What is the best source of digestible protein? A comprehensive guide

October 11, 2025 •

4 min read

Research consistently shows that a protein's quality is defined by both its amino acid profile and its digestibility, which is why understanding what is the best source of digestible protein is crucial for anyone looking to maximize nutritional intake. For a long time, protein quality was assessed by the Protein Digestibility Corrected Amino Acid Score (PDCAAS), but modern science now relies on the more accurate Digestible Indispensable Amino Acid Score (DIAAS) to identify the highest quality sources.

Quick Summary

The best sources of digestible protein often include animal products like eggs and dairy, along with processed plant proteins like soy and pea isolates. The DIAAS metric, favored over older scoring systems, evaluates the true absorption of amino acids, offering a more precise measure of a food's protein quality.

Key Points

DIAAS is the superior metric: The Digestible Indispensable Amino Acid Score (DIAAS) is now the gold standard for measuring protein quality, as it more accurately reflects amino acid absorption in the small intestine than the older PDCAAS method.
Animal proteins offer high digestibility: Eggs, milk proteins (whey, casein), lean meat (chicken, beef), and fish consistently have high DIAAS and true digestibility scores, making them excellent sources of high-quality, easily absorbed protein.
Processing enhances plant protein digestibility: While some whole plant foods have lower digestibility due to fiber and antinutrients, processing them into isolates (like soy or pea protein) can significantly improve amino acid absorption.
Cooking affects digestibility: Moderate cooking generally improves protein digestibility by denaturing proteins and reducing antinutrients, but excessive heat can create undesirable chemical changes that hinder absorption.
Variety is key for plant-based diets: Since many plant proteins are incomplete, combining different sources like legumes and grains is the best strategy for vegetarians and vegans to ensure they consume all essential amino acids for high overall protein quality.
Digestibility isn't the only factor: The best protein choice also depends on individual health goals and dietary preferences. For gut-sensitive individuals, some animal proteins may be easier to digest, while others might prefer processed plant proteins for better tolerance.

Understanding Protein Digestibility and Quality

Protein is a crucial macronutrient made up of amino acids, which are the building blocks of muscle, enzymes, and hormones. Protein quality isn't just about the total quantity; it’s about the composition of amino acids and how easily the body can absorb and utilize them. The term "digestibility" refers to the proportion of consumed protein that is actually broken down and absorbed by the body.

For years, protein quality was measured using the PDCAAS, a system that had limitations, including its inability to account for the true amount of amino acids absorbed before they reached the colon. The Food and Agriculture Organization (FAO) of the United Nations now recommends the DIAAS as the superior method for measuring protein quality. DIAAS evaluates the digestibility of individual indispensable (essential) amino acids in the small intestine, providing a more accurate measure of protein bioavailability. A DIAAS score above 1.0 indicates a high-quality, excellent protein source that can even help compensate for lower-quality proteins in a diet.

Top Sources of Highly Digestible Protein

Several food sources consistently rank high for protein digestibility and quality. These are often categorized by their origin, with animal proteins generally having higher inherent digestibility than most whole plant proteins due to factors like fiber and antinutrients in plants.

Animal-based proteins

Eggs: Boiled whole eggs are a top-tier source, with a DIAAS score of 1.12 for individuals over three years old, due to their excellent balance of essential amino acids and high digestibility.
Milk Proteins (Whey and Casein): Dairy proteins are exceptionally digestible. Whey protein isolate and concentrate have high DIAAS scores (up to 1.25 for isolate, 1.33 for concentrate), while casein also scores very high. Whey is known for its faster digestion, making it ideal post-exercise, while casein digests more slowly.
Lean Meats and Fish: White meats like chicken breast (DIAAS 1.08) and pork tenderloin (DIAAS 1.17) are highly digestible. Fish, such as salmon (DIAAS 1.00) and tilapia, are also excellent, easily-digested protein sources.

Plant-based proteins

Soy Protein Isolate: This highly processed form of soy removes much of the fiber and antinutrients, boosting its digestibility and DIAAS score significantly (DIAAS up to 1.05).
Tofu and Tempeh: As minimally processed soy products, tofu and especially fermented tempeh offer good protein quality. The fermentation process in tempeh further aids digestibility.
Pea Protein Isolate: Isolate powders are more digestible than whole peas because the processing removes fiber and other compounds that hinder absorption. Pea protein isolate can achieve a high DIAAS score (0.82-0.89).
Quinoa: This seed is one of the few plant sources that contains all nine essential amino acids, making it a complete protein.
Hemp Seeds: Another complete plant protein, hemp seeds are highly digestible and rich in healthy fats and fiber.

