How to tell if protein is good quality?

According to the Food and Agriculture Organization (FAO), the quality of a protein is primarily determined by its amino acid composition and digestibility. This guide will explain how to tell if protein is good quality by breaking down the key factors that influence its nutritional value for your body.

Quick Summary

This article explains how to assess protein quality based on its essential amino acid content and digestibility. It compares animal and plant protein sources, details key scoring methods like DIAAS and PDCAAS, and provides practical advice for making informed dietary choices.

Key Points

• Amino Acid Profile: High-quality protein contains all nine essential amino acids (EAAs) that the body cannot produce on its own.

• Completeness: Animal sources like meat, fish, and eggs are complete proteins, while many plant sources are incomplete but can be combined to meet needs.

• Digestibility Matters: The body's ability to break down and absorb protein, known as digestibility, directly impacts its quality.

• DIAAS vs. PDCAAS: Use the Digestible Indispensable Amino Acid Score (DIAAS) as the most accurate metric, as it accounts for individual amino acid digestibility at the ileum.

• Look for Certifications: For supplements, check for third-party certifications like NSF Certified for Sport to ensure quality and safety.

• Ingredient Awareness: Be cautious of excess fillers, artificial additives, and protein spiking in supplements, which indicate lower quality.

• Whole Food First: Prioritize diverse whole-food protein sources for a balance of amino acids and other vital nutrients.

Understanding the Building Blocks: Essential Amino Acids

Protein quality is fundamentally determined by the amino acids it provides. Proteins are complex molecules made up of smaller units called amino acids. While the body can produce 11 of the 20 amino acids, the remaining nine, known as essential amino acids (EAAs), must be obtained from food. A protein source that contains all nine EAAs in sufficient amounts is called a "complete protein" and is generally considered to be of higher quality. Conversely, an "incomplete protein" lacks one or more of these essential building blocks.

The Importance of a Complete Amino Acid Profile

For building and repairing tissues, all nine EAAs must be present. If a protein source is low in a particular EAA, it becomes the "limiting amino acid," which restricts the body's ability to synthesize new proteins, even if other amino acids are abundant. This is a key reason why animal proteins are often regarded as higher quality; they naturally contain all nine EAAs in the right proportions for human needs. However, careful combination of different plant proteins can also provide all the necessary EAAs over the course of a day.

The Role of Digestibility

Amino acid composition is only half the story. A protein's quality is also affected by its digestibility—how effectively the body can break down and absorb its amino acids.

Several factors influence protein digestibility:

• Source: Animal proteins generally have higher digestibility (91–99%) than plant-based proteins (70–90%) due to the food matrix.
• Processing: Methods like heating can break down anti-nutritional factors in plant foods (e.g., trypsin inhibitors), improving digestibility. Conversely, overheating can damage amino acids, lowering quality.
• Anti-nutritional factors: Compounds found in some plants, like tannins or lectins, can interfere with protein digestion and absorption.

Scientific Scoring Methods: DIAAS and PDCAAS

Dietary protein quality is not just a matter of guesswork; it is measured using scientific scoring systems. The two most prominent are the Protein Digestibility Corrected Amino Acid Score (PDCAAS) and the newer Digestible Indispensable Amino Acid Score (DIAAS).

PDCAAS

• Calculation: Compares a food's amino acid profile to a standard reference pattern and corrects for fecal digestibility.
• Scoring: Ranges from 0.0 to 1.0, with values capped at 1.0. This capping was a major criticism, as it prevented differentiation between high-quality proteins.

DIAAS

• Calculation: Considered a more accurate and advanced method, DIAAS uses true ileal digestibility—measuring absorption at the end of the small intestine rather than through feces. It also assesses digestibility for each individual essential amino acid.
• Scoring: Scores are not capped at 1.0, allowing for better comparison and indicating a protein's potential to complement other foods. DIAAS also uses age-specific amino acid reference patterns.

Animal vs. Plant Protein: A Comparison

While animal proteins are typically considered superior in quality due to their complete amino acid profile and high digestibility, a well-planned plant-based diet can easily meet all protein requirements.

Feature	Animal Protein	Plant Protein
Essential Amino Acid Profile	Complete; contains all nine EAAs in optimal proportions.	Often incomplete, meaning they are low in one or more EAAs.
Digestibility	Very high (91–99%).	Generally lower (70–90%), but can be improved with processing.
Sourcing & Cost	Varies, but often a primary and concentrated source.	Can be more affordable, especially legumes and grains. Requires variety for completeness.
Fiber & Nutrients	No fiber, but rich in other nutrients like B12, iron, and zinc.	High in fiber, vitamins, and antioxidants; often lower in saturated fat.
Environmental Impact	Significantly higher environmental footprint than most plant-based options.	Generally has a lower environmental impact.

