Is There Such a Thing as High Quality Protein?

October 17, 2025 •

4 min read

According to a 2024 analysis using the Digestible Indispensable Amino Acid Score (DIAAS) metric, some dairy and animal proteins can have significantly higher quality scores than many plant-based proteins. This confirms that a quantifiable difference exists, addressing the central question: Is there such a thing as high quality protein? The answer is yes, and it depends on a few key factors that influence how your body uses the protein you consume.

Quick Summary

The concept of high-quality protein is defined by its amino acid profile and digestibility. Complete proteins, found predominantly in animal sources, contain all essential amino acids, leading to higher quality scores. Different scoring methods exist, with the DIAAS offering a more accurate measurement than the older PDCAAS by assessing ileal digestibility.

Key Points

Protein quality is a real and measurable concept: It's defined by a protein's essential amino acid (EAA) composition and digestibility, not just the quantity.
DIAAS is the gold standard for measuring protein quality: The Digestible Indispensable Amino Acid Score (DIAAS) is a modern, superior metric that uses ileal digestibility and provides a more accurate, uncapped score than the outdated PDCAAS.
Animal proteins often have higher quality scores: Animal-based sources like dairy, eggs, meat, and fish are generally 'complete' proteins with excellent digestibility and high DIAAS scores.
Plant proteins can be high-quality, too: Complete plant proteins exist (e.g., soy, quinoa), and combining 'incomplete' sources (e.g., rice and beans) throughout the day can provide a complete EAA profile.
Consider your goals when choosing protein sources: For needs like muscle growth or preventing age-related muscle loss, prioritizing higher-quality, fast-digesting proteins like whey is particularly beneficial.
Processed food affects protein quality: High heat or storage can sometimes reduce a protein's digestibility, lowering its overall quality.

What Defines Protein Quality?

Protein quality is not a matter of opinion but is determined by specific nutritional and biological factors. A protein is considered high-quality based on its amino acid composition and how easily the human body can digest and absorb those amino acids.

Essential Amino Acids: The Building Blocks

There are 20 amino acids that form the building blocks of protein, but only nine are considered “essential” (EAAs) because the human body cannot produce them on its own. A protein source is deemed “complete,” and therefore of higher quality, if it contains all nine EAAs in sufficient amounts and proportions to meet human needs.

Histidine: Crucial for enzyme synthesis and nerve cell protection.
Isoleucine: Important for muscle metabolism and immune function.
Leucine: A key activator of muscle protein synthesis and repair.
Lysine: Important for calcium absorption and hormone production.
Methionine: Necessary for metabolism and detoxification.
Phenylalanine: Precursor for neurotransmitters and other amino acids.
Threonine: Important for skin and connective tissue.
Tryptophan: Precursor for serotonin, which regulates mood and sleep.
Valine: Aids in muscle growth and energy production.

The Role of Digestibility

The second factor in protein quality is digestibility—how efficiently the body breaks down the protein and absorbs its amino acids. Even if a protein has an excellent amino acid profile, a low digestibility score will reduce its overall quality because the body cannot fully utilize the amino acids. Factors like the food matrix, processing, and antinutritional compounds can all affect digestibility.

Quantifying Protein Quality: DIAAS vs. PDCAAS

Historically, various methods have been used to score protein quality, such as the Protein Efficiency Ratio (PER) and Biological Value (BV). Today, the most relevant metrics are PDCAAS and the newer, more accurate DIAAS.

Protein Digestibility Corrected Amino Acid Score (PDCAAS):

Measures protein quality by comparing a food's amino acid composition to a reference pattern, corrected for fecal digestibility.
Has a major limitation: scores are capped at 1.0. This means a protein that significantly exceeds all EAA requirements is scored the same as one that just barely meets them.
Uses fecal samples for digestibility, which can be less accurate due to microbial activity in the colon.

Digestible Indispensable Amino Acid Score (DIAAS):

The newer, superior standard recommended by the FAO in 2013.
Measures digestibility at the end of the small intestine (ileum) for each individual amino acid, providing a more precise assessment of absorbed amino acids.
Scores are not truncated at 100%, allowing for an accurate ranking of proteins based on their complete nutritional value. For instance, whey protein isolate might score 1.09, while soy protein isolate scores 0.90, revealing a more nuanced quality difference.
Scores >100% indicate excellent or high quality, 75-99% is good quality, and <75% indicates a protein that cannot make quality claims.

