Digestibility: Definition, Types, Determination Methods and Influencing Factors

The nutritional value of food depends not only on its chemical composition but also on the amount of nutrients or energy. Digestion is the fraction of nutrients or energy in swallowed food that is not excreted in the feces. Digestibility determines the amount of digestion and the relative measure of how much food and nutrients have been digested and absorbed by an animal.

Types of Digestibility

There are two types of digestibility, viz

1. Total or dry matter digestibility: This is used to determine the digestibility of the whole food or feed ingredients.

2. Nutrient digestibility: It determines the digestibility of food or special nutrients in food such as proteins, lipids, amino acids or carbohydrates.

From the point of view of aquatic farming, it is more important to have a thorough knowledge of digestion than digestive physiology (e.g. digestibility assessment method, cultivable species, age, size, sex, stock, time and food intake, and quality of food). Although digestibility is influenced by the physiology or it influences on the physiology of the body. Nutritious food is the best food if it is easily digested and used. The digestibility of food depends on the nature of the food and the type of food ingredient. All digestibility values are expressed as a percentage of the basic food intake. In general, the digestion of fish meal is fast and is about 90%.  On the other hand, the digestibility of protein from different vegetable ingredients is more varied. Equally, the digestibility of large plants varies from species to species.

Determination of Digestibility

Digestibility of food is determined in two ways, viz

  1. Direct method
  2. Indirect method

Direct Method: The direct method determines the amount of food taken and the amount of feces excreted and its ratio is expressed in percent which is considered to be the digestibility of food or nutrients. Although it is very difficult to accurately determine the amount of food and feces taken. Therefore, there is a defect in determining the digestibility of this method.

Perez et al. described the standard method to determine the apparent nutrient digestibility coefficients of the diets in the following classical formula:

Indirect Method: In 1917, the Swedish scientist Edin first used the indirect method of determining digestion. He conducted such research on cattle using markers or marked objects. The substance thus identified is immature and is mixed with very small amounts of evenly tested food. Such markers have no effect on the anatomy of the animal’s digestive tract. There are two types of markers, namely external or internal markers.

External markers used in food are Cr2O3, FeO, SiO3, Polypropylene etc. Among these markers, Cr2O3 is used in higher rates in fish.

The internal markers used to determine digestion are crude fiber (CF), HROM (Hydrolysis-resistant Organic Matter) and HRA (Hydrolysis-resistant Ash). Ash is rarely used to determine the digestibility of natural food items. CF and HROM are objects of the same group. The constituents of HROM are cellulose and chitin and the main constituents of CF are cellulose and lignin. Digestion is determined from the following specific values:

Here nutrient means any nutrient such as carbs or sugars or lipids. Therefore such digestion only determines easy digestion. It accurately determines the total digestibility and digestibility of nutrients which are usually expressed in percentages.

In determining the actual protein digestibility of the food, a controlled diet other than protein and similar experimental foods with other ingredients are taken. The difference between these two types of foods determines the actual protein digestibility of the experimental food and is expressed in percentages.

Apparent Digestibility (AD)

The term apparent digestibility is used since feces are contaminated with endogenous material, and hence the values are not “true” values. The apparent digestibility (AD) can be calculated by comparing the ratio of the marker in the food and feces to a specific nutrient.

The following formula is used for determining apparent digestibility (AD):

AD (percent) = [1 – (F/D x Dm/Fm)] x 100 Where AD = apparent digestibility F = percent nutrient or energy in feces D = percent nutrient or energy in diet Dm = percent marker in diet Fm = percent marker in feces.

Apparent nutrient digestibility (%)=100×NI-NENI

Where, NI represented the nutrient intake and NE expressed the nutrient excreted.

The following formulas are also used to determine the dry matter (DM) digestibility:

Digestibility of the feed:(amount of feed eaten – amount of feces ) / amount of feed eaten 


1 – (amount of feces / amount of feed eaten) * 100

Digestibility of different nutrients (multiply with 100 to get the results in percentages):

(Nutrient content in the eaten amount of the feed – Nutrient content in the feces)

 / Nutrient content in the eaten amount of the feed

Digestibility of the feed and nutrients when using the differential method:

(amount of test feed eaten – (amount of feces – undigested basic, non-test feed))

/ amount of test feed eaten

Ingredient digestibility of food: In case of food ingredients, it is very important to have knowledge about the digestibility of whole food or even the main ingredients in food. The method of determining the digestibility of an ingredient in food is also mentioned below:

  • In this case, a food will be used which is clearly known to digest the total and nutrient content. This type of food is called Reference diet (RD).
  • 20-30% of all the food items to be tested are mixed with the prescribed food. This type of food is called test diet (TD).
  • The digestibility of the total and nutrient content of the experimental food is clearly determined.

Apparent total digestibility of the ingredient) =

Apparent nutrient digestibility of the ingredient =

Factors Influencing Digestibility

1. Like metabolism, digestibility are influenced by environmental influences.

2. Digestion changes with changes in diet and intestinal structure. In the case of grass carp (Ctenopharyngodon idella) larvae only eat zooplankton but from the fingerling stage they start eating grass as food.

3. Although environmental influences and dietary changes did not occur, differences in digestion were observed on the same day (De Silva & Perera 1983). No correlation was found between such varied diet intakes.

Table: The factors that clearly affect total digestion and nutrient digestion are given below-



All those researchers have noticed the influence of the influencer

Dosage / size of food


Clarias gariepinus

Henken et al. 1955

Cyprinus carpio

Von Congnet et al. 1987

Salmo gairdneri

Windell et al. 1978

Species size, age, amount of stock

Salmo gairdneri

Hastings 1969, Windella et al. 1978

Salmo gairdneri

Rychly and Spanilof 1979, Cho et al. 1976

Digestive elements like Proteins, lipids, fiber etc.

Oreochromis niloticus

De Silva and Perera 1984

Salmo gairdneri

Beamish and Thomas 1984

Salmo gairdneri

Hanley 1987

Oreochromis niloticus

De Silva and Perera 1984

Types of nutrients such as protein

Salmo gairdneri

Nose and Toyama 1966

Physical condition of food

Salmo gairdneri

Bergot and Breque 1983

Protein-energy ratio

Cyprinus carpio

Von Congnet et al. 1987

Temperature, salinity etc.

Salmo gairdneri

Windell et al. 1978

Colisa fasciatus

Pandey and Singh 1980

Oreochromis niloticus

De Silva and Perera 1984

Source: De Silva 1989