, but in reality, we eat mixed meals. If your total daily protein intake is adequate and your diet is varied (i.e., includes a range of protein sources that complement each other), then small differences in quality usually don’t play a major role, even for vegans.
How we measure protein quality
Protein synthesis is a key biological process driving adaptations to training—from muscle growth and strength gains to a range of structural and functional changes in the body. How effectively a protein supports this process depends not only on the amount consumed (in grams), but also on the quality of the protein source.
In practice, protein quality is usually determined by three main factors:
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Content of essential amino acids (EAA) - these are amino acids that the body cannot produce on its own and must obtain through diet.
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Leucine content– leucine is often viewed as the “trigger” amino acid that initiates the anabolic response.
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Bioavailability– this refers to how much of the ingested amino acids are actually available for protein synthesis after digestion and absorption.
These three factors together determine how effective a protein source is at stimulating protein synthesis.
The key question is: how do we measure these properties accurately enough to distinguish between "low" and "high" quality proteins?
Below you will find an overview of the methods used to assess protein quality.
1) Biological value (BV) – “how much nitrogen remains in the body”
BV is based on nitrogen balance—that is, how much of the ingested amino acids (a source of nitrogen) is incorporated into body tissues. It’s measured on a scale from 0 to 100, with a higher number indicating a higher quality protein. Historically, egg protein was considered the gold standard with a BV of 100, although we now know it may not actually be the highest‑quality protein. Some sources, such as whey protein, can even score above 100 (e.g. BV 104).
However, BV has its limitations. It mainly considers how much nitrogen is ingested versus excreted.
It does not take into account digestibility, nor how effectively the body uses the amino acids.
2) Amino Acid Score (AAS) – does not take digestibility into account
AAS is expressed on a scale from 0 to 1, and for high‑quality proteins, it can even exceed 1 (again: higher = better quality).
This score compares the ratio of essential amino acids in a given protein to those in a reference protein defined by the WHO (initially in 1985, with an updated standard from 2007). The reference pattern is based on the nutritional needs of children aged 1–3 years, as they have the highest amino acid requirements (adults require slightly less).
The resulting score is determined by the“limiting amino acid”—that is, the essential amino acid in the shortest supply relative to the reference.
However, AAS does not consider digestibility, meaning it doesn't account for how much of the protein is actually absorbed and utilised by the body.
3) PDCAAS – AAS × (faecal) digestibility
PDCAAS builds upon AAS by including a measure of digestibility. It has been used by the WHO since 1993 and was, for many years, the most widely adopted method, though it is increasingly being replaced by DIAAS today.
What does it add compared to AAS?
In addition to the ratio of essential amino acids (AAS), PDCAAS also takes into account protein digestibility throughout the entire digestive tract.
The score is defined from 0 to 1 (higher = better usability). For some proteins, it can be calculated as >1, but in practice, the value is capped at 1 because it is assumed that “above 1” does not confer additional benefits.
Why can PDCAAS be misleading?
Precisely because of the 1.0 cap, some sources may appear “almost the same.” For example, in the table, soy protein also scores very high, although differences between sources may be masked by this.
4) DIAAS – the current gold standard
Because of the limitations in how digestibility is assessed in PDCAAS, a newer index was proposed in 2013: DIAAS.
How is DIAAS different?
DIAAS evaluates digestibility only in the small intestine. This matters because amino acids absorbed here are considered available for constructive metabolism—that is, they can be used to build body proteins.
Unlike PDCAAS, DIAAS is not capped at an upper limit of 1.0, which allows it to better distinguish between high‑quality protein sources. For example, using PDCAAS, whey protein isolate and soy isolate appear almost identical. With DIAAS, however, the difference is much clearer (approximately 1.09 vs. 0.90). This helps explain why soy protein, despite being labelled “complete” under PDCAAS, does not consistently perform as well as whey protein in research studies.
Complete vs. incomplete protein
When discussing protein quality (using BV, AAS, PDCAAS, or DIAAS), you’ll often come across the simplified distinction between complete and incompleteproteins. These terms are common in popular science writing because they’re easy to understand—but it’s important to know what they mean, and what they don’t.
1) What is a “complete” protein
A complete protein is one that contains all essential amino acids (EAA) in sufficient amounts (in the ratios the body needs for building its own proteins).
