Which of the following determines protein quality?

November 10, 2025 •

3 min read

According to the Food and Agriculture Organization (FAO), protein quality is a critical factor in meeting nutritional needs. The ability of a food protein to meet the body's metabolic demands is not just about quantity; it is also about its inherent quality. So, which of the following determines protein quality? The answer involves a combination of amino acid composition and digestibility.

Quick Summary

Protein quality is primarily determined by its amino acid profile, specifically the content of essential amino acids, and its digestibility. Animal-based proteins are generally higher quality due to complete essential amino acid profiles and higher digestibility compared to many plant-based sources. Modern scoring methods like DIAAS provide a more accurate assessment than older methods.

Key Points

Amino Acid Profile: Protein quality is fundamentally determined by its essential amino acid (EAA) composition, with high-quality proteins containing all nine EAAs in sufficient amounts for human needs.
Digestibility: How easily a protein is broken down and absorbed by the body is a critical determinant of its quality, impacting the bioavailability of its amino acids.
Limiting Amino Acid: The EAA present in the lowest quantity relative to the body's requirements is the limiting factor for protein synthesis and defines the protein's overall value.
DIAAS Score: The Digestible Indispensable Amino Acid Score (DIAAS) is the most current and accurate method for measuring protein quality, assessing ileal digestibility for each EAA.
Processing and Food Matrix: How food is processed, including cooking and sterilization, can alter protein structure, affecting digestibility and the availability of amino acids.
Source of Protein: Animal-based proteins are generally considered high-quality due to their complete EAA profile and higher digestibility, while many plant proteins may require complementing.

Essential Amino Acid Profile

Protein quality hinges on its amino acid composition, particularly the nine essential amino acids (EAAs) that the human body cannot produce. A high-quality protein provides sufficient amounts and appropriate ratios of all nine EAAs.

The Role of Limiting Amino Acids

The EAA present in the lowest amount relative to the body's needs acts as the "limiting amino acid," restricting the protein's overall quality. Plant proteins often have limiting amino acids, such as methionine and cysteine in legumes or lysine in grains. Combining different plant sources can create a complete EAA profile.

Leucine's Importance in Protein Quality

Leucine, a branched-chain amino acid, is particularly important as it helps regulate muscle protein synthesis. Proteins rich in leucine are thought to enhance muscle growth, which is beneficial for athletes and older adults. This demonstrates that beyond having all EAAs, the specific balance of amino acids matters.

Protein Digestibility and Bioavailability

For protein to be useful, it must be digested and absorbed. Digestibility refers to the proportion of protein absorbed from food. Factors like the food source, anti-nutritional compounds, and processing methods affect digestibility. Bioavailability represents the absorbed amino acids available for synthesis.

Differences Between Animal and Plant Protein

Animal proteins are typically more digestible than plant proteins due to fewer anti-nutritional factors. The table below compares these protein types:


Feature	Animal-Based Protein	Plant-Based Protein (Many)
Essential Amino Acids	All nine are present, making them "complete".	Often low in one or more EAAs, making them "incomplete".
Digestibility	High digestibility (e.g., egg, whey, meat) due to structure.	Lower digestibility due to fiber and anti-nutrients.
Processing Impact	Quality can be reduced with excessive heat processing.	Digestibility can be improved by methods like soaking, sprouting, and fermentation.
Complementary Needs	Not typically required for a complete profile.	Complementing with other plant sources is recommended.
Primary Function	Excellent for supporting rapid protein synthesis and growth.	Useful for combining with other sources to meet requirements.

Modern Methods for Measuring Protein Quality

While methods like PER and BV were used historically, modern approaches provide a more accurate assessment.

Protein Digestibility Corrected Amino Acid Score (PDCAAS)

PDCAAS was the standard method, multiplying the lowest amino acid score by true fecal digestibility. Its limitation was capping scores at 1.0, failing to differentiate among high-quality proteins.

Digestible Indispensable Amino Acid Score (DIAAS)

The FAO recommends DIAAS as the preferred method. It measures individual EAA digestibility in the ileum, providing a more accurate measure of absorbable amino acids. DIAAS scores are not capped, allowing for better distinction between high-quality sources and reflecting the value of proteins in mixed diets.

Processing and Food Matrix

Food processing impacts protein quality. Excessive heat can degrade amino acids like lysine, while moderate heat can improve digestibility by reducing anti-nutrients. The overall food matrix, including fiber, fat, and carbohydrates, also affects digestion and nutrient availability.

Conclusion: The Holistic View of Protein Quality

Protein quality is determined by its essential amino acid profile, digestibility, and processing. Modern methods like DIAAS offer a more precise assessment of how the body uses protein. Consuming a variety of high-quality protein sources ensures adequate EAA intake. Understanding these factors helps in making informed dietary choices for better nutrition.

Which of the following determines protein quality? - Factors to Remember

Essential amino acid content is fundamental.
Digestibility is crucial for amino acid absorption.
Limiting amino acids restrict overall quality.
Leucine content is important for muscle synthesis.
DIAAS is the most accurate quality measure.
Processing affects protein quality.

Frequently Asked Questions

A complete protein is a source of protein that contains all nine essential amino acids (EAAs) in sufficient quantities to meet the body's needs. Animal products like meat, eggs, and dairy are classic examples of complete proteins.

Digestibility determines how much of the protein's amino acids can actually be absorbed and used by the body. A protein with a perfect amino acid profile is of little value if the body cannot effectively break it down and absorb its components.

PDCAAS (Protein Digestibility Corrected Amino Acid Score) and DIAAS (Digestible Indispensable Amino Acid Score) are both scoring systems for protein quality. DIAAS is considered more accurate because it measures ileal digestibility of individual amino acids and does not cap high-quality proteins, unlike the older PDCAAS method.

Yes, plant-based proteins can be high-quality. While many are incomplete on their own, combining different plant sources, such as rice and beans, can provide a complete amino acid profile. Some plant proteins, like soy, are naturally complete.

Cooking can have both positive and negative effects. Moderate heat can improve digestibility by destroying anti-nutritional factors. However, excessive heat can cause damage to sensitive amino acids, like lysine, and reduce the overall protein quality.

No. While quantity is important, eating more low-quality protein may not be as effective as consuming less high-quality protein. The quality and balance of amino acids, along with digestibility, are key to how efficiently the body uses the protein.

Leucine, a branched-chain amino acid (BCAA), acts as a key signaling molecule that stimulates muscle protein synthesis. Protein sources rich in leucine can provide a more robust anabolic response, which is crucial for muscle repair and growth, especially after exercise.