The Problem of Truncated Scores
One of the most criticized limitations of PDCAAS is the truncation or capping of scores at 1.0. A score of 1.0 indicates that a protein provides 100% or more of the essential amino acids required per unit of protein after digestion. However, this practice creates a major flaw in comparing proteins because it prevents differentiation between proteins that significantly exceed the requirement and those that barely meet it. This limitation makes PDCAAS a poor comparative tool for high-quality proteins.
Inaccurate Digestibility Measurement (Fecal vs. Ileal)
PDCAAS relies on true fecal digestibility, often measured in rats. Fecal measurements, taken at the end of the digestive tract, can overstate protein quality because bacteria in the colon can break down unabsorbed amino acids, making them appear absorbed. Ileal digestibility provides a more accurate assessment of amino acids absorbed by the human body. Research indicates fecal measurements are particularly inaccurate for plant proteins.
Failure to Account for Anti-Nutritional Factors
Plant proteins often contain anti-nutritional factors like trypsin inhibitors and lectins that hinder amino acid digestion and absorption. PDCAAS does not effectively account for these, leading to inflated scores and overestimations of nutritional value, especially for plant-based sources. While processing can reduce these factors, they are not always eliminated. The DIAAS method is designed to be more accurate in considering these factors.
Single Reference Pattern for All Ages
The PDCAAS scoring pattern is based on the essential amino acid needs of 2- to 5-year-old children. This single benchmark doesn't reflect the varied amino acid requirements for all age groups and physiological states, such as adults, pregnant women, the elderly, or athletes. This limits its applicability for personalized nutritional guidance.
PDCAAS vs. DIAAS: A Comparative Table
| Feature | PDCAAS (Protein Digestibility-Corrected Amino Acid Score) | DIAAS (Digestible Indispensable Amino Acid Score) |
|---|---|---|
| Scoring | Truncated at 1.0, failing to differentiate higher quality proteins. | Uncapped, allowing for more accurate ranking of all protein qualities. |
| Digestibility | Measures fecal digestibility, often overestimating protein quality. | Measures ileal digestibility, providing a more accurate measure of amino acid absorption. |
| Amino Acid Basis | Uses a single crude protein digestibility value. | Uses individual amino acid digestibility values for greater accuracy. |
| Anti-nutrients | Does not adequately account for the impact of antinutrients. | More accurately reflects the effect of antinutrients on absorption. |
| Model | Based on rat studies for digestibility measurement. | Recommends using pigs, whose digestive systems are more similar to humans. |
| Reference Pattern | Uses a single pattern based on the needs of preschool children. | Provides age-specific reference patterns for different demographics. |
How Processing Affects Bioavailability
Food processing, especially high heat, can damage proteins and reduce amino acid bioavailability, such as lysine being made less available through the Maillard reaction. PDCAAS doesn't adequately account for these changes, potentially overestimating processed food protein quality. It also overlooks that free amino acids, sometimes added as supplements, may not have the same absorption kinetics as protein-bound amino acids.
Conclusion: Shifting to a Modern Metric
PDCAAS was an improvement over older methods, but its significant flaws, including truncated scores, inaccurate fecal digestibility, ignored anti-nutritional factors, and a single reference pattern, provide an incomplete assessment of protein quality. The FAO recommended replacing PDCAAS with the Digestible Indispensable Amino Acid Score (DIAAS) in 2013. DIAAS offers a more scientifically accurate evaluation using ileal digestibility for individual amino acids and providing uncapped scores, making it a superior method for modern nutritional science. {Link: FAO https://www.fao.org/ag/humannutrition/35978-02317b979a68a571e220a068007a82b.pdf}.
Lists
Reasons DIAAS is Superior to PDCAAS
- DIAAS provides uncapped scores, allowing for proper ranking of all protein qualities.
- Ileal digestibility measurement in DIAAS is more accurate than fecal measurements used in PDCAAS.
- DIAAS considers individual amino acid digestibility, rather than an average crude protein value.
- Age-specific reference patterns in DIAAS cater to diverse population needs.
- DIAAS better accounts for the effects of processing and antinutrients on protein quality.
PDCAAS's Major Flaws
- Truncation of scores at 1.0 limits comparative power and undervalues high-quality sources.
- Fecal digestibility, often from rat studies, inaccurately represents human absorption.
- Anti-nutritional factors are largely ignored, leading to overestimated protein quality.
- The use of a single reference pattern fails to address the varied needs of different populations.
- Doesn't adequately capture changes to protein bioavailability from processing.
Conclusion
The limitations of PDCAAS demonstrate why modern nutrition science requires more sophisticated and accurate measurement tools. While PDCAAS was a foundational method for protein quality assessment, its inherent flaws, from score truncation to flawed digestibility measurement, highlight its shortcomings. The shift to DIAAS represents a crucial evolution in our understanding of protein quality, providing a more reliable metric for consumers, manufacturers, and dietitians alike. A focus on methods like DIAAS that provide a more complete and accurate picture of protein nutritional value is essential for promoting optimal health and dietary choices.
