Understanding the Fundamentals of Protein Quality
Protein quality is not just about the total amount of protein in a food, but about how effectively the body can use that protein. This depends on two primary factors: the food's amino acid profile and the protein's digestibility. Proteins are made of building blocks called amino acids, and the body needs nine specific ones, known as indispensable or essential amino acids (IAAs), to function correctly. If a food lacks a sufficient amount of any one of these IAAs, it is considered a 'limiting amino acid,' which can reduce the overall usability of the protein. The various scoring systems were developed to quantify this nutritional value and allow for comparisons.
Protein Digestibility Corrected Amino Acid Score (PDCAAS)
Developed in 1989, PDCAAS was the gold standard for evaluating protein quality for many years. Its calculation hinges on two main components: the Amino Acid Score (AAS) and the true protein digestibility. The AAS is determined by comparing the food's essential amino acid profile to a reference pattern. The lowest ratio for any single essential amino acid becomes the AAS. The PDCAAS is calculated by multiplying the AAS by true fecal digestibility. A limitation is that scores are capped at 1.0.
Digestible Indispensable Amino Acid Score (DIAAS)
In 2013, the FAO recommended DIAAS as a replacement for PDCAAS, citing more accurate methodology. DIAAS measures the true ileal digestibility of individual indispensable amino acids, reflecting absorption at the end of the small intestine. It also uses updated reference patterns for different age groups. The final DIAAS is the lowest value calculated for any of the IAAs. Unlike PDCAAS, DIAAS values are not truncated and can exceed 100%, allowing for better ranking of high-quality proteins. Scores are categorized: <75% (no claim), 75–99% ('good source'), and ≥100% ('superior quality').
Older Methods for Protein Evaluation
Biological Value (BV)
BV measures the proportion of absorbed protein incorporated into the body.
Protein Efficiency Ratio (PER)
PER is an older method based on animal weight gain relative to protein consumption. It is less relevant for human nutrition.
Net Protein Utilization (NPU)
NPU measures the percentage of ingested nitrogen retained in the body.
DIAAS vs. PDCAAS: A Head-to-Head Comparison
| Feature | PDCAAS | DIAAS |
|---|---|---|
| Digestibility Source | Fecal nitrogen in rats | True ileal digestibility of individual amino acids |
| Digestibility Specificity | Single digestibility value | Specific digestibility value for each indispensable amino acid |
| Reference Pattern | Single pattern | Separate patterns for infants, children, and adolescents/adults |
| Truncation | Scores capped at 1.0 (100%) | Scores are not truncated, can exceed 100% |
| Accuracy | Can overestimate absorption | More accurately reflects absorption and utilization |
| Acceptance | Widely used, still in some regions | Recommended by the FAO/WHO as the new standard |
How to Determine Your Protein's Rating: A Practical Guide
For consumers, laboratory analysis is impractical. Informed decisions can be made by:
- Identifying the protein source: Check the nutrition label.
- Looking for official or tested scores: Some manufacturers list PDCAAS or DIAAS scores.
- Using general rating tables: Refer to published data from reliable sources.
- Considering complementary proteins: Combine different sources for a complete amino acid profile.
- Looking beyond the single score: Consider processing and other health benefits of whole foods.
Limitations and Practical Application of Protein Ratings
Protein ratings have limitations in real-world diets. DIAAS scores can vary by age group, and processing can affect digestibility. For most people, a varied diet is more important than a single meal's rating. Higher-rated sources are more relevant for specific goals like muscle synthesis. While ratings are a powerful comparison tool, they must be considered within the context of a complete diet and health status. The FAO continues to update recommendations for accurate protein quality measures.
Conclusion: Choosing the Right Protein Assessment Method
Understanding how to calculate protein rating provides valuable insight into protein quality. DIAAS offers greater accuracy by addressing PDCAAS limitations. For consumers, using ratings as a guide and combining sources are practical strategies.