High-Quality Protein Sources: Animal vs. Plant

Animal-based proteins are generally considered high-quality because they are complete proteins with high digestibility. However, certain plant-based options are also complete, and combining others can achieve a high overall protein quality in the diet.

Animal-Based Sources

Meat, Poultry, and Fish: Excellent and highly digestible sources, offering all essential amino acids.
Eggs: Often used as a benchmark for protein quality due to their near-perfect amino acid profile and high digestibility.
Dairy: Milk, yogurt, and cheese provide high-quality, complete proteins like casein and whey, with whey being particularly fast-digesting.

Plant-Based Sources

Complete Plant Proteins: While less common, some plant sources contain all nine essential amino acids. Examples include soy (tofu, edamame), quinoa, and hemp seeds.
Complementary Proteins: The strategic combination of incomplete proteins can create a complete amino acid profile. Examples include pairing grains (low in lysine) with legumes (low in methionine), such as rice and beans, or combining lentils with millet.

Comparison Table: Protein Quality Scores (DIAAS)


Food Source	DIAAS Score (Example)	Animal vs. Plant	Key Characteristics
Whey Protein Isolate	109%	Animal	Very high quality, high leucine content, fast-digesting.
Beef	91-99%	Animal	Highly digestible, rich in all EAAs.
Milk Protein Concentrate	>100%	Animal	Excellent quality, complete amino acid profile.
Soy Protein Isolate	90%	Plant	Complete protein source, but lower overall score than whey.
Pea Protein	~83%	Plant	Good quality, often lower in methionine + cysteine.
Chickpeas	~76%	Plant	Good quality, often limiting in methionine.
Oat Protein	~68%	Plant	Lower quality, limited by lysine.
Wheat	<50%	Plant	Lower quality, limited by lysine and lower digestibility.

Maximizing Protein Intake for Optimal Health

Recognizing the differences in protein quality allows you to make informed dietary choices based on your goals. For example, athletes and older adults may need to prioritize higher-quality, more anabolic protein sources, like whey, to maximize muscle protein synthesis and combat age-related muscle loss. For the average, healthy adult, meeting protein requirements is achievable through a variety of mixed protein sources throughout the day, whether from animal or plant sources.

Conclusion

There is undoubtedly such a thing as high-quality protein, defined by its essential amino acid content and its digestibility. Animal sources tend to be higher-quality due to their complete amino acid profiles and excellent digestibility, as reflected by higher DIAAS scores. However, plant-based proteins can still meet needs effectively through varied dietary patterns and the strategic combination of complementary sources. Understanding protein quality is a powerful tool for optimizing nutrition, whether you're building muscle, preventing age-related decline, or simply maintaining overall health.

Understanding Dietary Protein Quality: Digestible Indispensable Amino Acid Score

Frequently Asked Questions

Complete proteins contain all nine essential amino acids (EAAs) that the body cannot produce, while incomplete proteins lack one or more of these EAAs. Most animal proteins are complete, while most plant proteins are incomplete.

No, plant protein is not necessarily inferior. While animal proteins typically have a higher DIAAS score, a well-planned, diverse diet with complementary plant proteins can easily meet all essential amino acid needs. In some cases, processed plant protein isolates can even rival the quality of some animal proteins.

Yes, it is entirely possible to get all essential amino acids from a vegan diet. By eating a variety of plant-based protein sources, such as legumes, grains, nuts, and seeds, throughout the day, you can ensure your body receives a complete amino acid profile.

Complementary proteins are two or more incomplete protein sources that, when combined, provide all nine essential amino acids. For example, pairing beans (rich in lysine) with rice (rich in methionine) creates a complete protein source.

For maximizing muscle protein synthesis, particularly after exercise, research suggests that fast-digesting, leucine-rich proteins like whey and milk protein are highly effective. However, other complete proteins like casein and soy can also be highly effective for muscle growth and recovery.

Cooking can affect protein quality, either positively or negatively. While proper cooking can improve digestibility by denaturing the protein, processes involving excessive heat or browning (like the Maillard reaction) can sometimes damage amino acids and reduce overall protein quality.

Yes, protein quality still matters even with high intake. Although a high protein intake can compensate for lower quality, prioritizing high-quality sources is especially important for specific goals like preserving muscle mass during weight loss or supporting the increased needs of older adults and athletes.