Understanding What Is True About Protein Quality

November 25, 2025 •

4 min read

In recent nutritional studies, the quality of protein is understood to be determined by both its amino acid profile and its digestibility. So, what is true about protein quality? It’s a measure of how effectively the body can utilize a food's protein to build and repair tissues, a metric that goes far beyond simply counting grams.

Quick Summary

Protein quality is a measure of how well a protein source provides all essential amino acids in a digestible, usable form for the body.

Key Points

Amino Acid Profile: High-quality protein contains all nine essential amino acids (EAAs) that the body cannot produce on its own.
Protein Digestibility: The ease with which the body breaks down and absorbs a protein's amino acids is a major factor in its quality.
DIAAS is Superior: The Digestible Indispensable Amino Acid Score (DIAAS) is a more accurate measure of protein quality than the older PDCAAS method.
Animal vs. Plant Protein: Animal-based proteins are generally higher quality due to their complete EAA profile and higher digestibility, though some plant sources like soy and quinoa are complete.
Combining Proteins: To ensure all EAAs are met on a plant-based diet, different incomplete protein sources, like rice and beans, should be consumed throughout the day.
Quality Over Quantity: For specific needs such as muscle repair or for the elderly, focusing on high-quality protein allows for more efficient EAA intake without excessive calories.

What Defines Protein Quality?

To understand what is true about protein quality, one must look beyond total protein grams. The concept hinges on two primary factors: the amino acid composition and the protein's digestibility. A high-quality protein provides all the necessary building blocks—amino acids—in the correct proportions and can be efficiently broken down and absorbed by the body.

The Amino Acid Profile: The Building Blocks

Proteins are composed of 20 different amino acids, nine of which are considered "essential" or "indispensable" because the human body cannot produce them and must obtain them from food. For any protein synthesis to occur, all nine essential amino acids (EAAs) must be present in sufficient amounts. If a diet lacks even one EAA, the body's ability to create new proteins becomes limited.

Essential Amino Acids (EAAs)

Histidine: Plays a key role in immune response, digestion, and nerve protection.
Isoleucine: A branched-chain amino acid (BCAA) involved in muscle metabolism and concentrated in muscle tissue.
Leucine: Another BCAA that is a critical trigger for muscle protein synthesis and repair.
Lysine: Important for protein synthesis, calcium absorption, and hormone production.
Methionine: Essential for metabolism, detoxification, and tissue growth.
Phenylalanine: Precursor to neurotransmitters like tyrosine and dopamine.
Threonine: A component of structural proteins like collagen and elastin.
Tryptophan: Precursor to serotonin, regulating mood, sleep, and appetite.
Valine: The third BCAA, involved in muscle growth and energy production.

Complete vs. Incomplete Proteins

Protein sources are often categorized as either complete or incomplete based on their EAA content. This distinction is critical for dietary planning, particularly for individuals following plant-based diets.

Complete Proteins: Contain all nine essential amino acids in adequate amounts.
- Animal Sources: Meat, poultry, fish, eggs, and dairy are classic examples.
- Plant Sources: A few plant-based foods, such as soy (tofu, tempeh), quinoa, and buckwheat, are also complete proteins.
Incomplete Proteins: Lack or are very low in at least one essential amino acid.
- Examples: Most legumes, nuts, seeds, and grains fall into this category.

Combining Complementary Proteins

While most plant proteins are incomplete, they can be combined to form a complete amino acid profile. This was once thought necessary at every meal but is now known to be achievable by consuming a variety of protein sources throughout the day.

Beans (low in methionine) with rice (low in lysine)
Hummus and whole wheat pita
Peanut butter on whole-grain bread

The Role of Digestibility

For a protein to be truly high-quality, its amino acids must be digestible and absorbable. Digestibility varies between protein sources due to several factors.

Source: Animal proteins are generally more digestible (95-98%) than plant proteins (70-85%). This is partly due to plant cell walls and antinutritional factors.
Processing: Cooking can improve digestibility by deactivating antinutrients like trypsin inhibitors in legumes. However, over-processing with high heat can make some amino acids, like lysine, unavailable through the Maillard reaction.
Food Matrix: The presence of fiber and other compounds in whole plant foods can also lower protein digestibility compared to highly processed isolates.

