How Do You Check Protein Quality? A Deep Dive into Nutritional Metrics

October 13, 2025 •

3 min read

Over one billion people worldwide face protein inadequacy, with the quality of protein being a crucial factor, especially in resource-poor areas. This critical aspect of nutrition goes beyond simply counting protein grams; understanding how do you check protein quality is essential for optimizing your diet for muscle synthesis, tissue repair, and overall health.

Quick Summary

This article explores the modern methods used to determine protein quality, focusing on the Digestible Indispensable Amino Acid Score (DIAAS) and its predecessor, the Protein Digestibility Corrected Amino Acid Score (PDCAAS). It details the key factors of amino acid profile and digestibility, explains how to combine proteins, and provides practical advice for evaluating protein sources in your diet.

Key Points

Amino Acid Profile is Key: A high-quality protein provides a complete and balanced profile of the nine essential amino acids (EAAs).
DIAAS is the Modern Standard: The Digestible Indispensable Amino Acid Score (DIAAS) is the most accurate method, measuring amino acid absorption in the small intestine and providing a non-truncated score.
Digestibility is Crucial: How easily your body can break down and absorb protein impacts its quality; animal proteins are generally more digestible than plant proteins.
Complementary Proteins Work: Plant-based eaters can achieve a high-quality protein intake by combining different plant sources that provide the full spectrum of EAAs.
Read Beyond the Protein Grams: Relying solely on the total protein grams on a label can be misleading. Consider the source and, if possible, the DIAAS or PDCAAS score for a more accurate assessment.

Understanding Protein Quality: More Than Just the Quantity

Protein is a cornerstone of a healthy diet, but not all protein is created equal. The biological value of a protein source depends on two primary factors: its amino acid composition and its digestibility. The human body requires a specific set of nine essential amino acids (EAAs) that it cannot synthesize on its own, and a high-quality protein provides these in sufficient quantities and proportions. Furthermore, the protein must be easily digestible so the body can effectively absorb and utilize the amino acids.

The Evolution of Protein Quality Scoring

For decades, scientists have sought a standardized method to rank protein quality, leading to the development of systems like the Protein Digestibility Corrected Amino Acid Score (PDCAAS) and the newer Digestible Indispensable Amino Acid Score (DIAAS).

The PDCAAS Method

Introduced in 1993, PDCAAS compared a protein's amino acid profile to a reference and adjusted the score based on fecal digestibility. However, it had limitations, including truncated scores capped at 1.0, which didn't differentiate truly superior proteins, and the use of fecal digestibility, which could overestimate true digestibility.

The DIAAS Method: A Superior Standard

The FAO recommended DIAAS in 2013 to improve upon PDCAAS. DIAAS provides a more accurate assessment by measuring digestibility at the end of the small intestine (ileum) for individual amino acids. Its scores are not capped, allowing for better ranking of high-quality proteins. DIAAS also uses age-specific amino acid reference patterns for greater relevance.

Comparison of PDCAAS vs. DIAAS


Feature	PDCAAS (Older Standard)	DIAAS (Current Standard)
Digestibility Measure	Fecal digestibility of crude protein	Ileal digestibility of individual amino acids
Sampling Location	Feces	End of small intestine (ileum)
Max Score	Truncated at 1.00	Non-truncated (can exceed 100%)
Reference Pattern	Based on 2-5 year old children	Based on age-specific human requirements
Comparative Value	Less useful for comparing high-quality proteins	Provides a more accurate ranking and differentiation

The Role of Essential Amino Acids and Digestibility

Protein quality is defined by its essential amino acid (EAA) profile and its digestibility. Complete proteins, like most animal sources and some plant foods such as soy and quinoa, contain all nine EAAs. Incomplete proteins, common in most plant sources, lack sufficient quantities of one or more EAAs. Digestibility is crucial for the body's use of amino acids, with animal proteins generally being more digestible than plant proteins due to fewer anti-nutritional factors. Processing can also impact digestibility.

Practical Ways to Assess Protein Quality

Here are practical guidelines to help you make informed choices about protein quality.

For Omnivores

Prioritize Lean Animal Sources: Meat, fish, poultry, and eggs offer complete, highly digestible protein.
Focus on Nutrient Density: Choose animal proteins rich in other nutrients like iron and vitamin B12.

For Vegans and Vegetarians

Combine Complementary Proteins: Pairing different plant protein sources throughout the day, like rice and beans, ensures you get all EAAs.
Use Complete Plant Sources: Include soy and quinoa, which have a full EAA profile.
Opt for Protein Isolates: In supplements, isolates (e.g., soy or pea protein isolate) often have improved digestibility.

Interpreting Nutrition Labels

Total protein grams on a label don't fully indicate quality. Look for a DIAAS score if available, or consider brands that provide adjusted values or full amino acid profiles as a sign of transparency.

Conclusion

Evaluating protein quality involves more than just counting grams. The DIAAS method represents a modern, more accurate approach by considering essential amino acid profiles and ileal digestibility. By understanding these factors, consumers can make better dietary choices, whether through animal sources or combining plant proteins, to support their health goals. For further information, the Food and Agriculture Organization of the United Nations is a reliable resource.

Frequently Asked Questions

Complete proteins contain all nine essential amino acids that your body needs, typically found in animal products like eggs and meat. Incomplete proteins, common in many plant sources, are deficient in one or more essential amino acids.

The DIAAS is a more accurate measure because it uses ileal (small intestine) digestibility instead of fecal digestibility, providing a better estimate of what the body actually absorbs. It also doesn't truncate high scores, allowing for better differentiation between high-quality protein sources.

No, consuming complementary proteins doesn't need to happen in the same meal. Your body maintains a pool of amino acids, so eating a variety of plant proteins over the course of the day is sufficient to meet your EAA requirements.

Yes, processing can affect protein quality. For example, excessive heat can cause damage to amino acids, reducing their availability, while other processes like isolating protein can improve digestibility.

Not always. While many animal proteins have high digestibility and complete amino acid profiles, some plant proteins like soy and quinoa are also considered complete. Protein quality depends on the specific source, processing, and how it's prepared.

A limiting amino acid is an essential amino acid that is present in the lowest amount relative to a reference standard in a given protein source. This is the amino acid that restricts the body's ability to synthesize new proteins.

Look for third-party certifications and check if the brand reports a DIAAS or PDCAAS score, or lists the full amino acid profile. Transparent brands will provide this information, and isolates generally indicate improved digestibility due to processing.