Protein is one of the three essential macronutrients, playing a vital role in virtually every biological process within the body. From building and repairing tissues to producing hormones and enzymes, its functions are vast and critical for human health. But as nutritional science has evolved, we've learned that not all proteins are created equal. The 'quality' of a protein refers to its amino acid profile and how well it is digested and absorbed by the body. For anyone following a specific diet, such as plant-based, or for those with increased protein needs, knowing how to assess this quality is paramount.
The Foundations of Protein Quality
To understand protein quality, you must first know what proteins are made of: amino acids. There are 20 different amino acids that the body uses, but only nine of these are considered 'essential'. The body cannot produce these nine essential amino acids (EAAs), so they must be obtained through food. The presence and proportion of these EAAs in a food source are key indicators of its quality.
- Complete Proteins: A food source containing all nine essential amino acids in adequate amounts is considered a complete protein. These are typically found in animal-based products, though a few plant-based options also fall into this category.
- Incomplete Proteins: Most plant-based foods are considered incomplete proteins because they are low in or lack one or more of the nine EAAs. A varied diet, however, can provide all necessary amino acids by combining different incomplete protein sources throughout the day.
Key Metrics for Assessing Protein Quality
Nutritional scientists have developed several methods to measure protein quality, moving beyond older systems to more accurate modern metrics.
- Protein Digestibility Corrected Amino Acid Score (PDCAAS): For years, the PDCAAS was the gold standard recommended by the FAO/WHO and adopted by the US FDA. It evaluates a protein's quality by comparing its EAA profile to a reference pattern and correcting it for fecal digestibility. However, the method has significant drawbacks, including a score truncation at 1.0, which makes it difficult to differentiate between high-quality proteins like whey and casein. The use of fecal digestibility, which includes nitrogen from gut bacteria, can also overestimate a protein's true absorbability.
- Digestible Indispensable Amino Acid Score (DIAAS): Proposed by the FAO in 2013, the DIAAS is the new, more accurate method for evaluating protein quality. It addresses the limitations of PDCAAS by focusing on the digestibility of individual indispensable (essential) amino acids measured at the end of the small intestine (the ileum), which provides a more precise reflection of absorption. The DIAAS also does not truncate its scores at 1.0, allowing for a better comparative ranking of high-quality proteins.
- Biological Value (BV): An older metric, BV measures the percentage of absorbed protein that is retained and utilized by the body. It doesn't consider digestibility, which limits its usefulness for evaluating overall dietary protein.
- Net Protein Utilization (NPU): Similar to BV, NPU measures the proportion of dietary protein retained, but it also accounts for digestibility. Both BV and NPU have largely been superseded by more advanced methods like DIAAS for regulatory purposes but are sometimes still mentioned.
Comparing Protein Quality Metrics: DIAAS vs. PDCAAS
Understanding the differences between the current and former standard is crucial for interpreting food labels and nutritional data.
| Feature | PDCAAS | DIAAS |
|---|---|---|
| Digestibility Measurement | Fecal digestibility of overall protein | Ileal digestibility of individual essential amino acids |
| Score Truncation | Scores are capped at 1.0 | Scores are not truncated, allowing for better comparisons |
| Amino Acid Focus | Overall amino acid profile vs. reference pattern | Digestibility of each indispensable amino acid individually |
| Recommended By | US Food and Drug Administration (FDA) | Food and Agriculture Organization (FAO) |
| Accuracy | Can overestimate protein quality | Provides a more accurate measure of amino acid absorption |
How to Apply Protein Quality to Your Diet
For the average person consuming a varied diet, the body naturally handles protein requirements. The myth that complementary proteins must be consumed at the same meal is outdated; combining different protein sources over the course of a day is sufficient. The focus should be on dietary diversity and incorporating both complete and incomplete proteins.
- Animal-Based Proteins: Meat, fish, eggs, and dairy are considered high-quality proteins due to their complete EAA profile and high digestibility.
- Plant-Based Proteins: While often incomplete individually, they can be paired to create a complete amino acid profile. Combining legumes (like beans and lentils), grains (like rice and quinoa), and nuts and seeds throughout the day ensures all EAAs are consumed.
Examples of complementary plant-based protein pairs:
- Rice and beans
- Hummus and whole-wheat pita
- Peanut butter on whole-grain toast
- Cornbread and black bean chili
- Lentil soup with whole-grain bread
Factors That Influence Protein Quality Beyond the Score
Beyond the EAA profile and digestibility scores, several other factors can influence the quality of the protein you consume.
Food Processing
The way food is processed can significantly impact protein quality. High-heat cooking or processing methods, for instance, can reduce the bioavailability of certain amino acids. Excessive heat in soy processing, for example, can decrease the availability of lysine. Conversely, cooking can sometimes improve digestibility by deactivating anti-nutritional factors present in raw foods, such as trypsin inhibitors in soybeans.
Anti-Nutritional Factors
Certain compounds in plant-based foods, known as anti-nutritional factors, can interfere with protein digestion. Examples include trypsin inhibitors and phytic acid. While these are often neutralized by proper cooking, their presence in unprocessed foods can lower the bioavailability of protein. This is one reason why plant proteins typically have lower digestibility ratings than animal proteins.
Individual Health Factors
Protein needs and utilization efficiency can also vary based on age, health status, and physical activity level. Vulnerable populations, such as older adults or those with certain health conditions, may have reduced appetite and higher protein requirements to combat issues like sarcopenia (age-related muscle loss). For these individuals, choosing protein sources with the highest DIAAS values can be especially important. Consult a healthcare provider or registered dietitian for personalized advice.
Conclusion: Making Informed Protein Choices
Ultimately, understanding how to check protein quality involves looking beyond simple protein content. A protein's true value lies in its essential amino acid composition and its digestibility. The DIAAS metric provides the most accurate measure currently available, but for most people with a balanced, varied diet, a high-quality protein intake is achieved naturally. Focus on consuming a diverse range of protein sources—both animal and plant-based—to ensure you meet all your EAA needs. For specific dietary goals or conditions, paying closer attention to DIAAS values and food processing methods can help optimize your protein intake for better health and nutrition. For more information, you can explore reliable resources such as the Harvard T.H. Chan School of Public Health's nutrition guides.