Modern Methods for Scoring Protein Quality

To account for both amino acid content and digestibility, scoring systems have been developed. The current standard is the Digestible Indispensable Amino Acid Score (DIAAS), which replaced the older Protein Digestibility Corrected Amino Acid Score (PDCAAS).


PDCAAS vs. DIAAS: A Comparison of Protein Quality Scores	Feature	PDCAAS (Older Method)	DIAAS (Modern Method)
Digestibility Measurement	Fecal digestibility of crude protein.	True ileal digestibility of individual amino acids.
Scope	Measures overall protein digestibility.	Measures individual amino acid digestibility, which is more accurate.
Scoring	Values capped at 1.0, meaning high-quality proteins are indistinguishable.	Values not capped, allowing for better differentiation between high-quality proteins.
Accuracy	Prone to overestimating protein quality due to microbial activity in the large intestine.	More accurate reflection of amino acids absorbed by the body.
Use Case	Historically used for food labeling and regulations in the US.	Recommended by the FAO as the superior method for all ages.

Quality Over Quantity for Optimal Health

For many people, especially those in Western countries, simply getting enough total protein is not the issue; ensuring a sufficient supply of bioavailable EAAs is. For populations with higher protein needs—such as the elderly, who experience anabolic resistance, or athletes—relying on low-quality protein sources can require significantly more food and calories to get the same benefit. Therefore, prioritizing high-quality protein, which often comes with a complete EAA profile and higher digestibility, is more efficient for meeting specific physiological needs without excessive caloric intake.

Furthermore, the “protein package” matters. Animal-based protein foods come with a different nutritional profile (e.g., saturated fat) than plant-based proteins (e.g., fiber and antioxidants). Health guidelines from sources like the American Heart Association emphasize consuming a variety of healthy protein sources for overall cardiovascular health. A balanced diet that includes a mix of protein sources can provide a complete nutrient profile while managing caloric intake effectively.

For more in-depth nutritional guidance, consulting resources such as Frontiers in Nutrition can offer further insights into this complex topic.

Conclusion

What is true about protein quality is that it is a multi-faceted concept defined by both its amino acid composition and its digestibility. Animal-based proteins are typically complete and highly digestible, while most plant-based proteins are incomplete but can be combined to meet EAA needs. Newer scoring systems like DIAAS offer a more accurate way to measure protein quality, helping people, especially those with specific needs, optimize their diets for better health and physiological function.

Frequently Asked Questions

Protein quality is a measure of how effectively a protein source provides the nine essential amino acids (EAAs) that the body needs but cannot produce on its own, along with how easily the protein can be digested and absorbed.

The two main factors determining protein quality are the amino acid profile, specifically the content of the nine essential amino acids, and the protein's digestibility, or how efficiently the body can break down and absorb it.

Generally, animal proteins are considered higher quality because they are typically complete (containing all EAAs) and are more digestible. However, some plant proteins like soy and quinoa are complete, and consuming a variety of plant proteins can ensure a complete EAA profile.

PDCAAS (Protein Digestibility Corrected Amino Acid Score) is an older scoring system that measured digestibility based on fecal matter and capped scores at 1.0. DIAAS (Digestible Indispensable Amino Acid Score) is the newer, more accurate method recommended by the FAO, measuring true ileal digestibility and not capping high scores.

While it was once thought that complementary proteins had to be consumed in the same meal, modern understanding shows that consuming a variety of plant-based protein sources throughout the day is sufficient to provide all essential amino acids. Combining sources like beans and rice or hummus and pita are still excellent strategies.

Cooking can either increase or decrease protein quality depending on the method. Moderate heat can improve digestibility by deactivating antinutrients in plant foods. However, excessive heat can cause reactions that make amino acids, particularly lysine, less available for absorption.

Older individuals may experience anabolic resistance, where the body's protein synthesis is less responsive to nutritional intake. Consuming high-quality, readily digestible proteins with a complete EAA profile, especially sources high in leucine, can more effectively support muscle protein synthesis and combat age-related muscle loss.