Understanding the Gold Standard: DIAAS vs. PDCAAS
Protein quality is not just about the total grams of protein in a food. It is fundamentally determined by the content and digestibility of the nine essential amino acids (EAAs) that our bodies cannot produce themselves. For decades, the Protein Digestibility Corrected Amino Acid Score (PDCAAS) was the industry standard. However, the Food and Agriculture Organization (FAO) formally recommended replacing it with the superior Digestible Indispensable Amino Acid Score (DIAAS) in 2011.
There are several key differences that make DIAAS a more accurate measure:
- Method of Measurement: PDCAAS measured digestibility from fecal samples, which could lead to overestimations due to bacterial fermentation in the large intestine. DIAAS, however, measures digestibility at the end of the small intestine (ileum), providing a more precise reflection of true absorption.
- Scoring Cap: PDCAAS scores were capped at 1.00, meaning it could not differentiate between proteins that exceeded essential amino acid requirements. DIAAS scores can exceed 100% (or 1.00), allowing for better comparison of exceptionally high-quality proteins.
- Amino Acid Specificity: DIAAS calculates the digestibility of each individual essential amino acid, whereas PDCAAS used a single value for crude protein digestibility.
Top-Tier Animal Protein Sources
Animal-derived proteins are consistently found to have the highest quality scores, primarily because they naturally contain all essential amino acids in proportions that closely match human physiological needs. Some of the top contenders based on DIAAS data include:
- Eggs: Boiled whole eggs have a documented DIAAS of approximately 1.12, categorizing them as an excellent-quality protein source. Their high digestibility and well-balanced amino acid profile make them a benchmark for protein quality.
- Dairy Proteins: Different forms of dairy protein frequently score very high. Milk protein concentrate (MPC) has been reported with a DIAAS of 1.20, while whey protein concentrate and casein also boast scores over 1.0. This is one reason dairy supplements are popular in sports nutrition.
- Meat: Sources like beef and pork also register high on the DIAAS scale, generally scoring over 1.0 and confirming their status as complete, highly digestible proteins.
Why are these foods so high quality?
These animal proteins are naturally bioavailable and contain a complete array of all nine essential amino acids. The DIAAS score evaluates how efficiently these amino acids are absorbed and utilized by the body, and animal proteins typically excel in this regard due to their structural integrity and lack of significant antinutrients that can hinder digestion.
Understanding Plant-Based Protein Quality
While animal proteins typically take the top spots for a single food item, many plant-based options are also excellent sources, especially when combined strategically. Most plant proteins are "incomplete," meaning they are lower in one or more essential amino acids, often lysine in cereals or methionine in legumes.
Examples of plant protein scores and considerations:
- Legumes: Foods like peas, beans, and lentils are rich in protein but can be limiting in methionine. Pea protein concentrate, for example, often has a DIAAS below 0.85.
- Cereals and Grains: Grains like wheat and rice are typically limited in lysine. Combining them with legumes can create a complete protein meal.
- Quinoa and Soy: Soy protein isolate can achieve a DIAAS of around 0.90, which is considered 'high-quality'. Quinoa is one of the few plant-based foods often described as a 'complete' protein, though its DIAAS score is still generally lower than top animal sources.
- Potatoes: Surprisingly, potato protein has been shown to have a high DIAAS score (around 1.00), demonstrating its potential as a quality source, especially when combined with other proteins.
The Role of Protein Complementation
One of the most practical applications of the DIAAS methodology for plant-based diets is understanding protein complementation. By mixing different plant foods, you can ensure all essential amino acid needs are met. Cereal-based proteins that are low in lysine can be combined with legume-based proteins that are typically rich in lysine to create a higher-quality protein blend.
Comparison of Protein Quality (DIAAS)
| Food Source | DIAAS (Approximate) | Limiting Amino Acid | Quality Classification | Notes |
|---|---|---|---|---|
| Milk Protein Concentrate | 1.20+ | None | Excellent | Top-tier, often used in sports supplements. |
| Boiled Whole Egg | 1.12–1.13 | None | Excellent | A benchmark for natural, complete protein. |
| Beef | 1.09–1.12 | None | Excellent | Very high bioavailability and amino acid profile. |
| Whey Protein Isolate | 1.09 | Valine | Excellent | Fast-digesting, popular in supplements. |
| Soy Protein Isolate | 0.89–0.90 | Met+Cys | High | Leading plant-based source, but not quite excellent. |
| Chickpeas | 0.83 | Met+Cys | Good | Can be combined with grains to improve score. |
| Pea Protein Concentrate | 0.62–0.82 | Met+Cys | No Claim (Often) | Quality depends on processing and concentration. |
| Cooked Rice | 0.59–0.60 | Lysine | No Claim | Often paired with legumes for complementarity. |
| Cooked Kidney Beans | 0.58–0.59 | Met+Cys | No Claim | Good source of fiber and other nutrients. |
Factors Affecting Protein Quality
It's important to remember that a DIAAS value is just one piece of the nutritional puzzle. Several factors can influence the final quality of a protein you consume:
- Food Processing: Cooking, soaking, or germination can significantly impact the digestibility of a protein, especially in plant-based foods. For example, the heat from baking can decrease the bioavailability of lysine, which is susceptible to the Maillard reaction.
- Meal Composition: The presence of other foods, such as dietary fiber or antinutritional factors, can alter the digestibility of protein within a meal. DIAAS can be additive in mixed meals, meaning a full meal's quality can be higher than its individual parts.
- Individual Differences: Factors like age, health status, and physical activity levels can all affect an individual's specific amino acid requirements and utilization.
Conclusion
While a single, raw food item with the highest protein rating is challenging to pinpoint definitively due to different processing methods and age reference patterns, data consistently shows that animal proteins like eggs and milk protein concentrate are among the highest-quality single sources, often achieving excellent DIAAS scores over 1.1. However, excellent protein quality is accessible through various dietary patterns. Individuals following plant-based diets can achieve equally high protein quality by strategically combining different sources, leveraging the principles of protein complementation. By understanding the modern DIAAS metric and considering the digestibility and amino acid profiles of your food, you can make informed choices to ensure your body receives the highest quality protein for optimal health and function.
Visit the NIH to explore further research on protein quality and the DIAAS methodology.
