What are the methods of assessing protein quality?

January 4, 2026 •

3 min read

According to the Food and Agriculture Organization, the quality of a protein is primarily determined by its essential amino acid profile and its digestibility. Understanding the methods of assessing protein quality is crucial for making informed dietary choices, developing nutritional products, and ensuring adequate nutrition for various populations.

Quick Summary

This guide examines the various methodologies used to evaluate the nutritional value of dietary proteins, from traditional animal-based bioassays to advanced modern chemical scoring systems. It details the principles behind key assessment methods like DIAAS, PDCAAS, Biological Value, and the Chemical Score, highlighting their respective advantages, disadvantages, and applications in the fields of food science and human nutrition.

Key Points

DIAAS is the preferred method: The Digestible Indispensable Amino Acid Score (DIAAS) is now considered the most accurate method for assessing protein quality, replacing the older PDCAAS standard.
Digestibility is key: A protein's quality is determined by both its amino acid composition and its digestibility; a protein can be rich in essential amino acids but still low quality if it's poorly absorbed.
PDCAAS has limitations: The Protein Digestibility-Corrected Amino Acid Score (PDCAAS) is criticized for capping high scores at 1.0 and for relying on less accurate fecal digestibility data.
In vitro methods are useful for screening: Lab-based digestion models mimic human digestion for efficient testing and comparison of different protein sources and processing techniques.
Biological value measures retention: The Biological Value (BV) quantifies the proportion of absorbed protein retained by the body, with higher values indicating more efficient utilization.
Animal models are outdated: Older bioassays like the Protein Efficiency Ratio (PER), which used rats, are no longer considered reliable indicators for human nutrition due to species differences and methodological flaws.

The biological value of a food, which measures the proportion of absorbed protein that is incorporated into the body's proteins, can be significantly influenced by its amino acid profile and digestibility. For individuals, particularly those with specific dietary needs such as vegetarians, vegans, or athletes, understanding the various methods for assessing protein quality is essential to ensure a balanced and effective intake. Protein quality assessment has evolved considerably, moving from more rudimentary animal-based tests to sophisticated chemical scores that better reflect human nutritional requirements.

Bioassays: Animal-Based Methods

Historically, protein quality was often assessed using animal models. Two prominent methods were the Protein Efficiency Ratio (PER) and the Biological Value (BV).

Protein Efficiency Ratio (PER)

The PER measures protein quality based on weight gain relative to protein consumed in test subjects, typically weanling rats. Its limitation is that it doesn't distinguish between lean mass and fat gain, and the results have limited relevance for humans due to physiological differences.

Biological Value (BV)

Biological Value measures the proportion of absorbed protein that is retained and utilized by the body, based on nitrogen retention. Egg protein has a BV of 100 as a reference. A key limitation is that it doesn't account for pre-absorption digestion or the complexity of mixed diets.

Chemical Scoring: Modern Human-Centric Approaches

Modern methods combine chemical analysis of amino acids with digestibility data to provide more accurate scores for human nutrition.

Protein Digestibility-Corrected Amino Acid Score (PDCAAS)

Adopted by the FDA in 1993, PDCAAS considers both the essential amino acid profile relative to human requirements (Amino Acid Score, AAS) and a correction for fecal digestibility. A major limitation is that scores are capped at 1.0, preventing differentiation among higher quality proteins, and fecal digestibility can over-estimate true absorption.

Digestible Indispensable Amino Acid Score (DIAAS)

Recommended by the FAO in 2013, DIAAS is considered more accurate than PDCAAS. It measures the true ileal digestibility of individual indispensable amino acids, providing a better reflection of what the body actually absorbs. DIAAS scores are not capped, allowing for better ranking of high-quality proteins, and it uses age-specific amino acid requirements.

Comparison of Protein Quality Assessment Methods


Feature	Biological Value (BV)	Protein Efficiency Ratio (PER)	PDCAAS	DIAAS
Basis of Assessment	Nitrogen retained from absorbed protein.	Weight gain relative to protein intake in rats.	Amino acid score corrected by fecal digestibility.	Individual indispensable amino acid score corrected by ileal digestibility.
Reference Standard	Egg protein (BV=100).	Casein (PER=2.5).	Human amino acid requirements for 2–5 year-olds.	Age-specific human amino acid requirements.
Key Metric	Efficiency of absorbed nitrogen utilization.	Growth promotion in animal models.	Limiting amino acid availability adjusted for fecal digestibility.	Limiting digestible indispensable amino acid availability based on ileal sampling.
Accuracy	Prone to inaccuracy, especially at higher intake levels and for mixed diets.	Less applicable to human nutrition due to species differences.	Potential for overestimation due to fecal digestibility measurement.	Considered the most accurate and precise method for humans.
Scoring Range	Theoretical max 100, but can be over with protein combining.	Relative to casein control; can exceed 2.7.	Capped at 1.0; prevents differentiation among very high-quality proteins.	Not capped at 100%; allows for better ranking of high-quality proteins.

Advanced and In Vitro Methods

Other methods used include in vitro digestion models, which mimic human digestion in a lab, and mass spectrometry for detailed protein analysis. Nitrogen balance studies are also used but are more laborious.

Conclusion

Protein quality assessment methods have advanced significantly, moving from animal bioassays to more accurate chemical scoring systems like DIAAS. While PDCAAS was a standard for many years, DIAAS provides a more precise measure of digestibility and a less limited scoring system, making it the preferred method for assessing protein quality and comparing different protein sources. The choice of method depends on the application, from rapid lab screening to in-depth research. Understanding these methods is essential for interpreting nutritional information and making informed dietary choices.

Frequently Asked Questions

The main difference lies in their measurement of digestibility. PDCAAS uses less accurate fecal digestibility, while DIAAS measures true ileal digestibility, which more accurately reflects amino acid absorption. DIAAS also does not cap scores at 100%, allowing for better differentiation among high-quality proteins.

Vegetarians and vegans often rely on plant-based proteins, which can have lower scores on some assessment methods like DIAAS due to lower digestibility and potentially lacking some essential amino acids. Understanding protein quality helps combine different plant sources to ensure a complete and balanced amino acid profile.

The limiting amino acid is the essential amino acid found in the smallest amount in a food relative to human requirements. The overall protein quality score for methods like PDCAAS and DIAAS is limited by this particular amino acid, as it restricts the body's ability to synthesize new proteins.

In vitro methods, such as the internationally standardized INFOGEST model, mimic human digestion in a lab setting using enzymes. They are used to measure protein solubilization and the creation of peptides, allowing for efficient comparison of different food samples and processing effects.

Generally, high-quality proteins contain a well-balanced array of essential amino acids and are highly digestible. Examples include whey protein, casein, egg, and soy protein isolate. These sources often have high DIAAS scores and are efficiently used by the body.

Yes, complementary protein combining can improve overall protein quality. By mixing different sources, such as legumes and grains, which have different limiting amino acids, you can create a more complete protein profile that meets all essential amino acid needs.

The PER method, which measured weight gain in rats, has been largely replaced because it is not considered reliable for human nutrition. It does not account for differences in human metabolism and cannot distinguish between fat and lean body mass gain.