What is the Ileal Digestibility of Protein?

December 24, 2025 •

5 min read

Protein digestibility, the measure of how much protein is absorbed by the body, is far more accurately measured at the ileal level than at the fecal level. This is because the end of the small intestine (the ileum) is where protein absorption ceases, and undigested protein reaching the large intestine is metabolized by microbes, skewing total-tract measurements.

Quick Summary

This guide explains ileal digestibility of protein, covering why it is the gold standard for evaluating protein quality and bioavailability. It details the methodologies used to determine ileal digestibility and explores the factors influencing it, such as protein source, anti-nutritional factors, and food processing.

Key Points

Accuracy: Ileal digestibility is the gold standard for measuring protein absorption, as it measures uptake at the end of the small intestine before microbial activity in the large intestine affects results.
Types of Measurement: Apparent ileal digestibility measures net absorption, while true ileal digestibility is corrected for endogenous amino acid losses, providing the most accurate value.
Influence of Source: Animal proteins generally have higher ileal digestibility than plant proteins due to fewer anti-nutritional factors and less complex structures.
Processing Matters: Cooking, fermentation, and enzymatic hydrolysis are common processing methods that can significantly improve the ileal digestibility of proteins by altering their structure and inactivating inhibitors.
Modern Metrics: The Digestible Indispensable Amino Acid Score (DIAAS) is a modern, FAO-recommended system that uses true ileal amino acid digestibility for a more accurate assessment of protein quality compared to the older PDCAAS.
Endogenous Losses: Endogenous amino acid losses, from sources like digestive enzymes and intestinal cell turnover, are a key consideration that differentiates true and standardized digestibility from apparent digestibility.
Methodology: Measurement involves either invasive in vivo techniques (like intubation or ileostomates) or less precise in vitro lab simulations, with animal models like pigs often used to predict human digestibility.
Impact on Health: Higher ileal digestibility means more amino acids are absorbed and available for essential body functions, including muscle protein synthesis and tissue repair.

What Defines Ileal Digestibility?

Ileal digestibility refers to the proportion of ingested protein and amino acids that are digested and absorbed by the body before reaching the end of the small intestine (the ileum). It is the most accurate measure of protein and amino acid availability for metabolic use. This is in stark contrast to older, less precise methods that relied on fecal analysis, which incorrectly accounts for protein and amino acids modified or produced by gut bacteria in the large intestine.

The significance of ileal digestibility is recognized in modern nutritional science, including the adoption of the Digestible Indispensable Amino Acid Score (DIAAS) by the Food and Agriculture Organization (FAO). Unlike the older Protein Digestibility-Corrected Amino Acid Score (PDCAAS), DIAAS is based on true ileal digestibility and individual amino acid values, providing a more reliable indicator of protein quality.

Apparent vs. True Ileal Digestibility

When discussing ileal digestibility, it's important to distinguish between apparent and true measurements.

Apparent Ileal Digestibility (AID): This is calculated by subtracting the amino acids recovered in the ileal digesta from the amount consumed. It does not account for amino acids from endogenous sources (like digestive enzymes and shed intestinal cells) that are present in the digesta.
True Ileal Digestibility (TID): This is the most accurate measure, as it corrects for the endogenous amino acid losses present in the ileal digesta. By subtracting these endogenous losses, TID provides a value that truly reflects the digestibility of the dietary protein.

Factors Influencing Ileal Protein Digestibility

Several variables affect how well the body can digest and absorb protein at the ileal level.

Protein Source

Animal-based proteins typically have higher ileal digestibility than plant-based proteins. This is largely due to the presence of anti-nutritional factors (ANFs) and the less accessible protein structures found in many plant sources.

Animal Proteins: High-quality sources like whey, egg, and meat often have very high ileal digestibility values, as evidenced by numerous studies.
Plant Proteins: Legumes and cereals, for example, often have lower digestibility due to a denser protein matrix and the presence of ANFs. However, processing can significantly improve their digestibility.

Anti-Nutritional Factors (ANFs)

ANFs are compounds in food, particularly plants, that can interfere with protein digestion and nutrient absorption.

Protease Inhibitors: These compounds, such as trypsin and chymotrypsin inhibitors found in soybeans, can block digestive enzymes from breaking down proteins.
Phytates and Tannins: These can bind with proteins, making them less accessible to digestive enzymes.
Fiber: High fiber content can reduce the time food spends in the small intestine and physically hinder enzyme access to protein.

Food Processing

Cooking and other processing methods can drastically impact ileal digestibility by altering protein structure and reducing ANF content.

Heat Treatment: Techniques like cooking, autoclaving, and extrusion can denature proteins, unfolding their structure and making them more accessible to digestive enzymes. However, excessive heat can sometimes cause protein aggregation, which reduces digestibility.
Enzymatic Hydrolysis: Breaking proteins down into smaller peptides and amino acids improves digestibility and absorption efficiency.
Fermentation: This process uses microbes to break down complex proteins and reduce ANFs, enhancing digestibility.

