Understanding how our bodies break down and absorb protein is fundamental to nutrition. Protein digestibility refers to the proportion of protein from a food source that is absorbed by the body after passing through the digestive tract. This metric is a key component in determining the overall quality of a protein. A protein source with a perfect amino acid profile is useless if it cannot be digested and its amino acids absorbed. A variety of methods, both traditional and modern, are employed to quantify this crucial characteristic.
In Vivo Methods: The Gold Standard
In vivo, or 'in life,' methods are historically considered the gold standard for measuring protein digestibility because they involve living organisms and best replicate the complex physiology of digestion.
Animal Feeding Trials
- Rodent studies: Historically, rats were commonly used to determine protein quality measures like PDCAAS. The method involves feeding rats a diet with the test protein and a protein-free diet to calculate a 'true fecal protein digestibility' based on nitrogen balance. However, differences in rodent and human digestive systems and ethical concerns have pushed for newer, more accurate methods.
- Pig studies: Growing pigs are now often considered better models for human digestion, particularly at the ileal level, due to their physiological similarities. Digestion trials with pigs are used to determine true ileal digestibility coefficients for amino acids, which inform the DIAAS score.
Human Dual-Stable-Isotope Method
A minimally invasive dual-tracer method using stable isotopes has been developed for human studies. This technique involves feeding subjects intrinsically labeled test proteins along with a reference protein. By measuring the isotopic enrichment of amino acids in blood and breath samples, researchers can accurately measure protein digestion and absorption at the ileal level. This method is critical for validating other techniques and refining human protein requirements, especially for vulnerable populations.
In Vitro Methods: Laboratory Mimicry
Due to the high cost, ethical considerations, and time investment of in vivo trials, in vitro ('in glass') methods have become increasingly popular. These lab-based techniques simulate the human digestive process under controlled conditions.
The INFOGEST Static Protocol
The INFOGEST static digestion protocol is a harmonized, multi-compartmental model that has been developed and standardized by a global consortium of researchers.
- Oral Phase: Simulates the initial mixing with saliva and salivary enzymes like amylase.
- Gastric Phase: Mimics stomach digestion using gastric enzymes (like pepsin) and acidic conditions.
- Intestinal Phase: Simulates digestion in the small intestine with pancreatic enzymes (trypsin, chymotrypsin) and bile salts.
After digestion, analysis is performed using methods like total nitrogen (Kjeldahl or Dumas) or amino acid profiling (HPLC). The harmonized nature of this protocol significantly reduces experimental variability.
pH-Stat/pH-Drop Methods
These methods are based on measuring the change in pH during enzymatic hydrolysis. As proteins are broken down into peptides and amino acids, protons are released, causing the pH to drop. In the pH-stat method, alkali (like NaOH) is automatically titrated to maintain a constant pH, and the volume of alkali used is measured over time. These methods are rapid and inexpensive but can be less reliable for foods with high buffering capacity.
Multi-Enzyme Assays and Dialysis
Older multi-enzyme assays, such as those using pepsin and pancreatin, have also been employed to evaluate protein digestibility. Dialysis cell methods represent a non-static system where the digestion products are continually removed through a semi-permeable membrane to prevent enzyme inhibition.
Protein Scoring Systems: PDCAAS vs. DIAAS
Beyond simply measuring digestibility, various scoring systems combine digestibility data with the protein's amino acid composition to determine its overall quality. The two most prominent are the older PDCAAS and the newer DIAAS.
The PDCAAS Method
- The Protein Digestibility-Corrected Amino Acid Score (PDCAAS) was recommended by the Food and Agriculture Organization (FAO) and World Health Organization (WHO) in 1989.
- Calculation: A protein's amino acid score (based on its limiting amino acid) is multiplied by its fecal true digestibility, typically determined in rats.
- Limitations: Its use of fecal digestibility overestimates amino acid absorption because it doesn't account for microbial activity in the large intestine. Additionally, scores are capped at 1.0, meaning multiple high-quality proteins receive the same score.
The DIAAS Method
- The Digestible Indispensable Amino Acid Score (DIAAS) was recommended by the FAO in 2013 to replace PDCAAS.
- Calculation: This system uses true ileal digestibility values for individual indispensable amino acids, providing a more accurate measure of the amino acids absorbed by the body. Scores are not truncated at 1.0.
- Advantages: The DIAAS is considered more accurate and scientifically robust. It provides different reference patterns for three age groups, reflecting the changing nutritional needs across a lifespan.
Comparison of PDCAAS and DIAAS
| Feature | PDCAAS (older) | DIAAS (newer) |
|---|---|---|
| Digestibility Measure | Fecal digestibility (often in rats) | Ileal digestibility (individual amino acids) |
| Measurement Accuracy | Less accurate; can overestimate digestibility | More accurate; reflects actual amino acid absorption |
| Score Truncation | Yes, capped at 1.0 | No, scores can be above 100% |
| Based On | Overall crude protein digestibility | Individual indispensable amino acid digestibility |
| Reference Pattern | Single pattern based on preschool children | Multiple patterns for different age groups |
| Recommendation | Replaced by DIAAS | Currently recommended by FAO |
Factors Influencing Protein Digestibility
Protein digestibility is not a fixed number for a food source; it is influenced by several factors, including:
- Food Matrix: The structure of the food can protect proteins from enzymatic digestion. Plant cell walls, for instance, can encapsulate proteins, reducing their accessibility to digestive enzymes.
- Processing: Cooking, heat treatment, and extraction methods can either increase or decrease digestibility depending on the severity. Proper heating can deactivate anti-nutritional factors, while excessive heat can damage amino acids.
- Anti-nutritional Factors: Compounds found in plant-based proteins, such as tannins, phytates, and protease inhibitors, can bind to proteins or inhibit digestive enzymes, reducing absorption.
- Enzyme Specificity: Different enzymes cleave different peptide bonds. A more complete and diverse enzymatic digestion process improves overall digestibility.
Conclusion
Measuring protein digestibility is a critical, multi-faceted process essential for determining the nutritional quality of foods. While traditional in vivo methods offer high accuracy, the rise of modern in vitro models like the standardized INFOGEST protocol provides more ethical, cost-effective, and reproducible alternatives for routine testing. The evolution from the fecal-based PDCAAS to the ileal-based DIAAS represents a significant advancement towards more precise and physiologically relevant protein quality assessment. Acknowledging the factors that influence digestibility and utilizing the most appropriate measurement tools are vital for both food manufacturers and consumers to ensure optimal nutritional intake.
For more information on the official DIAAS recommendations, visit the Food and Agriculture Organization (FAO) of the United Nations: https://www.fao.org/home/en.