How to Assess Your Protein Sources

To evaluate the quality of protein in your diet, follow these practical steps:

• Read the nutrition label: Look for the total protein content per serving. For processed foods, check the ingredients list for the source of the protein.
• Check for completeness: If following a vegetarian or vegan diet, ensure you are combining complementary protein sources (e.g., rice and beans, hummus and pita bread) to obtain all nine essential amino acids.
• Consider digestibility: While ileal digestibility (used in DIAAS) is not on food labels, recognize that plant proteins are less digestible and may require slightly higher intake. Processing like cooking and sprouting can help.
• Look for quality certifications: When buying protein supplements, look for independent third-party certifications like NSF Certified for Sport or Informed-Sport. These verify that the product has been tested for banned substances and meets quality standards.
• Check the ingredients list (for supplements): A simpler, shorter ingredients list is generally better. Avoid products with excessive artificial additives, fillers, or sweeteners. Watch out for "protein spiking," where cheap amino acids like glycine are added to boost the total protein count on the label.

Conclusion

Determining the quality of protein goes beyond simply counting grams. It is a nuanced process involving the analysis of amino acid content and digestibility, as scientifically measured by metrics like DIAAS. For most people, a balanced diet including a variety of both animal and plant proteins is the most effective way to ensure high-quality intake. However, by understanding the principles of amino acid completeness and digestibility, those on plant-based diets can confidently meet their nutritional needs. Ultimately, being an educated consumer by reading labels, checking for certifications, and prioritizing whole food sources is the best way to ensure your protein is truly high quality.

References

• Envirocare Labs. (2025). How Amino Acid Analysis Drives Protein Quality Assessment. Retrieved from https://envirocarelabs.com/how-amino-acid-analysis-drives-protein-quality-assessment/
• Study.com. (2023). Complete Protein | Definition, Examples & Sources. Retrieved from https://study.com/learn/lesson/complete-protein-sources-examples.html
• Bandyopadhyay, S., et al. (2022). Evaluation of Protein Quality in Humans and Insights on Stable Isotopes. Advances in Nutrition. https://pmc.ncbi.nlm.nih.gov/articles/PMC9340995/
• WebMD. (2024). The Difference Between Animal Protein and Plant Protein. Retrieved from https://www.webmd.com/diet/difference-between-animal-protein-plant-protein
• Certified Laboratories. (2025). Understanding Protein Quality: The Role of PDCAAS. Retrieved from https://certified-laboratories.com/blog/understanding-protein-quality-the-role-of-pdcaas/
• Quora. (2016). Is there a way to assess the purity (and quality) of protein powder?. Retrieved from https://www.quora.com/Is-there-a-way-to-assess-the-purity-and-quality-of-protein-powder-How-do-they-review-protein-powder-brands

Frequently Asked Questions

What is the difference between complete and incomplete protein?

A complete protein contains all nine essential amino acids that the body cannot produce, such as those found in eggs, meat, and soy. An incomplete protein is low in one or more essential amino acids, but these can be combined to form a complete profile over time.

Are animal proteins always better quality than plant proteins?

Animal proteins generally have a complete amino acid profile and higher digestibility. However, a varied plant-based diet that combines different protein sources can provide all essential amino acids and be nutritionally complete. Some plant proteins like soy and quinoa are also complete.

What is DIAAS and why is it better than PDCAAS?

DIAAS (Digestible Indispensable Amino Acid Score) is the current standard for measuring protein quality, measuring true ileal digestibility for each essential amino acid. It is more accurate than the older PDCAAS, which was criticized for using cruder digestibility metrics and capping high scores at 1.0.

What is protein spiking?

Protein spiking is a deceptive practice where manufacturers add cheap, non-essential amino acids (like glycine or taurine) to supplements to artificially inflate the total protein count on the label. Checking for third-party testing can help avoid products that engage in this practice.

Can processing affect protein quality?

Yes, processing can affect protein quality. For some plant-based proteins, heating can remove anti-nutritional factors and improve digestibility. However, overheating can damage amino acids and reduce their availability.

How can vegetarians or vegans ensure they get high-quality protein?

Vegetarians and vegans should consume a wide variety of plant-based protein sources, such as legumes, whole grains, nuts, and seeds. Combining these foods, like rice and beans or peanut butter on whole-wheat bread, ensures all essential amino acids are consumed.

What should I look for on a protein supplement label to check for quality?

Check for third-party certification logos (e.g., NSF Certified for Sport). Look for a simple, minimal ingredients list with transparent sourcing. Be wary of proprietary blends and supplements with excessive fillers or additives.