Understanding the Building Blocks of Protein Quality
To truly understand how protein quality is measured, one must first appreciate its fundamental components. Proteins are large, complex molecules composed of smaller units called amino acids. There are 20 different amino acids, nine of which are considered essential, meaning the human body cannot produce them and they must be obtained from the diet. A high-quality or "complete" protein contains all nine essential amino acids (EAAs) in sufficient proportions to meet the body's needs.
The Nine Essential Amino Acids
- Histidine: Vital for growth and tissue repair.
- Isoleucine: Important for muscle metabolism and immune function.
- Leucine: Critical for muscle protein synthesis and energy production.
- Lysine: A precursor for carnitine, involved in energy production.
- Methionine: Precursor for cysteine and other compounds.
- Phenylalanine: Precursor for tyrosine, important for neurotransmitters.
- Threonine: Key for structural proteins like collagen.
- Tryptophan: Precursor for serotonin, a mood-regulating neurotransmitter.
- Valine: Important for muscle growth and repair.
The Evolution of Protein Quality Scoring
Over time, several methods have been developed to quantify and score protein quality, moving from basic animal studies to more precise human-focused metrics. The current gold standard, the Digestible Indispensable Amino Acid Score (DIAAS), represents the most accurate evaluation today.
DIAAS: The Digestible Indispensable Amino Acid Score
Endorsed by the Food and Agriculture Organization (FAO), the DIAAS is the superior method for determining protein quality because it accounts for individual amino acid digestibility at the end of the small intestine (ileum). This provides a more accurate measure of the amino acids actually absorbed and utilized by the body, as opposed to older methods that used total fecal digestibility. A key advantage of DIAAS is that its scores are not truncated at 100%, allowing for a clear ranking of superior protein sources.
PDCAAS: The Protein Digestibility Corrected Amino Acid Score
Before DIAAS, the PDCAAS was the most widely used metric for protein quality. It also compared the amino acid profile of a protein to a reference pattern and corrected for digestibility. However, PDCAAS has notable limitations:
- It used fecal digestibility, which overestimates the amount of protein the body can actually use.
- It truncated all scores at 100%, meaning that a high-quality protein with a score above 100% was simply reported as 100%, erasing the potential to rank proteins of excellent quality.
Other Historical Methods
While less common today, other methods provided foundational insights into protein quality:
- Biological Value (BV): Measures how efficiently the body utilizes absorbed protein for tissue formation.
- Protein Efficiency Ratio (PER): Based on the weight gain of test subjects (typically rats) in relation to their protein intake.
- Amino Acid Score (Chemical Score): A simple comparison of a protein's EAA profile to a reference pattern, without accounting for digestibility.
Factors Beyond the Score
Beyond the primary scoring metrics, several other factors influence protein quality and absorption. A protein's bioavailability, or how easily it can be absorbed and used by the body, is critical. Processing methods, such as heating or fermentation, can alter a protein's structure and affect its digestibility. Similarly, the presence of antinutritional factors, like trypsin inhibitors in some legumes, can interfere with protein digestion.
The Complementary Protein Strategy
For those following a plant-based diet, it is important to understand that many plant sources are considered "incomplete" proteins because they are low in one or more EAAs. This can be easily addressed by consuming complementary proteins throughout the day. For example, legumes (like beans) are low in methionine, while grains (like rice) are low in lysine. Combining these foods within a daily diet ensures all EAAs are consumed, thereby creating a complete protein profile. This strategy demonstrates that a diet's overall protein quality is not determined by a single food, but by the combination of sources consumed over time.
Comparing DIAAS and PDCAAS
| Feature | DIAAS (Digestible Indispensable Amino Acid Score) | PDCAAS (Protein Digestibility Corrected Amino Acid Score) |
|---|---|---|
| Digestibility Method | Ileal digestibility (measured at the end of the small intestine). | Fecal digestibility (measured in the large intestine). |
| Accuracy | More accurate, as it measures true absorption before the effects of large intestine bacteria. | Less accurate, as fecal measurements can overestimate amino acid absorption. |
| Scoring Range | Scores are not truncated and can exceed 100%, allowing for ranking of high-quality proteins. | Scores are truncated at 100%, making it impossible to distinguish between proteins of superior quality. |
| Reference Pattern | Uses updated, more accurate amino acid reference patterns. | Uses an older amino acid reference pattern. |
| Current Status | Adopted as the gold standard for protein quality evaluation. | Previously the standard, now largely superseded by DIAAS. |
Conclusion: Making Informed Dietary Choices
Understanding how do you measure protein quality is crucial for making informed dietary choices. While both PDCAAS and DIAAS provide valuable information, the DIAAS represents a more accurate and advanced metric for assessing a protein's nutritional value. Beyond the numbers, consider the complete nutritional profile of your food sources. By consuming a variety of protein-rich foods, including animal and/or complementary plant proteins, you can ensure your diet provides all the essential amino acids needed for optimal health, whether your goal is muscle growth, maintenance, or overall well-being. For further reading on the science behind protein quality evaluation, the NIH offers extensive resources on the subject.
