How to determine high quality protein?

October 24, 2025 •

4 min read

According to the Food and Agriculture Organization (FAO), not all proteins are created equal in their ability to support human nutrition. So, how to determine high quality protein for your dietary needs? The answer goes beyond simply looking at the total grams per serving and requires a deeper understanding of amino acid composition and digestibility.

Quick Summary

Evaluating protein quality depends on its complete amino acid profile and how well the body absorbs it. Modern methods like the DIAAS score provide a superior measure of a protein's true nutritional value compared to older metrics, allowing for more informed dietary choices.

Key Points

Complete Amino Acid Profile: High-quality proteins must contain all nine essential amino acids that the body cannot produce on its own.
Digestibility Matters: Even with a perfect amino acid profile, a protein's quality is limited by how efficiently your body can digest and absorb it.
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 method.
Prioritize Diverse Sources: Incorporating a mix of animal and plant proteins, or complementing incomplete plant proteins, is the best way to ensure you're getting a full spectrum of amino acids.
Vet Protein Supplements: When buying powders, look for transparent labeling and third-party certifications to confirm ingredient quality and accuracy.
Consider the Protein Package: Always look at the full nutritional profile of a protein source, including the fats, fiber, and micronutrients that come along with it.

The ABCs of Protein Quality: Amino Acids and Digestibility

Protein is a crucial macronutrient made up of smaller units called amino acids. For adults, there are nine essential amino acids (EAAs) that the body cannot produce on its own and must obtain from food. A protein is considered "complete" if it contains all nine EAAs in sufficient amounts to meet the body's needs. This is the first and most fundamental aspect of high-quality protein. However, a complete amino acid profile is only part of the story. The protein's digestibility—how efficiently the body can break it down, absorb it, and utilize it—is equally important. A protein may have a perfect amino acid profile on paper, but if it is poorly digested, its true nutritional value is diminished.

Amino Acid Profile: Complete vs. Incomplete

Complete proteins, which are typically found in animal products, are prized because they contain all nine EAAs. Examples of complete animal proteins include beef, poultry, fish, eggs, and dairy. Fortunately, plant-based eaters can also find complete protein sources, such as soy products (tofu, tempeh, edamame), quinoa, and buckwheat.

Many plant-based proteins, however, are considered "incomplete" because they are deficient in one or more EAAs. For instance, legumes often lack methionine and cysteine, while grains are typically low in lysine. The good news is that combining different plant proteins throughout the day can create a complete amino acid profile. This strategy is known as complementary proteins. For example, the classic combination of rice and beans, or whole-wheat pita with hummus, creates a complete protein meal.

Digestibility and Absorption Rates

The rate at which your body digests and absorbs protein can influence its effectiveness, especially for specific goals like muscle recovery after a workout. Proteins are often categorized as "fast" or "slow" absorbing.

Fast-Digesting Proteins: These are quickly broken down, causing a rapid rise in blood amino acid levels. Whey protein is a prime example, often used post-workout.
Slow-Digesting Proteins: These digest slowly, releasing amino acids over a longer period. Casein protein, found in dairy, is an example often consumed before sleep.

Factors like a food's fiber content and presence of anti-nutritional factors (e.g., phytates, tannins in some plants) can also impact its digestibility.

Protein Quality Scoring Methods: DIAAS vs. PDCAAS

The Protein Digestibility-Corrected Amino Acid Score (PDCAAS) was historically used, but it had limitations like using unreliable fecal digestibility and truncating high scores. In 2013, the FAO introduced the Digestible Indispensable Amino Acid Score (DIAAS) to provide a more accurate evaluation. DIAAS is now preferred because it measures digestibility in the small intestine, doesn't truncate scores, is based on individual amino acid digestibility, and uses updated human amino acid requirements. A DIAAS score of 100 or higher indicates an excellent, high-quality protein.

