How Do You Estimate Protein Quality? Understanding the Metrics

October 12, 2025 •

3 min read

Protein quality estimates are a cornerstone of nutritional policy worldwide, making it crucial to understand how do you estimate protein quality. The process involves evaluating a protein's essential amino acid profile and its digestibility, which provides insight into its overall nutritional value for the human body.

Quick Summary

This article explains how to measure protein quality using metrics like PDCAAS and the more advanced DIAAS. It breaks down the role of essential amino acids and digestibility, compares assessment methods, and discusses factors like cooking and protein complementation.

Key Points

Two Core Factors: Protein quality is fundamentally determined by a protein's essential amino acid (EAA) profile and its digestibility.
DIAAS is the Modern Standard: The Digestible Indispensable Amino Acid Score (DIAAS) is the most accurate metric, as it measures digestibility at the ileum and doesn't cap scores.
PDCAAS is Outdated: The older PDCAAS method is less accurate, largely due to its use of fecal digestibility measurement and a scoring system capped at 1.0.
Protein Complementation: Combining different protein sources, like rice and beans, is an effective strategy to obtain all essential amino acids, especially on a plant-based diet.
External Factors Matter: Processing methods (like cooking) and anti-nutritional factors (ANFs) present in foods can significantly influence a protein's overall quality.
Context is Key: For most healthy adults, dietary variety ensures adequate protein quality; metrics like DIAAS are most useful for specific health needs or product development.

Understanding Protein Quality: The Core Concepts

Protein quality is an index of how well a protein meets the body's requirements for indispensable (essential) amino acids and nitrogen. It is determined by the food's essential amino acid (EAA) profile and its digestibility. A high-quality protein is easily digested and contains all nine EAAs in the necessary amounts and proportions.

Essential Amino Acid Profile

Humans require nine EAAs from their diet. A protein is 'incomplete' if it lacks an adequate amount of one or more of these essential amino acids, common in many plant-based proteins. Examples include cereals being low in lysine and legumes potentially limited by sulfur-containing amino acids (methionine and cysteine). The most deficient EAA is called the 'first-limiting' amino acid.

Protein Digestibility

Digestibility is crucial; it measures how well the body can break down and absorb amino acids. Factors influencing digestibility include the source (animal proteins are generally more digestible), anti-nutritional factors (ANFs), and processing.

The Primary Assessment Methods: PDCAAS vs. DIAAS

The main methods for quantifying protein quality are PDCAAS and the newer DIAAS.

Protein Digestibility Corrected Amino Acid Score (PDCAAS)

Introduced in 1993, PDCAAS multiplies a food's amino acid score by its true fecal protein digestibility, based on the requirements of young children. Its limitations include:

Fecal Digestibility: It overestimates absorption by measuring fecal digestibility, which includes protein fermented in the large intestine.
Truncated Score: Scores were capped at 1.0, making different high-quality proteins appear equal.

Digestible Indispensable Amino Acid Score (DIAAS)

Recommended in 2013, DIAAS is the preferred standard. Its improvements over PDCAAS include:

Ileal Digestibility: DIAAS measures digestibility in the small intestine (ileum) for a more accurate absorption score.
Individual Amino Acid Digestibility: It uses specific digestibility percentages for each EAA.
Untruncated Scores: DIAAS doesn't truncate scores, allowing for a better ranking of high-quality proteins.

Comparing PDCAAS and DIAAS


Feature	PDCAAS	DIAAS
Digestibility Measurement	Fecal protein digestibility (total digestive tract)	Ileal amino acid digestibility (small intestine)
Digestibility Detail	Single digestibility percentage for the entire protein	Specific digestibility percentage for each indispensable amino acid
Bioavailability	Can be overestimated due to bacterial activity in the large intestine	More accurately reflects true amino acid bioavailability
Scoring	Truncated at 1.0, meaning multiple high-quality proteins receive the same score	Untruncated, allowing for a broader range of scores above 100% to reflect higher quality
Animal Model	Historically based on rat feeding studies	Prefers human studies, but also accepts swine data as a closer model to humans

Factors Influencing a Protein's Quality

Several factors can affect protein quality.

Processing Techniques: Cooking or fermentation can improve digestibility, but excessive heat can reduce the bioavailability of amino acids.
Anti-nutritional Factors (ANFs): Compounds in plant foods can hinder digestion and absorption. Proper cooking can deactivate these.
Food Matrix: The overall food composition can influence digestion.
Complementary Proteins: Combining different protein sources can ensure a complete EAA intake. This is particularly important for vulnerable populations.

The Importance of Protein Complementation

Combining plant-based proteins, like legumes and grains, helps provide all EAAs. A varied diet typically ensures adequate EAA intake throughout the day, which is vital for vegetarian and vegan diets.

Conclusion: Making Sense of Protein Quality

To estimate protein quality, assess both amino acid profile and digestibility. DIAAS offers a more precise evaluation than PDCAAS. For most healthy individuals, a varied diet provides sufficient protein quality. DIAAS is more useful for specific nutritional needs or product formulation. The key is understanding how different protein sources contribute to a healthy diet. A comprehensive understanding of these metrics aids informed dietary choices.

Understanding Dietary Protein Quality: Digestible Indispensable Amino Acid Score (DIAAS) and Other Methods

Frequently Asked Questions

Essential amino acids are the nine amino acids that the human body cannot synthesize and must obtain through dietary protein intake. These include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

Plant proteins often have one or more limiting essential amino acids. However, by combining different plant sources, such as grains and legumes, one can easily create a meal that provides a complete and high-quality protein profile.

DIAAS is more accurate because it measures amino acid absorption at the ileum (end of the small intestine), which is a better indicator of true digestibility than the fecal measurements used by PDCAAS. DIAAS also accounts for the digestibility of individual amino acids, unlike PDCAAS.

Cooking can either improve or decrease protein quality. Mild heating often improves digestibility by breaking down the protein structure. However, extreme heat, such as that caused by certain processing methods, can reduce the bioavailability of amino acids like lysine.

No, a lower score simply indicates that a single food may be limited in one or more essential amino acids. A varied diet that includes multiple protein sources can provide a complete amino acid profile, regardless of the individual scores of each food item.

Since DIAAS scores are not truncated at 1.0, a score above 100 means the protein source provides all essential amino acids in excess of the human requirement per gram of protein. These proteins can be particularly useful for complementing proteins with lower scores.

While most healthy people consuming a mixed diet don't need to track protein quality closely, it becomes more important for vulnerable groups like the elderly, who have higher protein needs, or individuals on restricted diets, like vegans, to ensure adequate intake of all essential amino acids.