Factors Influencing Protein Digestibility

Beyond the source itself, several factors can affect how well your body digests protein:

Processing Methods: Cooking and other heat treatments can either enhance or diminish digestibility. Moderate heat can denature proteins, making them more accessible to digestive enzymes, while very high heat can create cross-links that hinder absorption. Fermentation, used to make products like tempeh, also improves digestibility by breaking down complex proteins and reducing antinutrients.
Antinutrients: Compounds naturally found in many plants, like phytates and tannins, can bind to protein and inhibit its digestion. Soaking, sprouting, and cooking legumes are common techniques to reduce these effects.
Fiber Content: The high fiber content in many whole plant foods can also slow digestion and reduce the absorption of protein. While fiber is beneficial for overall gut health, a diet primarily based on unprocessed plant foods may require a slightly higher total protein intake to compensate for lower digestibility.

Comparison of Digestibility (DIAAS Scores)

The following table provides a comparison of Digestible Indispensable Amino Acid Score (DIAAS) values for some common protein sources, based on the requirements for individuals over three years old.


Food Source	DIAAS Score
Milk Protein Concentrate	1.41
Whey Protein Concentrate	1.33
Egg, boiled	1.12
Beef	1.12
Chicken breast	1.08
Soy Protein Isolate	0.98
Tofu	0.97
Chickpeas	0.85
Pea Protein Concentrate	0.73
Oats	0.57
Cooked Kidney Beans	0.51

The Role of a Varied Diet

While focusing on highly digestible sources is beneficial, a varied diet is key to ensuring a complete spectrum of nutrients. A vegetarian or vegan diet, often consisting of lower individual DIAAS scores, can still provide excellent protein quality by combining different plant sources. For instance, combining a legume (rich in lysine) with a grain (rich in methionine) within the same meal or throughout the day can ensure all essential amino acids are consumed. This principle of protein complementarity makes it possible to build a complete and highly nutritious dietary pattern, regardless of whether you primarily consume animal or plant-based proteins.

Ultimately, what is the best source of digestible protein depends on individual preferences, dietary restrictions, and health goals. For most, incorporating a mix of highly digestible animal and plant-based options is the most effective strategy. Eggs, milk, and lean meats offer a straightforward path to high-quality protein, while processed plant isolates and carefully combined whole plant foods provide equally valuable alternatives.

For more on the shift from PDCAAS to DIAAS, consider exploring FAO resources.

Conclusion

Choosing the best sources of digestible protein means looking beyond mere protein content to consider the bioavailability of the amino acids within. By understanding the advantages of sources like eggs, whey, and lean meats, alongside highly digestible plant options such as soy and pea protein isolates, individuals can make more informed choices. Whether your goal is muscle repair, weight management, or simply better overall health, prioritizing protein digestibility is a powerful strategy for optimizing your nutritional intake.

Frequently Asked Questions

DIAAS (Digestible Indispensable Amino Acid Score) is the more modern and accurate metric for protein quality, measuring digestibility at the end of the small intestine. PDCAAS, the older method, measures overall fecal digestibility and is considered less precise.

Animal proteins generally possess higher inherent digestibility compared to many whole plant proteins, primarily because of the fiber and antinutrients present in plants. However, processing can significantly improve the digestibility of plant proteins.

No, not all plant proteins have a high DIAAS score. Unprocessed whole legumes and grains, for instance, often have lower scores than processed isolates or animal proteins. Scores vary significantly depending on the specific plant and its processing.

Yes, cooking can affect protein digestibility. Moderate cooking, like boiling or steaming, can improve it by denaturing proteins. However, excessive heat can sometimes reduce digestibility by damaging amino acids.

Both whey and casein are highly digestible milk proteins, but whey is absorbed and utilized by the body faster than casein. This makes whey a popular choice post-exercise for rapid muscle protein synthesis, while casein provides a slower, more sustained release.

Vegans and vegetarians can ensure a high intake of digestible protein by consuming a variety of plant sources, such as combining legumes and grains to form a complete amino acid profile. Choosing processed plant protein isolates (like soy or pea) is also an effective strategy.

Highly digestible protein sources are generally beneficial for most people, especially athletes or individuals with compromised digestive health, as they ensure maximum nutrient absorption. However, a balanced diet incorporating a mix of sources is always recommended for comprehensive nutrition.