Protein Digestibility and Quality: What the Scores Actually Mean

Two foods can contain the same number of grams of protein without providing exactly the same amount of usable indispensable amino acids.

That is the problem protein-quality systems try to describe.

Protein quality is influenced by two related factors:

• the pattern of indispensable amino acids in the protein
• how well those amino acids are digested and made available

This is different from simply asking whether a food contains protein or whether its protein is "complete."

Protein amount and protein quality are different measurements

A nutrition label might tell you that a food contains 20 g of protein.

That number describes quantity.

It does not by itself tell you:

• the proportions of individual indispensable amino acids
• which indispensable amino acid is present in the lowest proportion relative to human requirements
• how much of those amino acids is digestible

Protein-quality methods attempt to add that information.

This does not mean the protein number on the label is useless. It means that protein quantity and protein quality answer different questions.

What does protein digestibility mean?

Digestibility describes the proportion of dietary protein or its amino acids that is digested and absorbed from the gastrointestinal tract.

Protein digestibility can differ because of:

• the structure of the food
• processing
• the surrounding food matrix
• fibre and certain antinutritional factors
• heat-induced chemical changes
• the particular protein source

Digestibility is therefore one reason that two foods containing the same nominal amount of protein may not provide identical amounts of digestible amino acids.

What does "protein quality" measure?

In protein-quality assessment, a protein is compared with a reference pattern of indispensable amino-acid requirements. (EFSA)

If one indispensable amino acid is present in a relatively low amount, it can become the limiting amino acid for that protein.

Quality scoring therefore considers more than total protein.

It asks how well the protein's indispensable amino-acid supply, after accounting for digestibility, matches a reference requirement pattern.

What is PDCAAS?

PDCAAS stands for Protein Digestibility-Corrected Amino Acid Score.

The method combines:

1. the amino-acid score of the protein
2. a measure of overall protein digestibility

In simplified terms, the amino-acid pattern is compared with a reference pattern and then corrected for digestibility.

PDCAAS became widely used for evaluating dietary protein quality and remains relevant in nutrition regulation and food assessment.

Why can PDCAAS reach 1.0 or 100%?

Traditionally, PDCAAS values are truncated at 1.0, or 100%.

That means proteins that would otherwise score above 100 are reported as 100.

As a result, two proteins can both receive the maximum truncated PDCAAS even if one supplies a larger amount of digestible indispensable amino acids beyond the reference pattern.

This truncation is one of the limitations that prompted work on newer methods.

What is DIAAS?

DIAAS stands for Digestible Indispensable Amino Acid Score.

FAO proposed DIAAS as a more advanced approach to evaluating protein quality.

Instead of applying one overall protein-digestibility value, DIAAS considers the digestibility of individual indispensable amino acids.

It uses amino-acid digestibility measured at the end of the small intestine — the terminal ileum — rather than relying on faecal crude-protein digestibility.

This distinction is important because amino acids that disappear or are transformed later in the large intestine do not necessarily represent amino acids available to the body in the same way.

PDCAAS vs DIAAS

The main differences can be summarized like this:

DIAAS addresses several recognized limitations of PDCAAS, but PDCAAS has not simply disappeared from practical or regulatory use.

What does a higher DIAAS mean?

A higher DIAAS indicates that a protein supplies a stronger combination of digestible indispensable amino acids relative to the reference amino-acid pattern used for the calculation.

For an individual food or ingredient, a score above 100 can indicate that the protein has enough digestible indispensable amino acids to potentially complement another protein with a lower score.

It should not be interpreted as meaning that the food contains "more than 100% protein."

The score describes protein quality relative to a reference pattern, not the percentage of the food that is protein.

The reference pattern matters

Protein-quality scores are not completely context-free numbers.

The calculation compares the food with an amino-acid reference pattern representing requirements.

Different life stages have different amino-acid requirements.

This is one reason a protein-quality score should be understood as a nutritional assessment method rather than a permanent universal ranking of foods.

Why individual amino acids matter

Proteins are built from amino acids.

Some amino acids must be supplied by the diet and are described as indispensable or essential amino acids.

A food may contain substantial total protein while providing one indispensable amino acid in a relatively low proportion.

That amino acid can limit the score.

This is related to, but not identical with, the everyday concept of complete and incomplete proteins.

