Why Protein Digestibility Matters
Protein is an essential macronutrient, but its utility to the body depends heavily on its digestibility. Digestibility refers to the proportion of protein that is absorbed and utilized by the body after digestion. A protein source with a high digestibility score provides more usable amino acids for tissue repair, muscle growth, and other vital functions compared to a source with a low score, even if both contain the same amount of crude protein by weight. Factors like the source of the protein, the presence of anti-nutritional factors (e.g., fiber or tannins), and food processing methods all influence digestibility. A key aspect of measuring digestibility is distinguishing between 'apparent' and 'true' values.
The Evolution of Measurement: From PDCAAS to DIAAS
Measuring protein quality has evolved, with the Digestible Indispensable Amino Acid Score (DIAAS) being introduced by the FAO and WHO to improve upon the older Protein Digestibility Corrected Amino Acid Score (PDCAAS).
Protein Digestibility Corrected Amino Acid Score (PDCAAS)
Established in 1993, PDCAAS combines a protein's amino acid profile with its fecal true digestibility. It involves calculating an Amino Acid Score (AAS) by comparing the limiting essential amino acid to a reference pattern, multiplying this by protein digestibility measured in rats, and capping the final score at 1.0, which can underestimate high-quality proteins.
Digestible Indispensable Amino Acid Score (DIAAS)
DIAAS offers a more precise evaluation by using ileal digestibility and avoiding score truncation. It measures the digestibility of individual indispensable amino acids at the end of the small intestine (ileum). The DIAAS is the lowest value among all indispensable amino acids. Scores are not capped at 100%, allowing for better differentiation, and the method suggests using pigs or human studies.
True vs. Apparent Digestibility
'True' and 'apparent' digestibility are key distinctions. Apparent digestibility compares protein intake to fecal excretion without accounting for metabolic nitrogen. True digestibility corrects for metabolic nitrogen, giving a better estimate of absorbed dietary protein and always resulting in higher values than apparent digestibility.
Comparison of PDCAAS and DIAAS
A comparison highlights the advancements with DIAAS:
| Feature | PDCAAS | DIAAS |
|---|---|---|
| Digestibility Measurement | Fecal digestibility (in rats) | Ileal digestibility (preferably in pigs) |
| Focus | Corrects the overall amino acid score using a single, overall protein digestibility figure | Measures the digestibility of each individual indispensable amino acid |
| Scoring | Truncated at a maximum score of 1.0, underrepresenting higher-quality proteins | Non-truncated, allowing for scores over 100% and providing a more accurate ranking |
| Accuracy | Prone to overestimation, particularly for lower-quality proteins | More accurate, reflecting the true amounts of amino acids absorbed |
| Animal Model | Rats | Pigs, or stable isotope tracer methods in humans |
| Use Case | Less accurate for mixed meals and for precise differentiation between high-quality proteins | Enables the calculation of complementary effects in mixed meals and provides a clearer quality distinction |
How to Improve Protein Digestibility
Improving protein digestibility is important, especially for plant-based sources. Techniques include thermal processing, enzymatic hydrolysis, fermentation, physical treatments, and protein complementation (combining sources).
In-Vivo vs. In-Vitro Methods
In-vivo methods using animals or humans are the most accurate but costly. In-vitro (test-tube) methods are faster and cheaper but less precise.
Conclusion
Understanding protein digestibility, particularly the shift from PDCAAS to the more accurate DIAAS, is vital for assessing nutritional value. Factors like food processing and distinguishing true from apparent digestibility influence this. Techniques to improve digestibility and protein complementation enhance protein intake, especially from plant-based foods. {Link: FAO report https://www.fao.org/ag/humannutrition/36216-04a2f02ec02eafd4f457dd2c9851b4c45.pdf} provides more on the evolution of protein quality assessment.
Keypoints
- DIAAS vs. PDCAAS: DIAAS is the newer, more accurate standard for protein quality, improving upon the older PDCAAS method.
- Digestibility Measurement: DIAAS uses ileal digestibility for precise absorbed amino acid values; PDCAAS uses less accurate fecal digestibility.
- Scoring Truncation: DIAAS allows non-truncated scores for better comparison of high-quality proteins, unlike PDCAAS.
- True vs. Apparent: True digestibility provides a more accurate measure of absorbed protein by accounting for endogenous losses.
- Improving Digestibility: Processing like heat, fermentation, and enzymatic hydrolysis can increase protein digestibility, particularly in plant foods.
- Complementary Proteins: Combining incomplete protein sources creates a complete amino acid profile.
