What is the food protein grading scale? An overview of protein quality metrics

January 9, 2026 •

3 min read

Every cell in the human body contains protein, but not all dietary protein is created equal. A food protein grading scale is a standardized method used by nutrition scientists and food manufacturers to evaluate the quality and bioavailability of protein sources for human consumption.

Quick Summary

Different scientific methods, such as DIAAS and PDCAAS, are used to measure a food's protein quality based on its amino acid profile and digestibility.

Key Points

DIAAS is the Modern Standard: The Digestible Indispensable Amino Acid Score (DIAAS) is the most accurate protein quality measure, using ileal digestibility and uncapped scoring.
PDCAAS is Outdated: The older Protein Digestibility Corrected Amino Acid Score (PDCAAS) is less accurate due to fecal digestibility measurement and score truncation at 1.0.
Protein Quality Depends on Two Factors: A protein's nutritional quality is determined by its essential amino acid content and how easily the body can digest and absorb it.
Combine Incomplete Proteins: Plant-based diets can get all essential amino acids by combining different sources, such as rice and beans.
High-Quality Protein Sources Vary: Animal products generally contain high-quality protein, but high-quality plant options like soy and quinoa also exist.
Cooking and Processing Affect Quality: Food preparation can influence a protein's digestibility and overall quality.

The Importance of Protein Quality

Protein is composed of building blocks called amino acids. Of the 20 amino acids the human body uses, nine are considered “essential” because they cannot be synthesized internally and must be obtained from food. Protein quality is determined by two main factors: its amino acid composition, particularly the ratio of essential amino acids, and its digestibility, which refers to how efficiently the body can break down and absorb the protein. A high-quality protein is one that is easily digested and contains a sufficient amount of all nine essential amino acids. Historically, various methods have been developed to grade protein quality, evolving over time to become more accurate and reflective of human nutritional needs.

The Older Standard: PDCAAS

From 1993 until recently, the Protein Digestibility Corrected Amino Acid Score (PDCAAS) was the official standard for protein quality evaluation, recommended by the Food and Agriculture Organization (FAO) and the World Health Organization (WHO). This method calculates a score by comparing a food's essential amino acid profile to a reference pattern and correcting for digestibility based on rat studies. A significant limitation of PDCAAS is that scores are capped at 1.0, preventing differentiation among the highest-quality proteins. The use of fecal digestibility is also considered less accurate than newer methods as it may overestimate absorbed amino acids.

The Modern Gold Standard: DIAAS

Introduced by the FAO in 2013, the Digestible Indispensable Amino Acid Score (DIAAS) is now considered the superior method for measuring protein quality. DIAAS improves upon PDCAAS by measuring amino acid digestibility at the end of the small intestine (ileal digestibility), providing a more accurate measure of absorption. Unlike PDCAAS, DIAAS scores are uncapped, allowing for better ranking of high-quality protein sources; for example, whey protein isolate scores higher with DIAAS than with PDCAAS. DIAAS also uses age-specific reference patterns, offering a more tailored assessment for different populations.

A Legacy of Methods

Prior to PDCAAS, methods such as the Protein Efficiency Ratio (PER) and Biological Value (BV) were used. PER measured weight gain in animals but had limitations. BV assessed nitrogen retention to determine how much absorbed protein was used by the body, but it didn't account for digestibility. These older methods have been replaced by more accurate approaches.

Complete vs. Incomplete Proteins and Complementary Combinations

Understanding protein quality involves the concepts of complete and incomplete proteins.

Examples of protein types:

Complete Proteins: Contain all nine essential amino acids in sufficient amounts. These are typically found in animal products and some plant-based foods like soy and quinoa.
Incomplete Proteins: Lack one or more essential amino acids, common in most plant sources.
Complementary Proteins: Combining incomplete proteins, like rice and beans, creates a complete amino acid profile.

Combining proteins for a complete profile:

Rice and beans
Hummus and whole-wheat pita bread
Peanut butter on whole-wheat bread
Corn tortillas with refried beans

PDCAAS vs. DIAAS: A Comparison Table


Feature	PDCAAS	DIAAS
Digestibility Measurement	Fecal digestibility in rats	Ileal digestibility in pigs or humans
Limitation	Score capped at 1.0, limiting differentiation of high-quality proteins	Uncapped score, allowing a more precise ranking of protein quality
Accuracy	Less accurate, can overestimate amino acid absorption due to microbial metabolism	More accurate, reflects the true absorption of amino acids by the body
Reference Pattern	Single pattern based on a 2-5 year-old child	Age-specific patterns for infants, children, and adults
Official Recommendation	Formerly recommended by FAO/WHO (1993-2013)	Recommended by FAO since 2013

The Consumer's Guide to Protein Grading

While the technical details of DIAAS and PDCAAS are primarily used in the food industry, consumers can benefit from knowing that protein quality is evaluated. A varied diet typically provides all necessary essential amino acids. However, individuals with higher protein needs, like athletes, might find DIAAS scores useful for comparing high-quality protein sources.

Find more information on healthy protein sources at the Harvard T.H. Chan School of Public Health's Protein page.

Conclusion

The food protein grading scale has progressed significantly, from older animal-based measures to the more precise, human-focused DIAAS. This modern standard addresses the limitations of the previous PDCAAS method, offering a more accurate understanding of how the body utilizes protein from different foods. For general health, a diverse diet with various protein sources is key, while understanding these grading scales can be particularly helpful for those with specific dietary requirements or interests.

Frequently Asked Questions

DIAAS is the newer, more accurate method that measures amino acid digestibility at the end of the small intestine (ileal digestibility) and does not cap scores. PDCAAS is the older method, which is less accurate because it relies on fecal digestibility and caps all scores at 1.0.

A capped score, like PDCAAS's 1.0, prevents consumers and manufacturers from distinguishing between proteins of the highest quality. With DIAAS, proteins that provide more than the required amount of essential amino acids can score higher than 100%, offering a clearer differentiation.

Older methods like Biological Value (BV) and Protein Efficiency Ratio (PER) are largely obsolete for human nutrition applications. They have been superseded by more accurate, human-focused methods like PDCAAS and now DIAAS.

A limiting amino acid is the essential amino acid present in the smallest quantity within a food relative to human needs. It determines the overall protein quality and score, as protein synthesis can only occur at the rate permitted by the availability of the most limited essential amino acid.

For most people following a varied and healthy diet, especially one that includes both animal and plant sources, essential amino acid needs are easily met. However, for those with specific goals like maximizing muscle protein synthesis, comparing DIAAS scores can offer more precise information.

Vegetarians and vegans can ensure high-quality protein intake by consuming a variety of plant-based protein sources throughout the day. Combining different incomplete proteins, such as beans and rice, can provide all essential amino acids.

Foods generally considered high-quality or complete proteins include animal sources like eggs, milk, meat, and fish. Plant-based complete proteins include soy (like tofu) and quinoa.