What Is the Best Way to Determine Protein Quality? Understanding the Science Behind Your Diet

October 20, 2025 •

3 min read

Protein quality isn't just about the quantity of protein in your food; it's also about the bioavailability of its essential amino acids. Learning what is the best way to determine protein quality is essential for optimizing nutrition, especially as dietary trends shift towards more plant-based sources.

Quick Summary

Protein quality evaluation has advanced from older methods like PDCAAS to the more accurate Digestible Indispensable Amino Acid Score (DIAAS), which accounts for individual essential amino acid absorption and bioavailability.

Key Points

DIAAS is the Modern Standard: The Digestible Indispensable Amino Acid Score (DIAAS) is the most accurate method for determining protein quality, surpassing the older PDCAAS by measuring true ileal digestibility and providing uncapped scores.
Completeness and Digestibility Matter: Protein quality depends on both the essential amino acid (EAA) profile and the digestibility of those amino acids. High-quality proteins provide all nine EAAs and are easily absorbed.
PDCAAS Has Limitations: The Protein Digestibility Corrected Amino Acid Score (PDCAAS) is less accurate due to capped scores (maximum 1.0) and the use of overall fecal digestibility, which can overestimate absorption.
Processing Can Affect Quality: The bioavailability of amino acids can be altered by food processing methods, antinutritional factors in plant foods, and the overall food matrix.
Combine Incomplete Proteins Strategically: Vegetarians and vegans can achieve adequate EAA intake by consuming a variety of complementary plant protein sources throughout the day, rather than needing to combine them in a single meal.
Choose High-Quality Sources for Specific Needs: Individuals with higher protein needs, like older adults and athletes, can benefit from prioritizing high-DIAAS protein sources to maximize the anabolic response.

Protein is a cornerstone of human nutrition, playing a vital role in building and repairing tissues, synthesizing hormones, and supporting immune function. The 'quality' of protein is determined by its amino acid profile and its digestibility. Essential amino acids (EAAs) are crucial because the body cannot produce them and they must come from food. High-quality protein sources contain all nine EAAs in sufficient amounts, and these amino acids are readily absorbed by the body.

The Evolution of Protein Quality Assessment

Early methods for evaluating protein quality, such as Biological Value (BV) and Protein Efficiency Ratio (PER), had limitations. BV didn't consider digestibility, while PER, based on rat growth, wasn't a reliable indicator for human metabolism. These methods paved the way for more advanced systems.

The Rise and Limitations of PDCAAS

Introduced in 1991, the Protein Digestibility Corrected Amino Acid Score (PDCAAS) assesses protein quality using the amino acid score (AAS) and fecal digestibility. The AAS compares a protein's EAA profile to a reference pattern for young children, and this is adjusted by overall fecal digestibility, often measured in rats. However, PDCAAS caps scores at 1.0, which means proteins of different quality levels can receive the same score. Additionally, using fecal digestibility can overestimate actual amino acid absorption due to bacterial activity in the large intestine.

Introducing the Gold Standard: DIAAS

Proposed in 2013, the Digestible Indispensable Amino Acid Score (DIAAS) is now considered a more accurate method for determining protein quality. DIAAS improves upon PDCAAS by measuring digestibility in the ileum, the part of the small intestine where amino acids are absorbed, providing a more accurate measure of what's available to the body. Unlike PDCAAS, DIAAS does not cap scores, allowing for a better comparison of high-quality proteins. It also assesses the digestibility of each EAA individually.

Comparison of PDCAAS and DIAAS


Feature	PDCAAS	DIAAS
Digestibility Measurement	Measures overall fecal digestibility	Measures individual amino acid ileal digestibility
Score Truncation	Scores are capped at 1.0, limiting differentiation	Scores are not capped, allowing for more accurate comparisons
Age Reference	Uses a single reference pattern for 2-5-year-olds	Provides age-specific reference patterns for different needs
Methodology	Less accurate due to fecal sampling and general protein digestibility	More precise, reflecting true amino acid absorption
Applicability	Still used for some regulatory labeling	Recommended as the gold standard by FAO

Practical Factors Influencing Protein Quality

Food processing methods, like heating, can impact protein structure and bioavailability. Antinutritional factors (ANFs) found in some plant foods can also interfere with protein digestion, although cooking can often reduce their effects. The overall composition of a meal, or the food matrix, can also influence how protein is absorbed.

The Role of Complete vs. Incomplete Proteins

Complete proteins contain all nine essential amino acids in sufficient quantities. Examples include most animal products and some plant-based options like soy, quinoa, and buckwheat. Incomplete proteins lack one or more EAAs; most plant sources fall into this category.

Combining incomplete proteins in every meal isn't necessary. The body utilizes an amino acid pool, so consuming a variety of protein sources throughout the day ensures you meet your EAA needs.

Putting Protein Quality to Use in Your Diet

For individuals with higher protein requirements, such as athletes or older adults, or those following vegetarian or vegan diets, considering protein quality is especially important. Using DIAAS scores can help guide the selection of high-quality protein sources to support muscle health. A diverse intake of protein sources is a practical way to ensure adequate EAA availability.

Conclusion

Determining protein quality has evolved significantly. The Digestible Indispensable Amino Acid Score (DIAAS) is currently considered the most reliable scientific method because it accurately measures amino acid digestibility in the small intestine and provides uncapped scores. While DIAAS may not be widely used on food labels yet, understanding its principles can inform dietary choices. Consuming a variety of protein sources, including both complete and complementary incomplete proteins, is a practical approach to ensure optimal protein intake and support overall health.

Learn more about protein sources

Frequently Asked Questions

Essential amino acids (EAAs) are nine amino acids that the human body cannot produce on its own and must obtain from food. Protein quality is largely determined by a food's EAA profile and how readily the body can digest and absorb them for processes like muscle repair and immune function.

DIAAS is more accurate than PDCAAS because it measures amino acid digestibility at the end of the small intestine (ileum), providing a better reflection of absorption. It also uses uncapped scores, offering a more nuanced comparison between high-quality protein sources that were all capped at 1.0 under PDCAAS.

Yes, it is possible to get all essential amino acids from a plant-based diet. While many plant foods are incomplete proteins, containing lower levels of certain EAAs, consuming a variety of plant protein sources throughout the day ensures a balanced intake.

No, it is a common misconception that complementary plant proteins must be consumed in the same meal. The body maintains a pool of amino acids, so combining different plant protein sources over the course of a day is sufficient to meet your EAA requirements.

Examples of complete proteins include animal-based products such as meat, poultry, fish, eggs, and dairy. Plant-based complete proteins include soy (like tofu and edamame), quinoa, and buckwheat.

Processing can both improve and harm protein quality. Methods like heating can deactivate antinutritional factors and increase digestibility, but excessive heating can cause damage to amino acids, particularly lysine, through reactions like the Maillard reaction.

High-quality proteins, especially those rich in key amino acids like leucine, can effectively stimulate muscle protein synthesis. This is particularly important for athletes seeking muscle recovery and growth and for older adults looking to counteract age-related muscle loss (sarcopenia).