What is Protein Quality?
Protein quality refers to how well a specific dietary protein can provide the amino acids required by the human body. Since the body cannot synthesize nine essential amino acids (EAAs), it must obtain them from food. A high-quality protein source contains all nine of these EAAs in sufficient amounts and proportions for optimal body function. Two key factors influence this quality: the amino acid profile and the digestibility or bioavailability of those amino acids. Animal-based proteins like meat, eggs, and dairy are typically considered high-quality, while many plant-based proteins are lower in one or more EAAs and thus are often categorized as 'incomplete' proteins. However, careful dietary planning, such as combining complementary plant proteins, can provide all necessary EAAs.
Scoring Methods for Measuring Protein Quality
To assess and compare the quality of different protein sources, several scoring methods have been developed over the years. These methods provide a standardized way to evaluate a protein's nutritional value and its effectiveness in supporting human health and growth.
Protein Digestibility-Corrected Amino Acid Score (PDCAAS) The PDCAAS method, developed by the FAO/WHO, was the preferred standard for many years and is still referenced on many food labels in the US.
- How it works: PDCAAS compares a food's amino acid profile to a reference amino acid profile and corrects for the protein's fecal digestibility.
- Limitations: This method has several known limitations, including relying on less-accurate fecal digestibility data and capping scores at 1.0, which means superior proteins do not receive higher scores that reflect their potential for complementing other proteins.
Digestible Indispensable Amino Acid Score (DIAAS) The DIAAS, recommended by the FAO in 2013, is a more advanced and accurate method for measuring protein quality.
- How it works: DIAAS corrects for the true ileal digestibility of each individual amino acid, measuring absorption at the end of the small intestine before microbial fermentation in the large intestine can occur and distort results.
- Advantages: It provides a more accurate estimate of amino acid bioavailability and does not truncate scores at 1.0, allowing differentiation between high-quality proteins. This is particularly important for accurately assessing mixed-diet protein quality.
Factors Influencing Protein Quality
Beyond the intrinsic amino acid profile, several external factors can affect the actual protein quality delivered by a food source:
- Cooking and Processing: While cooking generally improves protein digestibility by denaturing its structure, excessive heat can cause damage, leading to the Maillard reaction that makes certain amino acids unavailable for absorption.
- Antinutrients: Certain compounds, known as antinutrients, can interfere with protein digestion. For example, trypsin inhibitors in soybeans can block the enzymes needed for protein breakdown, and tannins can bind to proteins, reducing their bioavailability. Proper cooking and processing can significantly reduce these factors.
- Complementary Proteins: Combining different incomplete protein sources can create a complete amino acid profile. For example, eating rice (low in lysine) with beans (low in methionine) can compensate for the deficiencies in each to create a higher-quality protein meal.
- Dietary Context: Other components of a meal, such as fiber or other macronutrients, can influence the absorption kinetics of amino acids.
Comparison of Protein Quality Metrics
| Feature | PDCAAS (Protein Digestibility-Corrected Amino Acid Score) | DIAAS (Digestible Indispensable Amino Acid Score) |
|---|---|---|
| Digestibility Measurement | Fecal digestibility of crude protein (less accurate). | True ileal digestibility of individual amino acids (more accurate). |
| Accuracy | Overestimates the nutritional value of some proteins, especially plant-based. | Considered the current gold standard for measuring protein quality in humans. |
| Score Truncation | Scores capped at 1.0; cannot distinguish between excellent protein sources. | Scores can exceed 100%, allowing for better comparison of protein sources. |
| Reference Standard | Based on amino acid requirements of preschool-age children. | Based on amino acid requirements of different age groups. |
| Limiting Amino Acid | Focuses on the first limiting amino acid, potentially ignoring others. | Measures the ileal digestibility of all individual indispensable amino acids. |
Practical Implications of Protein Quality
For most people in high-income countries consuming a varied diet, meeting overall protein requirements is generally not a major concern. However, paying attention to protein quality becomes more critical for specific populations or goals:
- Older Adults: Anabolic resistance, or the reduced muscle protein synthesis response to dietary protein, means older adults may need higher intakes of high-quality, leucine-rich protein to prevent age-related muscle loss.
- Athletes and Bodybuilders: To maximize muscle protein synthesis after exercise, sufficient intake of high-quality protein, especially rich in leucine, is beneficial.
- Vegetarians and Vegans: Those on plant-based diets should be mindful of consuming a variety of protein sources to ensure they get all nine essential amino acids, as single plant proteins may be 'incomplete'.
Conclusion
The most comprehensive explanation for protein quality considers both the essential amino acid profile and the protein's digestibility. While older methods like PDCAAS provided a starting point, modern nutritional science favors the Digestible Indispensable Amino Acid Score (DIAAS) for its superior accuracy, particularly in measuring amino acid bioavailability. Ultimately, understanding these metrics helps individuals make more informed dietary choices to support their health, growth, and specific physiological needs. Whether through a diverse omnivorous diet or a thoughtfully planned plant-based one, ensuring a steady supply of high-quality protein is vital for optimal bodily function.