Animal vs. Plant Protein Digestibility


Factor	Animal Protein	Plant Protein	Notes
Protein Matrix	Generally less complex; higher digestibility.	Often a dense, complex structure; lower digestibility.	Processing can alter this significantly.
Anti-Nutritional Factors (ANFs)	Generally negligible amounts.	Can contain inhibitors (e.g., trypsin inhibitors, phytates) that hinder digestion.	Proper processing (cooking, soaking) can mitigate ANF effects.
Typical Digestibility	Higher, with values for egg and meat often exceeding 90%.	Lower on average, but varies widely by source and processing.	High-quality isolates (e.g., soy protein isolate) can rival animal proteins.
Bioavailability	Faster absorption kinetics for some proteins like whey due to high content of certain amino acids.	Absorption rate is generally slower.	Slower digestion can be beneficial for satiety.
Processing Impact	Cooking typically increases digestibility by denaturing proteins.	Processing methods like fermentation and hydrolysis are crucial to improving digestibility.	Excessively high heat can sometimes decrease digestibility.

How Is Ileal Digestibility Measured?

Accurately measuring ileal digestibility, especially the true value, requires specialized techniques that differentiate dietary protein from endogenous losses.

In Vivo Methods

In human studies, naso-ileal intubation or the use of ileostomates (individuals with a stoma) are the most direct methods.

Naso-ileal intubation: A tube is inserted to collect digesta directly from the terminal ileum of healthy volunteers.
Ileostomy Model: Digesta is collected from individuals with a stoma, which allows for robust and repeated testing. In animal nutrition, surgically implanted cannulas in pigs or other livestock models are used to collect ileal digesta.

In Vitro Methods

Due to the cost and invasive nature of in vivo studies, in vitro (laboratory-based) methods are often used as alternatives, though they have limitations.

Static In Vitro Models: These mimic the human gastrointestinal tract using fixed conditions (e.g., pH, enzymes, temperature) through oral, gastric, and intestinal phases.
Dynamic In Vitro Models: More complex and advanced systems like TIM-1 simulate the dynamic physiological processes of digestion, including peristalsis and gastric emptying, for more accurate predictions.

Standardized Ileal Digestibility (SID)

In animal feed formulation, standardized ileal digestibility (SID) has become the preferred metric. It corrects for the predictable, baseline (basal) endogenous losses, offering a more consistent and comparable metric for different feed ingredients. Unlike apparent digestibility, SID values are additive in mixed diets, making it invaluable for precise diet formulation.

Conclusion

Understanding what the ileal digestibility of protein is is crucial for accurate nutritional assessment. By measuring protein and amino acid absorption at the end of the small intestine, ileal digestibility provides a far more precise picture of a protein's nutritional value than outdated fecal methods. This metric is influenced by the protein source, the presence of anti-nutritional factors, and how food is processed. As a key component of modern protein quality scoring systems like DIAAS, ileal digestibility is a cornerstone of advanced nutritional science, enabling more effective diet planning for both humans and livestock. The pursuit of maximizing ileal digestibility through better food processing and informed dietary choices ultimately leads to improved protein utilization and better overall health.

For Further Reading

For a detailed overview of plant protein digestibility methods, including both in vivo and in vitro approaches, refer to the comprehensive review by MDPI.

Frequently Asked Questions

Ileal digestibility measures protein absorption at the end of the small intestine, while fecal digestibility measures what is excreted. Fecal measurements are less accurate because they do not account for microbial protein metabolism in the large intestine.

It provides a more accurate estimation of the amount of amino acids actually available for the body's metabolic processes. The large intestine's microbial activity and the production of microbial protein render fecal estimates misleading for determining true protein bioavailability.

DIAAS is a protein quality assessment method recommended by the FAO that uses true ileal digestibility values for individual indispensable amino acids. It offers a more precise evaluation of protein quality compared to the older PDCAAS.

Anti-nutritional factors (ANFs) such as protease inhibitors, phytates, and tannins can reduce ileal digestibility by either interfering with digestive enzymes or binding to proteins, making them less accessible for digestion and absorption.

Yes, processing methods like cooking, fermentation, and enzymatic hydrolysis can significantly improve protein digestibility. These processes can denature proteins, break them into smaller peptides, and deactivate anti-nutritional factors.

While animal proteins typically have higher average ileal digestibility, highly processed plant protein isolates (like soy protein isolate) can achieve comparable or even higher digestibility than some animal sources. Processing is a key variable.

These are amino acids that originate from the body, including digestive enzymes, mucus, and shed intestinal cells, rather than from the diet. True ileal digestibility corrects for these losses to provide a more accurate measure of dietary protein digestibility.

Measurements are performed using direct and invasive methods, such as naso-ileal intubation in healthy volunteers or through the use of ileostomates. In animal studies, cannulated pigs are often used as a reliable model.

Accurate assessment of ileal digestibility is critical for determining the true nutritional value of a protein. This information allows for more effective diet formulation, ensuring that the body receives the necessary amino acids for muscle growth, tissue repair, and overall health.