Which of the following is required to determine protein quality?

November 27, 2025 •

3 min read

According to the Food and Agriculture Organization (FAO), assessing protein quality requires considering its amino acid composition, especially the essential amino acids, along with its digestibility. This dual approach ensures a protein source can effectively contribute to human health and metabolic needs.

Quick Summary

Assessing protein quality requires evaluating its essential amino acid content relative to human requirements and its digestibility. Modern methods like DIAAS provide a more accurate evaluation than older measures like PDCAAS by focusing on ileal digestibility and avoiding score truncation.

Key Points

Essential Amino Acid Content: Protein quality is fundamentally determined by the presence and ratio of the nine essential amino acids (EAAs) that the body cannot produce itself.
Digestibility: How easily a protein can be broken down, absorbed, and utilized by the body is a critical factor for determining its nutritional value.
DIAAS Method: The Digestible Indispensable Amino Acid Score (DIAAS) is the most accurate current method, measuring the ileal digestibility of individual EAAs.
PDCAAS Method: The Protein Digestibility-Corrected Amino Acid Score (PDCAAS) is an older method that has limitations, including the use of less accurate fecal digestibility and a capped score of 1.0.
Protein Complementarity: Combining different protein sources, especially plant-based ones, can compensate for missing or limiting essential amino acids to achieve a complete amino acid profile.
Bioavailability: Beyond digestion, bioavailability considers the rate at which amino acids are absorbed and made available for protein synthesis, affecting metabolic responses.

Essential Amino Acid Composition: The Building Block Foundation

Protein is made up of amino acids, including nine 'essential' ones the body cannot produce. A high-quality protein source provides all essential amino acids (EAAs) in sufficient amounts, crucial for protein synthesis. A protein lacking one or more EAAs is considered incomplete.

The Concept of a Limiting Amino Acid

Protein synthesis is limited by the essential amino acid present in the smallest amount relative to the body's needs; this is the 'limiting amino acid'. For example, many plant proteins like legumes are low in methionine and cysteine, while grains are often low in lysine. Combining different plant protein sources helps ensure a complete EAA intake.

Digestibility and Bioavailability: What Your Body Can Actually Use

Digestibility, how well a protein is broken down and absorbed, is the second key factor in determining protein quality. A protein's nutritional value depends on how much of its amino acids can be effectively utilized by the body. Digestibility is influenced by:

Source: Animal proteins are generally more digestible than plant proteins due to factors like fiber and anti-nutritional compounds in plants.
Processing: Cooking can improve digestibility in some cases (e.g., soy), while excessive heat can reduce it.
Anti-nutritional factors: Plant compounds such as tannins and phytates can hinder protein digestion and nutrient absorption.

Protein Scoring Methods: Quantifying Quality

Standardized methods have been developed to measure protein quality, notably PDCAAS and DIAAS.

Protein Digestibility-Corrected Amino Acid Score (PDCAAS)

Until 2013, PDCAAS was the recommended standard. It assessed quality by comparing amino acid profiles to a reference and correcting for fecal digestibility. A major drawback is that scores are capped at 1.0, failing to distinguish between proteins of truly exceptional quality.

Digestible Indispensable Amino Acid Score (DIAAS)

Recommended by the FAO since 2013, DIAAS is considered more accurate. It improves upon PDCAAS by:

Measuring ileal digestibility: Assesses absorption in the small intestine for a more precise value.
Avoiding score truncation: Scores can exceed 100%, allowing for better comparison of high-quality proteins.
Using individual amino acid digestibility: Accounts for variations in the digestibility of different amino acids.

A Comparison of Protein Scoring Methods


Feature	PDCAAS (Protein Digestibility-Corrected Amino Acid Score)	DIAAS (Digestible Indispensable Amino Acid Score)
Digestibility Measurement	Fecal digestibility of crude protein	Ileal digestibility of individual amino acids
Accuracy	Less accurate; can overestimate protein quality	More accurate; reflects true amino acid absorption
Score Truncation	Capped at 1.0, limiting differentiation of high-quality proteins	Not truncated; can exceed 100%, providing a more precise scale
Reference Pattern	Based on amino acid requirements for 2- to 5-year-old children	Provides reference patterns for three different age groups
Recommended By	Formerly recommended by FAO/WHO (pre-2013)	Currently recommended by FAO

The Role of Protein in Human Health

Evaluating protein quality helps ensure dietary intake meets needs for growth, repair, and maintenance. Individuals like athletes and older adults may benefit from prioritizing high-quality, digestible proteins due to higher requirements. Foods with high DIAAS scores, such as eggs and whey, provide essential amino acids efficiently, which is important for those with limited caloric intake.

Conclusion: A Holistic View of Protein Quality

To accurately determine protein quality, both essential amino acid composition and digestibility are required. DIAAS, the current standard, offers a more accurate assessment by considering ileal digestibility and individual amino acid availability compared to the older PDCAAS method. Combining plant proteins is a strategy to achieve a complete essential amino acid profile. Understanding these factors is vital for effective dietary planning to support health and specific nutritional needs.

Learn more about DIAAS and the science of protein quality from the Frontiers in Nutrition Journal [https://www.frontiersin.org/journals/nutrition/articles/10.3389/fnut.2024.1389664/full].

Frequently Asked Questions

Essential amino acids are the nine amino acids that the human body cannot produce and must obtain from food. Non-essential amino acids can be synthesized by the body from other materials.

Protein digestibility is crucial because it determines how many amino acids from a food are actually absorbed and become available to the body for use in protein synthesis. A protein with a good amino acid profile is useless if it cannot be efficiently digested.

DIAAS is a more accurate measure than PDCAAS because it uses ileal digestibility, which reflects absorption in the small intestine, and it does not truncate scores, allowing for better comparison of high-quality proteins.

Yes, by combining complementary plant protein sources, such as rice and beans or lentils and whole wheat, you can achieve a complete essential amino acid profile.

A limiting amino acid is the essential amino acid present in the lowest concentration relative to the body's requirements within a given protein source. It acts as a bottleneck, restricting the body's ability to utilize the other amino acids for protein synthesis.

Generally, animal proteins are more digestible and contain all essential amino acids in higher ratios, leading to higher DIAAS scores. However, some plant proteins, like soy protein isolate, can also achieve very high scores.

A DIAAS score above 100% indicates that a protein source provides more than 100% of the essential amino acids required per unit of protein, distinguishing it as exceptionally high quality compared to proteins with a score just at or above the minimum.