How do I know the quality of protein? A comprehensive guide

October 11, 2025 •

4 min read

According to the Food and Agriculture Organization (FAO), protein quality is measured by how well a food meets the body's essential amino acid needs. Understanding how do I know the quality of protein? is key to maximizing nutritional intake for health and fitness goals.

Quick Summary

This article explains how to determine protein quality by evaluating its amino acid content, digestibility, and bioavailability. It compares animal versus plant protein sources and introduces key metrics like DIAAS.

Key Points

Amino Acid Profile: Protein quality is primarily determined by its essential amino acid (EAA) content and balance, not just the quantity.
DIAAS vs. PDCAAS: The Digestible Indispensable Amino Acid Score (DIAAS) is the most accurate protein quality metric, surpassing the older PDCAAS by measuring true ileal digestibility and individual EAAs.
Digestibility is Key: High-quality protein is highly digestible, meaning the body can absorb and use its amino acids efficiently. Animal proteins are typically more digestible than plant proteins.
Complementation for Plant-Based Diets: Vegans and vegetarians can achieve a complete amino acid profile by combining different protein sources, like pairing legumes with grains, over the course of the day.
Processing Matters: Food processing methods can either improve (isolates) or degrade (extreme heat) protein quality and digestibility.
Diversify Your Sources: For optimal nutrition, focus on eating a variety of protein sources rather than fixating on a single one, as a varied diet naturally provides a full spectrum of amino acids.

Understanding the building blocks: Amino acids

Protein is a crucial macronutrient made up of smaller units called amino acids. There are 20 different amino acids that the human body uses, and they fall into two categories: essential and non-essential.

Essential Amino Acids (EAAs): There are nine EAAs that our bodies cannot produce on their own, meaning they must be obtained through our diet. These are histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
Non-Essential Amino Acids: Our bodies can synthesize the remaining 11 amino acids, so we don't need to get them directly from food. However, some are considered conditionally essential during specific states like illness or trauma.

A protein source is considered 'complete' if it contains all nine essential amino acids in sufficient quantities. Animal proteins like meat, eggs, and dairy are typically complete, while many plant proteins are considered incomplete because they lack adequate amounts of one or more EAAs.

Why protein quality matters

Simply looking at the total grams of protein on a food label doesn't tell the whole story. The quality of that protein dictates how efficiently your body can use it for vital functions like building and repairing tissues, producing hormones, and supporting immune health. High-quality protein sources have a complete amino acid profile and high bioavailability, meaning the amino acids are easily absorbed and utilized by the body. Lower-quality proteins, which might be deficient in certain EAAs or harder to digest, require a higher total intake to achieve the same metabolic effect.

The gold standard: DIAAS vs. PDCAAS

For decades, the Protein Digestibility Corrected Amino Acid Score (PDCAAS) was the standard for measuring protein quality. However, its methodology had significant flaws, including overestimating the quality of certain proteins by using fecal digestibility and capping all scores at 100%. In 2013, the Food and Agriculture Organization (FAO) recommended the Digestible Indispensable Amino Acid Score (DIAAS) as a superior method.

How DIAAS works

The DIAAS provides a more accurate and advanced assessment of protein quality by:

Measuring true ileal digestibility: Instead of measuring digestibility from fecal samples contaminated by bacteria, DIAAS measures absorption at the end of the small intestine (the ileum), where absorption is considered complete.
Considering individual EAAs: It measures the digestibility of each individual EAA, providing a more detailed picture of how well the body can absorb all necessary components.
Removing truncation: Unlike PDCAAS, DIAAS values are not capped at 100%, allowing for a true ranking of high-quality protein sources.

Animal vs. plant protein: A digestibility comparison

Animal proteins generally have higher digestibility and bioavailability than plant proteins. This is partly due to their different amino acid sequences and the absence of certain antinutritional factors (ANFs) found in some plants.