A sufficient amount does not just mean they are present, but that there is relatively enough compared to the reference requirement (hence the concept of a reference protein in AAS/PDCAAS/DIAAS).
Typically, most animal‑based proteins are considered complete, including:
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whey, milk, yoghurt, cottage cheese
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eggs
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meat, fish
Among plant sources, the picture is more mixed. Some (e.g., soy) are also considered complete. However, even when plant proteins contain all EAAs, they may have a less favourable ratio or lower digestibility, which is reflected in metrics like DIAAS.
2) What is an “incomplete” protein
An incomplete protein is one that is relatively deficient in one or more essential amino acids. In practical terms, this means that protein synthesis is limited by the amino acid present in the smallest amount.
This brings us to a concept you already know from AAS:
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Limiting amino acid = the EAA that is most “undersupplied” compared to the reference. And it is this one that determines how efficiently a given source can be used for protein synthesis.
In practice, this most often applies to plant‑based proteins, where certain patterns are commonly seen:
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Cereals (wheat, rice, maize, oats)tend to be relatively low in lysine.
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Legumes (lentils, beans, chickpeas)tend to be relatively low in methionine (and sometimes cysteine).
Are plant proteins worthless, then?
No. Plant proteins are normally counted and the body can use them. The difference between “higher quality” and “lower quality” protein is most apparent when you eat small to medium protein servings or when you have a generally low protein intake during the day.
Gram for gram – is plant protein weaker?
Yes and no. Let’s compare two 10g servings:
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10g of “lower quality” protein (often from plant sources) may have fewer EAAs and less leucine. In small doses, your body might not be able to “extract” the full potential for muscle repair and growth.
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10g of “high‑quality” protein (e.g., whey, dairy, eggs) typically has a better EAA profile, more leucine, and higher digestibility. As a result, even a small portion is more likely to trigger a stronger muscle protein synthesis response.
But this doesn’t mean plant protein is ineffective. It just means that small servings may be less potent.
Can this be compensated for? Yes – with a larger portion or smarter composition
If you eat more of a less “high‑quality” protein, you often make up for its weaknesses. You simply provide enough EAAs and leucine, even from a lower‑quality source.
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instead of a small portion of plant protein, you have a larger portion
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or you combine sources (e.g., legumes + cereals)
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or you increase your total protein intake for the whole day—at least 1.6g/kg of body weight
If your intake is in the moderate to high range (around 1.6–2.2g/kg/day), then the differences between protein sources for supporting muscle adaptations often diminish.
Similarly, distributing protein across more meals helps: ~0.4g/kg per meal (typically 3–5 servings per day) is a practical guideline.
Vegans can benefit from high‑quality instant plant‑based protein powders, which are of high quality and good usability.
How to properly combine plant proteins?
For best results, it is good to combine different plant protein sources so they complement each other in amino acids (especially those that one source lacks).
Most often, legumes (lentils, beans, peas, chickpeas) are combined with cereals (rice, wheat, bread, tortilla, pita). Adding seeds and nuts also works great, further improving the overall amino acid profile.
You do not have to combine proteins in every meal.
A common myth is that sources must be combined in every meal.
However, the body has a sufficient amount of amino acids available during the day (the amino acid pool), so it is sufficient if suitable protein sources are included throughout the day—they do not necessarily have to be in one specific meal.
This is also supported by findings from studies, which show no difference in results when combining sources at every meal vs. only throughout the day.
Bottom line
Protein quality is a frequently discussed topic today, and several indices aim to quantify it. The most accurate at present is DIAAS, as it better reflects the number of amino acids actually absorbed. But it is important not to forget that this is an assessment of an isolated source—a single food. In practice, however, we eat complex meals in which different protein sources are naturally combined, and thus their “weaknesses” in the amino acid profile can complement each other.
You will also often hear claims that plant proteins are not complete and that their intake “does not count.” This is unnecessarily simplified. Yes, some plant sources may have lower scores according to quality indicators (typically due to a limiting amino acid or poorer digestibility), but that does not mean they are useless. If you consume enough protein during the day and combine sources sensibly, your body will assemble the necessary amino acids, and there is no reason to worry about plant proteins.
