Understanding the Nutrition Diet: What Is the Unit of Digestibility?

October 24, 2025 •

4 min read

Over 75% of a healthy diet is typically absorbed by the body, but this is never a perfect 100%. When we discuss the efficiency of this process, many people wonder, 'What is the unit of digestibility?' In fact, it is not a single unit but a ratio, or coefficient, expressed as a percentage that is crucial for understanding nutrition.

Quick Summary

Digestibility is not measured in a standard unit but as a percentage, known as the digestibility coefficient. This value reflects the proportion of ingested nutrients that are successfully absorbed by the body during the digestive process, influencing overall nutrient utilization.

Key Points

Not a Unit: Digestibility is not measured in a standard unit like mass or volume, but as a percentage or ratio called a coefficient.
What It Measures: The digestibility coefficient indicates the proportion of a nutrient absorbed by the body versus the amount consumed.
Apparent vs. True: Apparent digestibility is a simple intake-minus-fecal-output calculation, while true digestibility corrects for endogenous (non-dietary) losses for a more accurate result.
Measurement Methods: Techniques include total collection (for precision), using indigestible markers (for convenience), and in vitro or in situ lab simulations.
Factors Affecting It: Digestibility is influenced by diet composition (e.g., fiber content), food processing, age, and animal species.
Nutrient Specificity: Coefficients can be calculated for specific nutrients, such as crude protein, fat, or dry matter, to provide detailed nutritional insights.

Demystifying the Concept: No Single Unit Exists

In nutrition science, unlike measurements for mass (kilograms) or volume (liters), there is no single universal unit for digestibility. Instead, digestibility is quantified as a ratio or a digestibility coefficient, which is a unitless value expressed as a percentage (%). This coefficient is calculated by comparing the amount of a specific nutrient consumed in a diet to the amount of that same nutrient excreted in the feces. A higher digestibility coefficient indicates more efficient absorption of that nutrient by the body, meaning a larger portion of the consumed food was utilized rather than passed through the digestive tract.

For example, if you consume 100 grams of protein and excrete 20 grams of undigested protein in your feces, the apparent protein digestibility is calculated as follows:

$$ \text{Digestibility (%) = } \frac{(\text{Nutrient In} - \text{Nutrient Out})}{\text{Nutrient In}} \times 100 $$

In this case, the calculation would be: (100 g - 20 g) / 100 g * 100 = 80%. This means the protein in your diet has an 80% apparent digestibility.

Apparent vs. True Digestibility

When discussing digestibility, it's essential to differentiate between two primary types: apparent and true digestibility. Apparent digestibility is a simpler calculation but doesn't tell the whole story. It measures the difference between intake and fecal output, but feces contain more than just undigested food. The body naturally sheds cells from the intestinal lining and secretes enzymes and other substances into the digestive tract, all of which contribute to the fecal matter. These are known as endogenous losses.

True digestibility provides a more accurate measure by accounting for and subtracting these endogenous losses from the total fecal output. The coefficient of true digestibility will therefore always be higher than the apparent digestibility coefficient.

Comparison of Apparent vs. True Digestibility


Feature	Apparent Digestibility	True Digestibility
Calculation	(Intake - Fecal Output) / Intake	(Intake - (Fecal Output - Endogenous Losses)) / Intake
Accuracy	Less accurate, simpler to measure.	More accurate, requires complex correction for endogenous losses.
Scope	Measures what is retained in the body, which is what is used for dietary recommendations.	Represents the actual absorption of dietary nutrients.
Impacted by	Factors like diet composition, fiber content, and processing methods.	Less affected by the level of a specific nutrient in the diet.
Use Case	Widely used for practical nutritional evaluation in animals and humans.	Primarily used in research to understand metabolic processes.

How Digestibility Is Measured

The measurement of digestibility can be a complex process, particularly in human studies. For animals, controlled feeding trials are common. In humans, total collection studies can be challenging, so markers are often used.