For that topic specifically, see Complete vs Incomplete Proteins: What's the Difference?.

Does a lower score mean the protein is useless?

No.

A protein-quality score evaluates a specific characteristic of a protein. It does not determine the overall nutritional value of the food.

A food with a lower score can still contribute:

• protein
• indispensable amino acids
• energy
• fibre
• vitamins
• minerals
• other useful nutrients

Foods are eaten as part of diets, not as isolated laboratory protein samples.

That distinction becomes especially important when several protein sources are eaten across the day.

Protein complementation

Different proteins can have different limiting amino acids.

Combining or simply eating a variety of protein-containing foods can produce an overall amino-acid supply that is more balanced than the score of one isolated food might suggest.

For example, cereal proteins and legume proteins often have different amino-acid strengths and limitations.

This does not mean every meal must contain a carefully engineered combination.

For most people eating a varied diet with adequate total protein, protein quality should be considered in the context of the entire diet rather than one ingredient in isolation.

Processing can affect protein quality

Protein quality is not determined only by the original food source.

Processing can change digestibility and amino-acid availability.

Heating can improve digestibility in some foods by changing protein structure or reducing certain antinutritional factors.

But severe processing conditions can also reduce the availability of particular amino acids.

Lysine is one important example because reactions during heating can make some lysine less nutritionally available.

This is another reason protein quality cannot always be inferred perfectly from the raw ingredient alone.

Animal and plant proteins

Animal-source proteins often have high digestibility and indispensable amino-acid profiles that align well with human requirements.

Some plant proteins have lower digestibility or a more limiting indispensable amino-acid pattern.

However, this is not a useful reason to divide all foods into a simple "good protein" and "bad protein" hierarchy.

Plant foods vary widely, animal foods vary, processing matters and people consume combinations of foods.

A diet can obtain adequate protein from plant foods when total intake and food selection are appropriate.

Do you need to calculate DIAAS for your meals?

For ordinary food tracking, usually not.

DIAAS and PDCAAS are useful for:

• nutrition research
• protein-quality evaluation
• food formulation
• comparing protein ingredients
• certain regulatory applications

They are not normally numbers that an individual needs to calculate for breakfast, lunch and dinner.

For day-to-day nutrition, more practical questions are usually:

• Are you consuming enough total protein?
• Are you eating a varied diet?
• Are your protein sources appropriate for your needs and preferences?
• If your diet is highly restricted, are you obtaining sufficient indispensable amino acids?

Don't multiply every protein entry by a quality score

Suppose your food log shows 25 g of protein in a meal.

It is generally not useful to take a DIAAS or PDCAAS value from somewhere online and mechanically multiply the 25 g by that score to create a new "real protein" number.

Protein-quality calculations depend on the specific food, reference pattern, measurement method and context.

Mixed diets also introduce complementation between protein sources.

For everyday tracking, record protein consistently rather than inventing an adjusted protein total from isolated scores.

When protein quality deserves more attention

Protein quality can become more relevant when:

• total protein intake is close to minimum requirements
• the diet relies heavily on a narrow range of protein sources
• food availability is limited
• a diet excludes many common protein foods
• nutrition is being formulated for specific populations or clinical purposes

Individual medical or therapeutic nutrition needs should be discussed with an appropriately qualified healthcare professional.

What the scores are useful for

Think of PDCAAS and DIAAS as tools for answering a narrow technical question:

How well does this protein supply digestible indispensable amino acids relative to a reference requirement pattern?

They do not independently answer:

• Is this food healthy?
• Is this food environmentally sustainable?
• Is this the best protein for a particular person?
• How much protein should you eat?
• Is the whole diet nutritionally adequate?

Those are broader questions requiring additional information.

Practical takeaway

For everyday nutrition, protein quality is best understood rather than constantly calculated.

Remember four points:

1. total protein grams do not describe the entire amino-acid profile
2. digestibility influences how much amino acid becomes available
3. PDCAAS and DIAAS use different methods to account for protein quality
4. the quality of one isolated protein is not the same thing as the quality of an entire diet

If you already eat enough protein from a varied selection of foods, individual protein-quality scores usually provide context rather than a daily tracking target.

Sources

Evidence and authoritative guidance used for the nutrition and health information in this article:

• EFSA — Dietary Reference Values for protein