The Foundational Pillars of Protein Quality
Protein is a macronutrient composed of amino acids, often called the building blocks of life. While your body can produce eleven of the 20 amino acids it needs, the remaining nine are essential, or indispensable, and must be obtained through your diet. The definition of protein quality is anchored in two primary characteristics: the protein's essential amino acid (EAA) profile and its digestibility.
- Essential Amino Acid (EAA) Profile: High-quality proteins, also known as 'complete' proteins, contain all nine EAAs in adequate proportions. In contrast, 'incomplete' proteins, typically found in plants, are low in or lack one or more EAAs. For example, grains are often low in lysine, while legumes lack methionine. A diverse diet, however, can provide all necessary EAAs by consuming complementary proteins throughout the day.
- Digestibility: This refers to how well the body can break down and absorb the amino acids from a protein source. Even if a protein has a perfect amino acid profile, poor digestibility means fewer amino acids are available for the body to use, thereby reducing its quality. Animal proteins are generally more digestible than plant proteins due to the latter's complex fiber matrix and antinutrients.
Modern Metrics for Assessing Protein Quality
In nutritional science, several scoring systems have evolved to measure and compare protein quality more accurately. The most prominent modern metric is the Digestible Indispensable Amino Acid Score (DIAAS).
The Shift from PDCAAS to DIAAS
Historically, the Protein Digestibility-Corrected Amino Acid Score (PDCAAS) was the standard for evaluating protein quality, adopted by the FDA in 1993. However, the Food and Agriculture Organization (FAO) now recommends DIAAS as a superior method due to its improved accuracy.
Comparison of Protein Quality Metrics
| Feature | PDCAAS (Older Method) | DIAAS (Current Standard) |
|---|---|---|
| Digestibility Measurement | Uses true fecal nitrogen digestibility, which can overestimate absorption because some nitrogen is absorbed by gut bacteria. | Measures ileal digestibility, using samples taken at the end of the small intestine. This provides a more accurate value for amino acids absorbed by the body. |
| Scoring Scale | Scores are capped at 1.0, meaning proteins that provide more than 100% of amino acid requirements cannot be ranked higher than those that just meet the minimum. | Scores are not truncated, allowing for a more accurate ranking of higher-quality proteins. Scores above 100% can be achieved and are used to enrich other proteins. |
| Amino Acid Profiles | Compares the protein's EAA profile against a reference pattern for preschool-age children. | Provides age-specific reference patterns for infants, children, and adults, offering more tailored results. |
| Source for Assays | Typically relies on rat studies to determine fecal digestibility. | Uses animal models, preferably pigs, whose digestive system is more similar to humans. |
Factors Affecting a Protein's Quality
Beyond the intrinsic amino acid profile, several external factors can influence a food's protein quality before it is consumed:
- Processing and Cooking: The way food is prepared can affect protein structure and digestibility. For instance, processes like the Maillard reaction can reduce amino acid bioavailability. However, moderate cooking can denature proteins, making them easier to digest.
- Antinutrients: Compounds such as trypsin inhibitors, tannins, and phytates found in plant foods can interfere with protein digestion and absorption. Proper cooking and preparation methods, like soaking and fermentation, can minimize their impact.
- Food Matrix: The overall composition of the food affects protein quality. For example, fiber content in plant-based meals can reduce digestibility, whereas consuming different foods together can create a complementary protein profile.
Why Protein Quality is Important
For most people in developed nations with varied diets, protein quality is not a major concern, as they typically meet their needs through a mix of food sources. However, it becomes critical in specific populations and contexts:
- Malnutrition: In regions with limited food variety, a high-quality, easily digestible protein source can be crucial for meeting nutritional needs, especially for children who have higher requirements.
- Specific Dietary Patterns: Vegans and vegetarians must be mindful of consuming a variety of protein sources to ensure they get all nine EAAs throughout the day. Combining foods like rice and beans, or including complete plant proteins like soy and quinoa, is key.
- Higher Protein Needs: For athletes, older adults, and individuals recovering from illness, a higher intake of high-quality protein is needed to support muscle protein synthesis and recovery. In these cases, prioritizing high-DIAAS proteins or ensuring sufficient complementary protein intake is beneficial.
Conclusion
Protein quality is a complex metric determined by a food's essential amino acid profile and its digestibility. The shift from PDCAAS to the more accurate DIAAS reflects a more sophisticated understanding of how the human body processes and utilizes protein. While animal proteins are often considered 'complete,' a balanced and varied plant-based diet can easily provide all the necessary amino acids. Ultimately, focusing on both the quality and variety of protein sources is the best strategy for optimal health.
For a deeper dive into the science behind these metrics, you can refer to the FAO's report on Dietary Protein Quality Evaluation in Human Nutrition.