Feature	Animal Protein (e.g., Eggs, Milk, Beef)	Plant Protein (e.g., Legumes, Grains)
EAA Profile	Generally complete, containing all nine EAAs in balanced ratios.	Often incomplete, with one or more limiting EAAs (e.g., lysine in grains, methionine in legumes).
Digestibility	Very high (typically >95% ileal digestibility).	Lower due to antinutrients (e.g., phytates, fiber) and tougher plant cell walls.
Processing Impact	Less affected by typical processing like cooking, but can lose availability with extreme heat.	Processing methods like soaking, sprouting, and fermentation can significantly improve digestibility and reduce antinutrients.
Bioavailability	High; amino acids are easily absorbed and utilized by the body.	Lower than animal proteins, but a varied diet can compensate.
DIAAS Score	Typically high (e.g., whey protein isolate can exceed 100%).	Often lower than animal proteins in whole form, but isolates can score highly (e.g., soy isolate).

Practical steps for assessing protein quality in your diet

1. Prioritize complete proteins

Ensure your diet includes sources that contain all nine essential amino acids. For omnivores, this is straightforward with eggs, dairy, meat, and fish. Vegans can achieve this with sources like soy (tofu, tempeh), quinoa, and buckwheat.

2. Practice protein complementation

If you rely on plant-based sources, you don't need to eat all complementary proteins in the same meal, but you should aim for a varied diet throughout the day. Combining different incomplete proteins—such as grains and legumes (e.g., rice and beans), or legumes with seeds (e.g., hummus with whole-wheat pita)—can provide a full amino acid profile.

3. Consider protein supplements

If you have high protein needs (e.g., athletes, elderly) or follow a strict diet, supplements can be beneficial. Look for high-quality, highly digestible options like whey, casein, or purified plant-based protein isolates (e.g., pea protein isolate, soy isolate).

4. Factor in food processing

Different forms of food can affect protein bioavailability. For example, a protein isolate from soy or peas has higher digestibility than the whole bean or pea flour. Similarly, simple cooking methods like boiling can improve the digestibility of some plant foods by deactivating antinutrients.

5. Evaluate your overall diet

Consider the context of your entire diet, not just individual foods. A diverse and balanced diet rich in various whole foods will naturally provide a good mix of amino acids. For many, obsessing over the precise DIAAS score of every food is unnecessary, as long as overall intake is varied and sufficient. The ultimate goal is to meet your body's essential amino acid needs. For more specific dietary concerns, consult a healthcare provider or a registered dietitian.

Conclusion

Knowing the quality of protein involves looking beyond the macronutrient count to understand its essential amino acid profile and digestibility. While DIAAS is the most accurate scientific metric for evaluation, practical strategies like incorporating complete proteins, complementing incomplete sources, and diversifying your diet are effective for most people. Animal proteins often boast superior digestibility, but a well-planned plant-based diet can easily meet all essential amino acid needs. By understanding these factors, you can make informed choices to optimize your nutrition and support overall health.

Complete Vegan Protein Sources

Soy Products: Tofu, tempeh, and edamame.
Grains: Quinoa and buckwheat.
Seeds: Chia and hemp seeds.
Combinations: Rice and beans, or hummus and pita bread.
Protein Powders: High-quality pea or soy protein isolates.

Frequently Asked Questions

Essential amino acids are the nine amino acids that your body cannot produce and must obtain through diet. They are crucial for building new proteins, repairing tissues, and other vital functions.

DIAAS measures protein quality more accurately by using true ileal digestibility and assessing each individual essential amino acid, while the older PDCAAS method uses less precise fecal digestibility and truncates scores at 100%.

While many individual plant proteins have lower digestibility and lack one or more essential amino acids, a varied and balanced plant-based diet can provide all the necessary amino acids. Processed isolates can also achieve high quality scores.

Protein complementation is the practice of combining different plant-based foods that are deficient in different essential amino acids to create a complete protein profile, such as eating rice with beans.

Yes, some processed protein, like soy or pea protein isolates, can have very high quality scores because the isolation process removes antinutrients and improves digestibility. However, other processing methods can cause damage.

No. Your body maintains a pool of amino acids, and as long as you consume a variety of protein sources throughout the day, you will get all the essential amino acids you need.

Animal proteins like whey, casein, and eggs often have some of the highest DIAAS scores. Some refined plant protein isolates, such as those from soy or pea, can also score very highly.