What Defines High Protein Quality?
High-quality protein is defined by two key factors: its essential amino acid (EAA) profile and its digestibility. The body needs a specific set of EAAs, which it cannot produce on its own, to build and repair tissues. A protein source is considered 'complete' if it provides all these EAAs in sufficient amounts. Digestibility measures how efficiently the body can absorb and utilize the amino acids from a food. These two concepts are combined into standardized scoring methods that allow nutritionists and consumers to compare different protein sources.
The Shift from PDCAAS to DIAAS
The Protein Digestibility Corrected Amino Acid Score (PDCAAS) was the long-standing standard for measuring protein quality, adopted by health authorities like the World Health Organization (WHO) and the U.S. Food and Drug Administration (FDA). The PDCAAS compares a food's amino acid profile to a standard reference pattern and corrects for fecal digestibility, with a maximum score of 1.0. However, this method has limitations, including the truncation of scores at 1.0, which makes it impossible to differentiate between high-quality proteins that exceed requirements.
In 2013, the FAO recommended replacing PDCAAS with the Digestible Indispensable Amino Acid Score (DIAAS) to address these flaws. DIAAS offers a more accurate reflection of a protein's nutritional value for several reasons:
- Measures ileal digestibility: DIAAS assesses the digestibility of each individual EAA at the end of the small intestine (the ileum), where most absorption occurs. This is more precise than PDCAAS's reliance on fecal digestibility, which can be inflated by bacterial activity in the large intestine.
- Scores are not capped: Unlike PDCAAS, DIAAS values are not truncated at 1.0. This allows for a more granular ranking of truly high-quality proteins that provide EAAs in excess of human requirements.
- Age-specific reference patterns: DIAAS uses different EAA reference patterns for various age groups, from young children to adults, providing more relevant scores for different populations.
So, What is the Highest Protein Digestibility Score?
Based on the more advanced DIAAS methodology, milk protein concentrate and whey protein complex typically achieve the highest protein digestibility scores. Dairy and eggs consistently rank among the top-tier sources, with multiple studies showing scores well over 100 for key protein components like whey and casein. For instance, a complex of whey proteins has been reported to reach a DIAAS of 1.61, while cooked whole eggs have a DIAAS around 1.13 and casein around 1.09. A DIAAS score over 100 is considered 'excellent' quality, signifying that the protein can fully meet physiological needs from that source alone.
It is important to note that specific scores can vary slightly based on processing and the age group reference pattern used. For adults, these scores confirm the superior quality of these animal-derived proteins, making them highly effective for muscle repair, growth, and overall protein synthesis.
Protein Quality Scoring Comparison: DIAAS vs. PDCAAS
| Feature | DIAAS (Digestible Indispensable Amino Acid Score) | PDCAAS (Protein Digestibility Corrected Amino Acid Score) |
|---|---|---|
| Digestibility Measurement | Measures ileal digestibility of individual amino acids (more accurate). | Measures fecal digestibility of overall protein (less accurate). |
| Scoring Range | Not capped, allowing for scores >100, which accurately ranks superior proteins. | Capped at 1.0, making it impossible to differentiate between excellent and adequate sources. |
| Reference Pattern | Uses different, updated EAA reference patterns for specific age groups. | Based on the EAA requirements of 2-5 year-old children, which may not apply to all populations. |
| Ranking Accuracy | Provides a more precise and accurate ranking of high-quality protein sources. | Less precise for high-quality proteins due to score truncation. |
| Relevance to Humans | Measurement in the ileum more accurately reflects human protein absorption. | Measurement in feces can overestimate absorption due to microbial activity. |
Practical Application of Protein Digestibility
For a nutrition diet, understanding protein digestibility means focusing on both quantity and quality. While animal proteins like dairy, eggs, and meat provide exceptionally high-quality and easily absorbed protein, plant-based sources can still be highly effective when combined strategically.
Maximizing Protein Quality on a Plant-Based Diet
Most plant proteins are considered 'incomplete' because they are low in one or more EAAs. However, by combining different plant sources, you can create a complete protein profile. This concept of amino acid complementarity is well-supported by DIAAS research, which shows that combining sources like legumes and cereals can result in excellent protein quality. For example, a meal of beans and rice, traditionally eaten together in many cultures, effectively combines amino acids to form a complete protein. Soy protein isolate is a rare example of a plant protein with a high DIAAS, often approaching or exceeding 100 for adults.
Food Preparation and Digestibility
The way food is prepared can also affect protein digestibility. Cooking generally improves protein digestibility by denaturing proteins, making them easier for enzymes to break down. However, some processing methods can have a negative impact. For instance, studies have shown differences in the DIAAS of milk proteins depending on heat treatment and processing.
Top Sources of Digestible Protein
- Dairy: Whey, casein, milk protein concentrate, and whole milk consistently achieve high DIAAS values, demonstrating excellent protein quality.
- Eggs: Cooked whole eggs provide a complete and highly digestible protein, serving as a benchmark for high quality.
- Meat: Lean beef, chicken breast, and fish are also excellent sources of highly digestible, complete protein.
- Soy: Soy protein isolate stands out among plant-based options, offering a high DIAAS that rivals many animal proteins.
- Blends: Combining complementary plant-based protein sources, such as pea and rice protein, can significantly increase the overall DIAAS of a meal.
Conclusion
The Digestible Indispensable Amino Acid Score (DIAAS) is the most accurate and modern method for determining protein quality and what is the highest protein digestibility score. While older PDCAAS data is still widely cited, DIAAS provides a more granular and realistic assessment of how well the body can absorb and use amino acids. For those seeking the highest quality protein for muscle building and repair, dairy products (especially whey and casein) and eggs are top contenders, with scores often exceeding 100. However, strategic pairing of plant-based proteins can also create meals with an excellent DIAAS. Focusing on a varied diet rich in high-quality protein, whether from animal or complementary plant sources, is crucial for supporting overall health and nutritional needs.
For more information on nutrition diet guidelines, you can visit the Food and Agriculture Organization of the United Nations website(https://www.fao.org/ag/humannutrition/35978-02317b979a686a57aa4593304ffc17f06.pdf).