Protein Source Comparison Table


Protein Source	DIAAS Score*	Amino Acid Profile	Absorption Rate	Notes
Whey Protein Isolate	1.09+	Complete	Fast	Rapid muscle protein synthesis; low lactose.
Milk	1.00+	Complete	Mixed (Whey + Casein)	Contains both fast and slow-digesting proteins.
Egg	1.00+	Complete	Slow	High bioavailability; whole food source.
Beef	1.00+	Complete	Moderate	High in iron and B12; also contains saturated fat.
Soy Protein Isolate	0.90+	Complete	Moderate-Fast	Excellent plant-based complete protein source.
Pea Protein	0.70+	Incomplete (low in methionine)	Moderate	Often combined with other proteins to complete profile.
Rice and Beans (Combined)	Varies	Complete	Moderate	Excellent example of complementary protein for vegans.
Lentils	0.40+	Incomplete (low in methionine)	Moderate	High in fiber; benefits from pairing with grains.

*Note: DIAAS scores can vary depending on specific processing methods and reference patterns used. Scores are examples and should be verified for specific products.

Practical Steps to Identify High-Quality Protein

For most people, a varied diet is the best way to ensure adequate intake of high-quality protein. Focus on incorporating a mix of both animal and plant-based proteins to benefit from their unique nutrient profiles. When purchasing protein supplements, take these steps to ensure you're getting a quality product:

Look for Transparency: Reputable brands provide detailed information on their sourcing and manufacturing processes and often disclose the full amino acid profile.
Check for Certifications: Since supplements are not FDA-regulated, look for third-party certifications (e.g., NSF Certified for Sport, Informed-Choice) that verify the product's contents and safety.
Examine the Ingredients: Read the ingredient list to check for excessive fillers, artificial additives, or hidden sugars. Ensure the primary protein source is clearly stated, such as "whey protein isolate" rather than a vague "protein blend".
Evaluate Cost per Gram: Extremely cheap products may use inferior protein sources or fillers. Calculate the cost per gram of protein to compare value across brands.
Prioritize Whole Foods: While supplements are convenient, remember that the 'protein package' from whole foods often includes other beneficial nutrients like fiber, vitamins, and minerals that aren't found in powders.

Conclusion

High-quality protein is defined by its complete amino acid profile and its high level of digestibility, measured most accurately today by the DIAAS method. While animal proteins like eggs and dairy naturally offer a high-quality profile, many plant proteins like soy and quinoa are also excellent. Plant-based diets can also achieve high protein quality through the complementary pairing of different food sources like legumes and grains. The best strategy for a nutrition diet is to diversify your protein intake and make informed choices by understanding the science behind protein quality. For further reading, consult the Food and Agriculture Organization's official website for more information on protein assessment methodology.

Frequently Asked Questions

Essential amino acids are the nine amino acids that your body cannot produce and must be obtained from your diet. Non-essential amino acids can be synthesized by your body, so dietary intake is not as critical for meeting your needs.

Animal proteins are generally considered complete and more digestible, but many plant proteins like soy, quinoa, and buckwheat are also complete proteins. Combining different plant-based foods can provide a complete amino acid profile, and the fiber in plants offers additional health benefits.

Complementary proteins are two or more incomplete protein sources that, when combined, provide all the essential amino acids your body needs. For example, combining rice (low in lysine) with beans (high in lysine) forms a complete protein meal.

DIAAS is more accurate because it measures digestibility in the small intestine, providing a true measure of absorbed amino acids, unlike PDCAAS which used fecal digestibility and overestimated values. DIAAS also does not truncate its scores, allowing for a better ranking of high-quality proteins.

While the body does not have a strict absorption limit, consuming more protein than needed at once simply slows down the digestion process. The amino acids are used for other metabolic purposes, not necessarily muscle building.

Check for transparent labeling that shows the full amino acid profile and confirms the protein source. Look for third-party certifications like NSF or Informed-Choice to ensure the product has been tested for safety and purity.

The "protein package" refers to all the other nutrients that come along with your protein source, such as the fiber, fats, and vitamins in a whole food like lentils versus the minimal extra nutrients in a processed protein powder.