Which is the Highest Quality Protein Source? An In-Depth Guide

October 8, 2025 •

4 min read

According to the Food and Agriculture Organization, the Digestible Indispensable Amino Acid Score (DIAAS) is the most accurate method for determining which is the highest quality protein source, replacing the older PDCAAS system. Protein quality is determined by the balance of essential amino acids and digestibility, both crucial for a food's nutritional value.

Quick Summary

This article explains how protein quality is measured using the DIAAS scoring system, comparing top animal-based options like eggs and milk with plant-based sources such as soy and quinoa, while outlining the factors defining a protein's nutritional value.

Key Points

Eggs and Milk Products High DIAAS: Animal proteins like eggs and dairy (whey, casein) consistently rank highest due to their complete amino acid profile and high digestibility.
DIAAS vs. PDCAAS: DIAAS is the updated, gold-standard measure for protein quality, offering a more accurate score by measuring ileal digestibility and avoiding score truncation.
High-Quality Plant Options: Complete plant proteins exist, such as soy (tofu, edamame), quinoa, and hemp seeds, making it achievable for vegetarians and vegans to get high-quality protein.
Complementary Proteins: Combining different plant foods, like rice and beans, provides a complete amino acid profile to maximize nutritional benefit.
Vary Your Sources: The best strategy is to consume a variety of protein sources, both animal and plant-based, to ensure a full spectrum of nutrients and other health benefits.
Absorption Rate Matters: For athletes, absorption speed is relevant, with fast-digesting whey ideal for post-workout and slow-digesting casein best before bed.

Understanding Protein Quality

Protein is a macronutrient vital for building and repairing tissues, producing hormones, and supporting immune function. All proteins are made of building blocks called amino acids. Of the 20 amino acids, nine are 'essential' because the body cannot produce them and must obtain them from food. The quality of a protein is judged by its amino acid profile and digestibility.

For decades, the Protein Digestibility Corrected Amino Acid Score (PDCAAS) was the standard for measuring protein quality. However, the Food and Agriculture Organization (FAO) now recommends the Digestible Indispensable Amino Acid Score (DIAAS) as the superior method. DIAAS is more accurate because it measures amino acid digestibility at the end of the small intestine (ileal digestibility), giving a better representation of amino acid absorption. Unlike PDCAAS, DIAAS scores are not capped at 1.00, allowing for a more precise ranking of high-quality proteins. A DIAAS score over 100 indicates a protein can supply the full amount of indispensable amino acids needed when consumed at the daily protein requirement.

Factors Determining Protein Quality

Amino Acid Profile: The presence of all nine essential amino acids is paramount. A protein containing all nine is called a 'complete' protein.
Digestibility: This refers to how well the body can break down and absorb the protein. Animal proteins tend to be highly digestible.
Bioavailability: The efficiency with which the body can utilize the absorbed amino acids is also a key component of quality.

High-Quality Animal-Based Proteins

Animal sources are typically considered the gold standard for protein quality due to their complete amino acid profile and high digestibility. Here are some of the highest quality options:

Eggs: Often cited as the perfect protein source, eggs have a well-balanced amino acid profile and are highly bioavailable. A hard-boiled egg can have a DIAAS score of 1.13.
Milk Protein (Whey and Casein): Milk contains two main proteins: whey and casein. Both are complete proteins, but they differ in absorption speed. Whey is a 'fast' protein, quickly absorbed to rapidly boost muscle protein synthesis, making it ideal for post-workout recovery. Casein is a 'slow' protein, providing a sustained release of amino acids over several hours, which is excellent for muscle repair during sleep. Milk protein concentrate can have an exceptionally high DIAAS of 1.18.
Lean Meats: Chicken breast, turkey, and lean beef are all excellent sources of complete, high-quality protein. They also provide essential nutrients like iron and zinc.
Fish: Fatty fish like salmon and tuna, along with white fish like halibut, offer high-quality protein and beneficial omega-3 fatty acids.

High-Quality Plant-Based Proteins

For those on vegetarian or vegan diets, high-quality protein is easily achievable. While many plant proteins are incomplete on their own, some are complete, and strategic combinations can create a full amino acid profile over the course of a day.

