How to tell if protein is high-quality?

October 11, 2025 •

3 min read

According to the Food and Agriculture Organization of the United Nations, a new scoring system called DIAAS has been recommended to accurately quantify dietary protein quality. This guide will explain how to tell if protein is high-quality and help you make more informed dietary choices by focusing on essential amino acid content and digestibility.

Quick Summary

Assessing protein quality involves evaluating a food's essential amino acid profile and digestibility. Metrics like the Digestible Indispensable Amino Acid Score (DIAAS) offer a sophisticated measure by focusing on amino acid absorption in the small intestine. This provides a clear framework for distinguishing between high-quality sources, which are efficiently utilized, and lower-quality ones.

Key Points

Complete Amino Acid Profile: High-quality protein sources contain all nine essential amino acids in sufficient amounts, which is crucial for optimal bodily functions.
Digestibility and Bioavailability: An easily digestible protein that is readily absorbed by the body is more effective. Animal proteins and some plant proteins like soy and quinoa have high digestibility.
DIAAS is the Modern Standard: The Digestible Indispensable Amino Acid Score (DIAAS) is the most accurate metric for evaluating protein quality, superseding the older PDCAAS score.
Combine Incomplete Proteins: To maximize the value of plant-based protein, combine complementary sources like grains and legumes to ensure you get all essential amino acids throughout the day.
High-Quality Protein Benefits: Prioritizing high-quality protein supports muscle growth, aids in weight management by increasing satiety, and contributes to better bone health.
Processing Matters: How food is processed affects protein quality. Look for minimally processed options and understand that cooking can sometimes improve, but excessive heat can also harm, protein value.

The Foundational Pillars of Protein Quality

Protein quality is determined by its amino acid profile and how effectively the body can absorb and utilize it. Humans need 20 amino acids, nine of which are essential and must be consumed through food. A high-quality protein, or "complete protein," provides all nine essential amino acids (EAAs) in sufficient amounts.

The two main factors are:

Amino Acid Profile: This refers to the types and ratios of amino acids present, including essential amino acids like leucine.
Digestibility and Bioavailability: This measures how well amino acids from the protein source are absorbed.

Scoring Systems for Assessing Protein Quality

Objective measures have been developed using scoring systems like PDCAAS and DIAAS.

The Older Method: PDCAAS The Protein Digestibility Corrected Amino Acid Score (PDCAAS) was the previous standard, comparing a protein's amino acid profile to a reference, corrected for fecal digestibility. It capped scores at 1.0, making it unable to differentiate between superior proteins, and underestimated quality by using fecal digestibility.

The Modern Standard: DIAAS The FAO now recommends the Digestible Indispensable Amino Acid Score (DIAAS) as a more accurate method. Key benefits of DIAAS include:

Ileal Digestibility: It measures amino acid digestibility at the end of the small intestine for a more precise absorption value.
No Score Truncation: Scores are not capped at 1.0, allowing for better ranking of high-quality proteins.
Age-Specific Reference Patterns: It uses different patterns based on age groups with varying amino acid needs.

Practical Ways to Identify High-Quality Protein

To identify high-quality protein in your diet, consider these practical steps.

Prioritize Complete Protein Sources Focus on foods that are complete protein sources. These often include animal-based foods, but some plant-based options also provide all nine essential amino acids.

Animal Sources: Meat, poultry, fish, eggs, and dairy are excellent sources.
Plant Sources: Quinoa, soy, and buckwheat are complete plant proteins.

Combine Complementary Proteins For those consuming incomplete plant proteins, combining different sources throughout the day (not necessarily in the same meal) ensures intake of all essential amino acids.

Combinations: Examples include rice and beans or hummus and whole-grain pita.

Consider Processing and Preparation How a protein is prepared impacts its digestibility. Cooking can enhance the digestibility of some plant proteins, but excessive heat can degrade amino acids. Minimally processed proteins are generally preferred.

Comparison of Common Protein Sources

Understanding the quality differences between protein sources can guide your dietary choices.


Protein Source	Type	Amino Acid Profile	Digestibility/Bioavailability	DIAAS Score (Example)	Notes
Whey Protein	Animal (Dairy)	Complete	High	1.09+	Fast-digesting.
Casein	Animal (Dairy)	Complete	High (slow)	1.18+	Slow-digesting.
Egg	Animal	Complete	High	1.01+	Reference standard.
Beef	Animal	Complete	High	1.11+	Good source of iron and zinc.
Soy Protein Isolate	Plant	Complete	High	0.89-0.99	High-quality plant option.
Pea Protein	Plant	Incomplete (low in methionine)	Moderate	0.65-0.82	Often blended.
Rice Protein	Plant	Incomplete (low in lysine)	Moderate	0.37-0.59	Best combined with lysine-rich sources.
Quinoa	Plant	Complete	Good	0.68-0.79	Whole-grain option.
Cooked Beans	Plant	Incomplete (low in methionine)	Moderate	0.59-0.65	Pairs well with grains.

The Benefits of Focusing on High-Quality Protein

Prioritizing high-quality protein offers several health benefits.

1. Enhanced Muscle Growth and Repair: Rich in essential amino acids like leucine, high-quality proteins stimulate muscle protein synthesis, important for muscle building and preventing muscle loss.

2. Increased Satiety and Weight Management: Protein is more filling than carbs or fat, which helps control appetite and supports weight management.

3. Support for Bone Health: Adequate high-quality protein intake is linked to better bone density and reduced osteoporosis risk, especially with age.

4. Optimized Metabolism and Energy: Protein is essential for numerous metabolic processes, supporting overall vitality.

5. Improved Body Composition: A diet high in quality protein helps maintain a favorable lean muscle mass to body fat ratio.

Conclusion

Beyond simply counting protein grams, focusing on protein quality – its amino acid profile and digestibility – is crucial for optimizing nutritional intake. Utilizing metrics like DIAAS and incorporating a variety of high-quality sources, including understanding how to combine complementary plant proteins, can enhance muscle growth, aid weight management, and improve overall health. Making informed protein choices is key to a healthier lifestyle. For more information on selecting high-quality proteins like dairy, eggs, and lean meats, you can consult resources from the U.S. Dairy Association.

Frequently Asked Questions

A complete protein contains all nine essential amino acids that the human body cannot produce on its own. Incomplete proteins lack one or more of these essential amino acids. Animal products like meat, eggs, and dairy are typically complete, as are plant sources like soy and quinoa.

DIAAS (Digestible Indispensable Amino Acid Score) is the current standard for measuring protein quality, recommended by the FAO. It is superior to the older PDCAAS because it measures amino acid digestibility at the end of the small intestine for higher accuracy and does not cap high-quality protein scores.

Vegans and vegetarians can achieve high-quality protein intake by consuming a variety of plant-based sources throughout the day to form complementary protein combinations. Plant-based complete proteins like soy, quinoa, and buckwheat are also excellent options.

Yes, cooking can affect protein quality. While it can improve the digestibility of some plant proteins by breaking down cell walls, excessive heat can denature protein structures and reduce the biological value, particularly in processed foods.

Most animal proteins are considered high-quality because they are complete proteins with high digestibility. However, specific types and preparation methods can influence the overall quality. Lean cuts and low-fat dairy are often recommended.

Leucine is one of the nine essential amino acids and plays a critical role in stimulating muscle protein synthesis, the process of building new muscle tissue. Protein sources with a favorable leucine content are considered more effective for muscle growth.

Protein digestibility is extremely important. A protein source is only as good as the amino acids your body can actually absorb and use. A highly digestible protein provides more usable amino acids per gram, making it more effective for tissue repair and other functions.