DIAAS: Protein Quality Score Explained

What is the DIAAS?

The DIAAS (Digestible Indispensable Amino Acid Score) is a scientifically recognized method for evaluating the nutritional quality of dietary proteins. It was introduced by the Food and Agriculture Organization of the United Nations (FAO) in 2013 and is now considered the gold standard for protein quality assessment.

Unlike its predecessor, the PDCAAS (Protein Digestibility Corrected Amino Acid Score), the DIAAS accounts for the digestibility of each individual essential amino acid at the level of the ileum (the final section of the small intestine). This provides a more accurate picture of how much of each amino acid the body can actually absorb and use.

How is the DIAAS Calculated?

The DIAAS value is calculated by multiplying the amount of each essential amino acid in a food by its true ileal digestibility, then dividing by the reference value for that amino acid for a specific population group.

• DIAAS = (mg of a digestible indispensable amino acid per g of protein) / (mg of the same amino acid in the reference protein)
• The lowest individual amino acid score determines the overall DIAAS of the food, reflecting the concept of the limiting amino acid.
• A DIAAS value of 1.0 or above indicates that the protein supplies all essential amino acids in sufficient quantity and digestibility to meet human requirements.

What Does the DIAAS Measure?

The DIAAS evaluates two critical aspects of protein quality:

• Amino acid profile: Does the protein contain all nine essential amino acids (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine) in adequate amounts?
• Ileal digestibility: How much of each amino acid is actually absorbed in the small intestine and made available to the body?

The use of true ileal digestibility is what differentiates the DIAAS from the PDCAAS, which relies on fecal digestibility and therefore tends to overestimate the protein quality of certain foods.

Reference Values and Age Groups

The FAO has defined specific reference amino acid patterns for different population groups:

• Infants (0–6 months): Highest requirements for essential amino acids
• Young children (6 months–3 years): Reference group used for many food evaluations
• Older children (3–10 years), adolescents (11–17 years), and adults (18+ years): Lower but age-adjusted requirements

DIAAS Values of Selected Foods

Animal-based protein sources generally achieve higher DIAAS values than plant-based sources:

• Whey protein: DIAAS ≈ 1.09 (excellent)
• Whole egg: DIAAS ≈ 1.13 (excellent)
• Milk: DIAAS ≈ 1.14 (excellent)
• Cooked soybeans: DIAAS ≈ 0.91 (good)
• Pea protein: DIAAS ≈ 0.82 (good)
• Wheat: DIAAS ≈ 0.40–0.45 (low; limiting amino acid: lysine)
• Corn: DIAAS ≈ 0.40 (low)

Importance for Nutritional Practice

The DIAAS has meaningful practical implications for various population groups:

Vegans and Vegetarians

Plant-based proteins often have lower DIAAS scores and may fall short in certain essential amino acids. However, strategic protein combining (e.g., legumes with grains) can complement amino acid profiles and help achieve a complete essential amino acid pattern throughout the day.

Athletes and Active Individuals

For muscle building and recovery, proteins with a high DIAAS are preferred, as they deliver all essential amino acids -- particularly the branched-chain amino acids (BCAAs) leucine, isoleucine, and valine -- with high bioavailability.

Children and Older Adults

These groups have elevated requirements for high-quality protein. A high DIAAS value in consumed proteins is especially important for supporting growth, maintaining muscle mass, and promoting overall health.

DIAAS vs. PDCAAS: A Comparison

The DIAAS has largely replaced the older PDCAAS as the preferred method for protein quality assessment. Key differences include:

• Measurement method: DIAAS uses ileal digestibility (more precise); PDCAAS uses fecal digestibility (less precise).
• Truncation: PDCAAS values are capped at 1.0; DIAAS allows scores above 1.0, better reflecting exceptionally high-quality proteins.
• Precision: DIAAS calculates a score for each individual amino acid; PDCAAS applies a single overall digestibility value.

Limitations of the DIAAS

Despite its advantages, the DIAAS has some limitations:

• Measuring true ileal digestibility requires complex animal studies or human studies involving ileostomy patients.
• The score does not account for non-essential amino acids or other bioactive compounds present in the protein food.
• Food processing methods (such as heating or fermentation) can alter DIAAS values and are not always standardized across studies.

References

1. FAO (Food and Agriculture Organization of the United Nations): Dietary protein quality evaluation in human nutrition. FAO Food and Nutrition Paper 92. Rome, 2013.
2. Marinangeli C.P.F., House J.D. - Potential impact of the digestible indispensable amino acid score as a measure of protein quality on dietary regulations and health. Nutrition Reviews, 2017; 75(8): 658–667.
3. Rutherfurd S.M. et al. - Protein Digestibility-Corrected Amino Acid Scores and Digestible Indispensable Amino Acid Scores Differentially Describe Protein Quality in Growing Male Rats. Journal of Nutrition, 2015; 145(2): 372–379.