Which Protein is High Quality? An Expert Guide to Protein Sources and Scores

January 11, 2026 •

3 min read

Did you know that protein from whole eggs is so highly digestible and packed with all nine essential amino acids that it is often used as a gold standard to measure the quality of other proteins?

Quick Summary

Assessing protein quality requires evaluating its essential amino acid content and digestibility. Scores like PDCAAS and the newer DIAAS provide scientific measures for comparing protein sources to meet human nutritional needs.

Key Points

Complete vs. Incomplete: A complete protein has all nine essential amino acids, while an incomplete one lacks at least one.
Measurement Matters: Protein quality is scientifically measured by digestibility and amino acid content, using metrics like PDCAAS and the more advanced DIAAS.
Animal Sources Reign Supreme: Eggs, whey, and meat are classic high-quality proteins due to their complete amino acid profile and high digestibility.
Plant-Based Options Exist: Soy and quinoa are examples of complete plant proteins, and vegans can combine incomplete proteins (e.g., rice and beans) to cover all amino acid bases.
Consider Digestion Rate: Whey protein is fast-digesting, ideal for post-workout, while casein is slow-digesting, providing sustained amino acid release.
Holistic Nutrition: The best approach is a varied diet incorporating both high-quality animal and plant sources for a comprehensive nutrient intake, considering the entire food 'package'.

Protein is a critical macronutrient, but not all protein is created equal. The quality of a protein source is determined by its amino acid profile and its digestibility. Understanding these factors can help you make more informed dietary choices to support everything from muscle repair and hormone production to overall health. This guide breaks down what high-quality protein truly means, how it's measured, and where to find the best sources.

The Definition of High-Quality Protein

At its core, a protein's quality is defined by two main criteria: its completeness and its digestibility.

The Nine Essential Amino Acids

Proteins are made up of building blocks called amino acids. While the human body can produce 11 of the 20 necessary amino acids, nine are considered 'essential' because they must be obtained through food. A protein source that contains adequate levels of all nine essential amino acids is called a 'complete protein'. Most animal proteins—including meat, eggs, and dairy—are naturally complete. Essential amino acids include histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.

Digestibility and Absorption

The second measure of protein quality is digestibility—how efficiently the body can break down, absorb, and utilize the amino acids. Animal proteins generally have high digestibility, while some plant-based proteins can be less digestible. Processing methods like cooking can improve digestibility in plant sources by destroying protease inhibitors.

How Protein Quality is Measured

Protein quality has been historically measured using the Protein Digestibility Corrected Amino Acid Score (PDCAAS). This method compares a food's amino acid profile to a standard and corrects for digestibility, with a maximum score of 1.0. Eggs, milk, whey, and soy protein all score 1.0 on the PDCAAS scale. However, PDCAAS doesn't differentiate between proteins that exceed human requirements.

A more accurate method, the Digestible Indispensable Amino Acid Score (DIAAS), was proposed by the FAO in 2013. DIAAS measures individual amino acid digestibility in the small intestine, providing a better reflection of absorption. Unlike PDCAAS, DIAAS scores are not capped, allowing for more precise comparisons, for example, showing whey protein has a higher DIAAS than soy protein isolate despite both having a PDCAAS of 1.0.

Complete vs. Incomplete Proteins

Complete Proteins: Contain all nine essential amino acids in sufficient quantities. Sources include animal products, soy, and quinoa.
Incomplete Proteins: Lack one or more essential amino acids. Most plant proteins like beans, lentils, nuts, and grains are incomplete. Vegans and vegetarians can achieve a complete amino acid profile by combining different incomplete protein sources throughout the day, such as rice and beans.

High-Quality Protein Sources at a Glance

This table provides a comparison of popular protein sources, illustrating their key characteristics.


Source	Type	Amino Acid Profile	Digestion Rate	Typical PDCAAS/DIAAS	Key Nutrient Benefits
Whey Protein	Animal (Dairy)	Complete, high in BCAAs (leucine)	Fast (1-2 hours)	PDCAAS 1.0, High DIAAS	Supports rapid muscle repair post-workout.
Casein Protein	Animal (Dairy)	Complete, high in glutamine	Slow (7-8 hours)	PDCAAS 1.0, High DIAAS	Sustained release of amino acids, ideal for overnight muscle maintenance.
Eggs	Animal	Complete, perfect ratio	Highly Digestible (95%+)	PDCAAS 1.0, benchmark for quality.	Rich in choline, vitamin D, and antioxidants.
Lean Beef	Animal	Complete, high in iron and zinc	Moderately Digestible	PDCAAS 0.92	Excellent source of heme iron and zinc.
Soy Protein	Plant	Complete	Digestible, varying speed	PDCAAS 1.0	Contains all essential amino acids, making it a valuable vegan option.
Quinoa	Plant	Complete	Moderately Digestible	N/A	Contains all essential amino acids, plus fiber.
Rice & Beans	Complementary Plant	Complete when combined	Moderately Digestible	PDCAAS of ~0.75 when combined.	Cost-effective and rich in fiber when paired.

Beyond the Numbers: Other Considerations

Beyond amino acid profiles and digestibility scores, the overall nutritional value of a food is important. Lean meats and fish provide high-quality protein along with healthy fats like omega-3s, while plant proteins offer fiber and other micronutrients. A diverse diet including both animal and plant protein sources is generally recommended for a wide range of nutrients. Opting for whole food sources over highly processed ones is also beneficial.

Conclusion

To determine which protein is high quality, assess its essential amino acid content and how well your body can digest it, using scores like PDCAAS and DIAAS as guides. While animal proteins often score highest, plant-based options like soy and quinoa are also complete proteins. Vegans and vegetarians can ensure adequate intake by combining various plant protein sources. Consider the full nutritional profile of foods for optimal health.

Frequently Asked Questions

The two main factors determining protein quality are the essential amino acid content and the protein's digestibility. A high-quality protein provides all nine essential amino acids and is efficiently absorbed and utilized by the body.

Most animal proteins are considered high quality because they are complete proteins, containing all nine essential amino acids. However, some animal products, particularly highly processed meats, may contain unhealthy levels of saturated fat or sodium.

Yes, soy protein is one of the few plant-based sources considered a complete protein, containing all essential amino acids. It has high digestibility and can be an excellent option for vegetarians and vegans.

By eating a varied diet that includes different plant protein sources, such as legumes, grains, nuts, and seeds, vegans and vegetarians can practice protein complementation to get all essential amino acids over the course of a day.

Both are milk-derived proteins, but they differ in absorption rate. Whey is fast-digesting, ideal for a quick amino acid spike, while casein is slow-digesting, providing a sustained release over several hours, which is beneficial before bed or during long fasting periods.

PDCAAS and DIAAS are scoring systems for evaluating protein quality. DIAAS is considered superior because it measures digestibility more accurately at the intestinal level and does not 'cap' the score at 1.0, providing a more precise ranking of high-quality proteins.

Both supplements and whole foods can provide high-quality protein. However, whole foods offer a broader spectrum of other nutrients like fiber, vitamins, and minerals that are important for overall health. Supplements can be a convenient way to boost protein intake, but they don't replace the benefits of a balanced diet.