Total Collection Method: This traditional approach involves measuring total food intake and total fecal output over a specific period. It is precise but can be labor-intensive and subject to errors. For human subjects, it's often more practical for small, controlled studies.
Marker or Indicator Method: This technique uses a non-digestible substance (a marker) that is added to the diet or occurs naturally. By analyzing the ratio of the nutrient to the marker in both the feed and the feces, digestibility can be calculated without needing to collect all fecal output. Common markers include acid-insoluble ash or external markers like chromic oxide.
In Vitro and In Situ Methods: These laboratory techniques simulate the digestive process outside of a living organism, offering a faster and less expensive way to estimate digestibility. For example, the in vitro method uses enzymes and rumen fluid in a test tube to digest feed samples, while in situ uses small bags of feed incubated in the rumen of a live animal.

Key Factors Influencing Digestibility

The digestibility of a nutrient or a diet is not a static value; it is influenced by numerous biological and dietary factors.

Dietary Fiber: High levels of dietary fiber, especially certain types like cellulose, can decrease the digestibility of other nutrients by speeding up transit time and physically blocking access to digestive enzymes.
Food Processing: Methods like cooking, grinding, or fermentation can break down complex food structures, increasing nutrient availability and digestibility. Conversely, excessive heat can reduce the digestibility of proteins by damaging amino acids.
Chemical Composition: The chemical makeup of a food, including its protein, carbohydrate, and fat content, plays a major role. For instance, some plants contain anti-nutritive factors like enzyme inhibitors or tannins that can lower protein digestibility.
Age and Species: Digestive efficiency varies between species and changes with age. For instance, ruminants are specialized to digest fiber, while monogastrics like humans and pigs have different capabilities. A young animal may have an underdeveloped digestive system compared to a mature one.
Level of Intake: Eating larger quantities of food in a shorter period can decrease overall digestibility, as food passes through the digestive tract more quickly, leaving less time for full absorption.

Conclusion

While there is no single unit of digestibility, the digestibility coefficient, expressed as a percentage, provides a robust and practical way to measure how efficiently our bodies absorb nutrients from our food. By understanding the distinction between apparent and true digestibility and the many factors that influence this process, we can make more informed dietary choices. Measuring digestibility is a cornerstone of nutritional science, helping to evaluate the quality of diets and the bioavailability of nutrients for both human health and animal husbandry.

Digestibility of different nutrients

Digestibility coefficients can be determined for specific nutrient classes, providing a more detailed picture of how a diet is utilized. These often-measured coefficients include:

Dry Matter Digestibility: Reflects the overall digestibility of the total food, excluding water.
Crude Protein Digestibility: Measures the absorption of dietary protein.
Crude Fat Digestibility: Measures the absorption of dietary fats and lipids.
Gross Energy Digestibility: Reflects the absorption of the food's total energy content.
For more detailed nutritional information, resources like ScienceDirect provide further insight into the complexities of nutrient digestibility.

Frequently Asked Questions

The digestibility coefficient is a ratio, expressed as a percentage, that quantifies how much of a specific nutrient from a diet is absorbed by the body.

Apparent digestibility calculates absorption based on the difference between nutrient intake and fecal output. True digestibility, a more accurate measure, also accounts for and subtracts endogenous substances (like intestinal cells and enzymes) from the fecal output.

To calculate apparent digestibility, you can use the formula: (Intake - Fecal Output) / Intake * 100%. For example, if 100g of a nutrient is eaten and 20g is excreted, the digestibility is 80%.

Markers are non-absorbable substances, like acid-insoluble ash or chromic oxide, used in digestibility studies to estimate nutrient absorption without requiring the collection of total fecal output.

Yes, dietary fiber can significantly affect digestibility. High levels of fiber can speed up the passage of food through the digestive tract, which may decrease the overall absorption of other nutrients.

Food processing methods like cooking, grinding, or fermenting can either increase or decrease digestibility. Some processes break down complex food structures, enhancing absorption, while excessive heat can damage nutrients and lower their digestibility.

The digestibility of protein, like all nutrients, is not 100% because some proteins may be less susceptible to enzymatic digestion and anti-nutritive factors can interfere with absorption. Additionally, the body itself contributes metabolic nitrogen to fecal matter.