Soy: Products like tofu, tempeh, and edamame are complete proteins and provide all nine essential amino acids. Soy protein isolate has a DIAAS of 0.898.
Quinoa: This gluten-free seed is a complete protein source, offering all essential amino acids.
Hemp and Chia Seeds: Both hemp and chia seeds are considered complete or near-complete proteins, packing a substantial amount of protein along with healthy fats and fiber.
Combining Incomplete Proteins: Common combinations like rice and beans or hummus with pita bread can provide a complete protein profile.

The Verdict: Which is the Highest Quality Protein Source?

Ultimately, there is no single 'best' protein source for everyone, but there are clear nutritional winners based on scientific metrics. Eggs and dairy stand out due to their high DIAAS scores and digestibility. However, relying on one source is not necessary. The highest quality protein is derived from a varied, nutrient-dense diet that aligns with individual goals and preferences. For a comprehensive overview of how DIAAS is measured and why it is the preferred method, the FAO's report provides extensive detail. Source: Protein quality as determined by the Digestible Indispensable Amino Acid Score (DIAAS) from the National Institutes of Health (NIH).

High-Quality Protein Sources: A Comparative Table


Source	Type	Digestibility/Absorption	DIAAS Score (Approx.)	Key Takeaway
Eggs	Animal (Complete)	High	1.13	Often called the 'perfect' protein due to balance and bioavailability.
Milk Protein Concentrate	Animal (Complete)	High	1.18	High-quality source of both slow (casein) and fast (whey) proteins.
Whey Protein	Animal (Complete)	Very Fast	>1.00	Ideal for rapid post-exercise recovery due to quick absorption.
Casein Protein	Animal (Complete)	Very Slow	>1.00	Sustained amino acid release, best for preventing overnight muscle breakdown.
Chicken Breast	Animal (Complete)	High	~1.08	Excellent lean source of complete protein.
Beef	Animal (Complete)	High	~1.11	High in bioavailable iron and zinc, choose lean cuts.
Soy Protein Isolate	Plant (Complete)	High	~0.90	An effective and complete plant-based option.
Quinoa	Plant (Complete)	Medium	~0.79	A whole grain source of all essential amino acids.
Pea Protein Isolate	Plant (Incomplete)	Medium	~0.82	A good plant-based source, but limited in methionine.
Cooked Lentils	Plant (Incomplete)	Medium	~0.58	Nutrient-rich, but benefits from combining with a grain.

Conclusion

The pursuit of the highest quality protein source is not about finding a single, magic food. Instead, it is a nuanced process involving the understanding of amino acid profiles, digestibility, and absorption timing. While animal products like eggs and dairy consistently show the highest DIAAS scores, excellent nutrition can be achieved through both animal and plant-based diets by focusing on variety and nutrient density. For optimal health and muscle support, consume a mixture of high-quality proteins throughout the day, and consider your individual dietary needs and exercise regimen. Combining different plant-based proteins strategically ensures a full spectrum of amino acids, while lean animal sources offer exceptionally high bioavailability. Prioritizing quality over quantity in a balanced diet is the ultimate key to success.

Frequently Asked Questions

No, not necessarily. While many individual plant proteins are 'incomplete' (lacking sufficient levels of one or more essential amino acids), a varied plant-based diet can easily provide all essential amino acids. Complete plant proteins like soy and quinoa are excellent alternatives to animal sources.

DIAAS is the current scientific standard for measuring protein quality. It assesses the digestibility of each essential amino acid at the end of the small intestine, providing a more accurate score than older methods like PDCAAS.

Digestibility affects how efficiently your body can absorb and use the amino acids from a protein source. Highly digestible proteins, like those found in eggs and dairy, are more bioavailable and thus considered higher quality.

Cooking can affect protein quality, but typically in minor ways. For example, high-heat processing can slightly reduce the bioavailability of certain amino acids, but for most whole foods, the effect is not significant.

Neither is definitively 'better,' but they have different functions. Whey is absorbed quickly and is ideal for post-workout recovery, while casein is absorbed slowly, providing a sustained release of amino acids, making it good for overnight muscle repair.

Yes, a well-planned vegan diet can provide all the highest quality protein. By combining various plant sources, especially those with complete profiles like soy and quinoa, and ensuring high digestibility, a vegan can meet all amino acid requirements.

A complete protein is a food source that contains all nine of the essential amino acids your body cannot produce on its own. Animal products, soy, quinoa, and hemp seeds are examples of complete proteins.