What is the best indicator for protein quality? Exploring Modern Metrics

October 12, 2025 •

3 min read

Over 700 million people globally were undernourished in 2023, partly due to low-quality protein sources in their diets. To determine a food's nutritional value, understanding what is the best indicator for protein quality? is crucial, as it goes beyond simply counting grams of protein.

Quick Summary

The digestible indispensable amino acid score (DIAAS) is the most accurate protein quality metric, recommended by the FAO to replace the outdated PDCAAS. DIAAS measures individual amino acid digestibility and does not truncate high scores, providing a more reliable evaluation of a protein's contribution to human nutritional needs.

Key Points

DIAAS is the gold standard: The Digestible Indispensable Amino Acid Score (DIAAS) is the most accurate and current method for measuring protein quality, recommended by the Food and Agriculture Organization (FAO).
PDCAAS is outdated: The older Protein Digestibility-Corrected Amino Acid Score (PDCAAS) is less accurate, largely due to its reliance on less precise fecal digestibility and the truncation of high scores at 1.0.
Ileal vs. Fecal Digestibility: DIAAS uses ileal (small intestine) digestibility measurements, providing a more accurate assessment of the amino acids the body actually absorbs, unlike the less reliable fecal measurements used by PDCAAS.
Non-truncated scores: Unlike PDCAAS, DIAAS scores are not capped, allowing for meaningful comparisons and ranking among high-quality proteins.
Amino acid profile matters: The quality of a protein depends on its amino acid profile, particularly the balance and quantity of its indispensable amino acids (IAAs).
Food matrix effects: A protein's digestibility is influenced by other components in the food, such as fiber and antinutritional factors.

The Importance of Protein Quality

Protein, a vital macronutrient, is essential for growth, tissue repair, immune function, and various metabolic processes. It is composed of amino acids, including nine indispensable amino acids (IAAs) that must be obtained from food. A protein's quality reflects its ability to supply these IAAs in the correct amounts and proportions. Animal-based foods like meat, eggs, and dairy generally offer high-quality or "complete" proteins, while many plant-based sources lack sufficient levels of one or more IAAs.

The Rise and Limitations of PDCAAS

The Protein Digestibility-Corrected Amino Acid Score (PDCAAS) was the standard for measuring protein quality from the early 1990s until 2013. This method compared a protein's amino acid profile to a reference pattern and adjusted for digestibility based on fecal measurements. However, PDCAAS had notable limitations, including capping scores at 1.0 (100%), which prevented differentiation among high-quality proteins. It also used less accurate fecal digestibility, potentially overestimating absorption due to bacterial activity, and its reference pattern was based on the high protein needs of young children, which might not be suitable for all age groups.

DIAAS: The New Gold Standard

In 2013, the Food and Agriculture Organization (FAO) recommended the Digestible Indispensable Amino Acid Score (DIAAS) as the replacement for PDCAAS. DIAAS offers a more precise evaluation by measuring the digestibility of individual IAAs at the ileum, providing a more accurate reflection of amino acids absorbed. Unlike PDCAAS, DIAAS scores are not truncated, allowing for better ranking of high-quality proteins. DIAAS also utilizes age-specific reference patterns, making it more relevant for different populations.

Comparing PDCAAS and DIAAS

This table highlights the fundamental differences between the two primary protein quality indicators.


Feature	PDCAAS	DIAAS
Digestibility Method	Measures total protein digestibility via fecal analysis.	Measures individual IAA digestibility at the ileum.
Accuracy	Prone to overestimation due to bacterial activity in the colon.	Considered the most accurate indicator available.
Score Truncation	Scores are capped at 1.0 (100%), preventing differentiation of top-tier proteins.	Scores are not truncated, allowing for better ranking and comparison.
Reference Pattern	Based on the amino acid needs of 2- to 5-year-olds.	Uses age-specific reference patterns for infants, children, and adults.
Complementarity	Fails to recognize the enhanced value of combining complementary proteins.	More accurately reflects the complementary effects of combining protein sources.

Factors Influencing a Protein's Quality

Several factors besides the scoring method influence protein quality, including how food is processed, which can affect digestibility by deactivating or damaging amino acids. The food matrix, or the overall composition of a food, can also impact protein absorption. Additionally, combining complementary plant proteins throughout the day can ensure all IAA needs are met.

Conclusion: The Gold Standard for Modern Nutrition

The Digestible Indispensable Amino Acid Score (DIAAS) has replaced older metrics like PDCAAS as the scientific gold standard for determining protein quality, offering a more accurate measure of how a protein source meets human nutritional needs. This emphasizes the value of high-quality animal proteins and the importance of strategically combining plant proteins for those following vegetarian or vegan diets. {Link: Frontiersin https://www.frontiersin.org/journals/nutrition/articles/10.3389/fnut.2024.1389719/full}

How to Improve Your Protein Quality

To enhance your dietary protein quality, consider combining different plant-based foods, optimizing processing methods like heating to improve digestibility, incorporating high-quality animal proteins, being mindful of the food matrix, and carefully reading labels as high protein content doesn't always indicate high quality.

The Evolution of Protein Quality Indicators

The shift from PDCAAS to DIAAS reflects a growing understanding of human nutrition. Using modern metrics helps consumers and dietitians make better decisions tailored to various nutritional needs {Link: Frontiersin https://www.frontiersin.org/journals/nutrition/articles/10.3389/fnut.2024.1389719/full}.

Frequently Asked Questions

DIAAS is superior because it measures amino acid digestibility at the end of the small intestine (ileal), which more accurately reflects what the body absorbs. It also does not truncate scores at 100%, allowing for clearer differentiation between high-quality proteins.

A DIAAS score indicates how well a protein source meets the body's indispensable amino acid (IAA) requirements. A score of 100% or more indicates excellent quality, while a score below 100% signifies a deficiency in at least one IAA.

Yes, some plant-based protein isolates, like soy, can achieve high scores. Furthermore, combining different plant proteins, such as legumes and grains, within a meal or over the course of a day can provide a complete and high-quality amino acid profile.

Processing can both improve and degrade protein quality. Methods like heating can increase digestibility by deactivating antinutritional factors, while excessive heating or certain chemical reactions can damage or decrease the availability of some amino acids.

Complementary proteins are two or more incomplete protein sources that, when combined, provide all the necessary indispensable amino acids. For example, eating rice (low in lysine) with beans (low in methionine) offers a complete protein meal.

Yes, DIAAS uses age-specific reference patterns to account for differing amino acid requirements, such as those for infants, children, and adults. This provides a more accurate and relevant assessment for various populations.

The 'food matrix' refers to the overall structure and composition of a food, including its fiber and other nutrients. This matrix can impact the rate and extent of protein digestion and absorption, influencing the overall availability of amino acids.