What are three ways of assessing protein quality?

October 11, 2025 •

3 min read

Did you know that not all protein is created equal? While the total amount of protein on a nutrition label is important, assessing protein quality based on amino acid content and digestibility is crucial for human health. The three ways of assessing protein quality detailed below provide a deeper look into a protein source's true value.

Quick Summary

DIAAS, PDCAAS, and Biological Value are key methods for evaluating a protein's nutritional quality. These approaches measure amino acid composition and absorption efficiency to determine true dietary value.

Key Points

PDCAAS (Protein Digestibility Corrected Amino Acid Score): An older method that compares a protein's amino acid profile to a child's needs and adjusts for fecal digestibility.
DIAAS (Digestible Indispensable Amino Acid Score): The current gold standard, endorsed by the FAO, that uses more accurate ileal digestibility for each individual amino acid.
Biological Value (BV): A historical measure of how much absorbed protein is retained and used by the body, based on nitrogen balance.
DIAAS is superior to PDCAAS because it measures digestibility in the small intestine, considers individual amino acids, and doesn't cap its score at 1.0, giving a more accurate protein ranking.
Protein digestibility is key: A protein's amino acid profile is only useful if those amino acids can be properly broken down and absorbed by the body.

Introduction to Protein Quality Assessment

Protein is a fundamental macronutrient, but the biological value and utility of protein vary widely depending on its source. Not all proteins contain the same balance and quantity of essential amino acids (EAAs), which are the building blocks that the body cannot produce on its own. The body’s ability to digest and absorb these amino acids also differs significantly across food sources. These two factors—amino acid profile and digestibility—are the cornerstones of protein quality assessment. To accurately measure a food's protein value, particularly for regulatory and labeling purposes, food scientists rely on several key methodologies. The three primary methods used for assessing protein quality are the Protein Digestibility Corrected Amino Acid Score (PDCAAS), the Digestible Indispensable Amino Acid Score (DIAAS), and Biological Value (BV).

The Protein Digestibility Corrected Amino Acid Score (PDCAAS)

The PDCAAS method, recommended by the FAO and WHO in 1989, became a widely used standard for determining protein quality. It involves calculating an Amino Acid Score by comparing the essential amino acid content to a reference pattern, typically based on the needs of young children. This score is then adjusted by a figure representing fecal true digestibility, often measured in rats.

Limitations of PDCAAS

PDCAAS has several limitations, including its score being capped at 1.0, which prevents differentiation between proteins of exceptionally high quality. Additionally, using fecal digestibility can overestimate the amino acids truly available to the body as it includes amino acids used by gut bacteria.

The Digestible Indispensable Amino Acid Score (DIAAS)

Proposed in 2013 by the FAO, DIAAS is considered a more accurate method for assessing protein quality and an improvement over PDCAAS. DIAAS measures digestibility at the ileum (end of the small intestine), providing a more precise value for absorbed amino acids. It also accounts for the digestibility of each individual amino acid and uses age-specific reference patterns. Unlike PDCAAS, the DIAAS score is not capped, allowing for better ranking of high-quality proteins.

Biological Value (BV)

Biological Value is an older method that assesses how efficiently absorbed protein is used by the body, based on nitrogen retention. It compares the amount of nitrogen ingested to the amount excreted to determine the percentage of absorbed protein retained.

BV’s Limitations

BV has limitations as it doesn't account for protein digestibility and can be influenced by total protein intake levels. Testing is often done under fasting conditions, which may not reflect typical dietary intake.

Comparison of Protein Quality Assessment Methods


Feature	PDCAAS	DIAAS	Biological Value (BV)
Digestibility Measurement	Fecal digestibility of total protein (often in rats).	Ileal digestibility of individual amino acids (in humans or pigs).	Overall nitrogen retention from absorbed protein.
Amino Acid Basis	Comparison to a general reference pattern for preschool children.	Comparison to age-specific reference patterns, accounting for each EAA.	Efficiency of nitrogen retention, not specific amino acid balance.
Scoring Cap	Capped at 1.0, limiting differentiation of high-quality proteins.	Not capped, allowing for more precise ranking of superior proteins.	Uncapped, with scores theoretically up to 100%, but not considering all factors.
Test Subject	Typically relies on rodent assays.	Preferably uses human or pig studies for better relevance to human digestion.	Historically used animal studies, but also human balance studies.
Accuracy	Prone to overestimation due to fecal measurement and capped score.	Considered the most accurate and reliable method, as endorsed by the FAO.	Less reliable for real-world dietary applications due to methodological issues.

The Evolution and Future of Protein Assessment

The progression from BV to PDCAAS and now DIAAS reflects advancements in nutritional science. DIAAS offers the most accurate assessment by using ileal digestibility and evaluating individual amino acids, providing better information for consumers and a superior tool for food formulation. Ongoing research continues to improve protein assessment methods. Learn more about the recommendations for DIAAS in human nutrition from the FAO.

Conclusion

Assessing protein quality involves considering both the amino acid profile and digestibility. PDCAAS, DIAAS, and BV are the primary methods used. DIAAS is currently considered the most accurate, improving upon the limitations of PDCAAS and the historical BV method. Understanding these methods helps in making informed decisions about dietary protein for optimal health.

Frequently Asked Questions

The main difference is the point of measurement for digestibility. DIAAS uses ileal digestibility (end of the small intestine), which is more accurate. PDCAAS uses fecal digestibility, which can overestimate absorption.

DIAAS is more accurate because it measures the digestibility of each individual indispensable amino acid at the ileum and does not truncate the final score at 1.0. This allows for a more precise ranking of high-quality protein sources.

While historically significant, BV is less commonly used today. It has been largely replaced by more comprehensive methods like PDCAAS and DIAAS because BV does not account for the food's digestibility.

Yes. This concept is known as protein complementation. By combining two or more plant proteins with different limiting amino acids, you can create a more complete amino acid profile, thereby increasing the protein's overall quality.

A limiting amino acid is an essential amino acid present in the lowest quantity relative to the body's requirements. This amino acid can limit the body's ability to synthesize new proteins.

Animal-based proteins, such as whey, casein, eggs, and meat, are generally considered higher quality. They typically have a complete amino acid profile and high digestibility, leading to high PDCAAS and DIAAS scores.

Ileal digestibility is more accurate because it measures the amino acids absorbed in the small intestine, where most absorption occurs. Fecal measurements can be contaminated by bacterial nitrogen, leading to an overestimation of the protein's value to the body.