Digestibility is the quantification of the digestive process, it gives a relative measure of the extent to which ingested food and its nutrient components have been digested and absorbed by the animal.

Only a proportion of ingested food is digested and its nutrients are absorbed, rest is voided as faeces.

A nutritionally balanced diet will only be a good diet if it also easily and effectively digested and utilized.

Therefore, when formulating a diet it is essential to have a knowledge on the digestibility of ingredients and final product.

INTRODUCTION :

Digestibility Coefficients : Digestibility coefficients (DC) allow the

quantification of digestibility. For a given food, the apparent digestibility

coefficient (ADC) and true digestibility coefficient (TDC) can be defined as –

ADC depends on the physiological state of fish and ingestion rate.

TDC depends mainly on the type of diet and digestive capacity of the species.

ADC =Ingested -faecalingested

*100

TDC =Ingested- (faecal- endogenous faecal)ingested

* 100

Ingredient Species of fish

Level of ingredient used

Control diet Digestibility Coefficient

Water hyacinthE.Crassipes(dried)

Labeo rohita 20% and 40% of total dietary protein

Fish meal supplying 100% of the dietary protein

APD is 71% and 68% for 20% and 40% inclusion levels resp. and 79 % for control.

Ipil-ipilLeucaena leucocephala(leaf meal)

L.rohita ,, ,, APD coefficients were 68 and 63% resp.

Mustard oil cakeBrassica juncea

C.carpio 25 and 50% of dietary protein

,, APD coefficients were 84 and 81% resp.

Linseed mealLinum usitatissimum

C.carpio ,, ,, APD coefficients were 85 and 78% resp.

Sesame mealSesamum indicum

C.carpio 25,50 and 70% of dietary protein

,, APD coefficients were 81,78 and 78% resp.

Determination Of Digestibility :

Unlike comparable digestibility studies on terrestrial animals, those on fish have an inherent difficulty by virtue of the medium in which they live.

Digestibility of a diet / feed can be determined directly or indirectly as follows –

Direct method : In this method, the quantity ingested (total

or nutrient) and faecal matter voided are determined, and the ratio gives the percentage digestibility of feed and / or the nutrients under consideration.

1) Leaching of nutrients2) Collecting faeces3) Reingestion

Difficulties -

In Direct method : In this method estimation of digestibility is done by the use of markers.

Markers : A marker is a material, thought to be

indigestible, which is introduced in small quantities and distributed evenly in the test diet , or an indigestible component of the diet itself. These are known as external and internal markers respectively.

IndigestibleShould not influence physiology of digestionShould move along the gut with foodShould not be toxic

Characteristics of marker -

Types of Markers :

Markers those introduced into diet. eg: cr2o3,feo, sio2,polypropylene.In these most commonly used is cr2o3.

Commonly utilized for digestibility estimations are crude fibre (cellulose & lignin) , hydrolysis resistant organic matter (cellulose & chitin) & hydrolysis resistant ash (HRA) occasionally ash has used.

External Markers :

Endogenous (or) Internal Markers :

Digestibility is estimated from the determined values as follows :

Dry matter digestibility (%)

(or)Total digestibility

= 100 - 100{% marker in diet%marker in feed}

Nutrient digestibility (%)

= 100 - 100{%marker in diet%marker in faeces*

% nutrient in faeces % nutrient in feed

}

Ingredient Digestibility : The procedure that is presently accepted for

estimating digestibility of a ingredient is – Use a diet for which apparent total and

nutrient digestibilities are known for the species/organism that is being investigated – this diet is referred to as reference diet.

Prepare a test diet mixing 20 – 30 % of the ingredient to be tested with the reference diet – this diet is test diet.

Determine apparent total & nutrient digestibility of the test diet.

Contd….

The apparent total and nutrient digestibility of the ingredient is estimated using the following equation {assuming that the test diet(TD) has been made up in a ratio of 8 : 2 of the reference diet (RD) : the ingredient }

Apparent total digestibility of the ingredient

= 10020

( dry matter digestibility of TD - 80100

dry matter digestibility of RD )

Apparent nutrient digestibility of the ingredient

= 100200(nutrient digestibility of TD -8

0100nutrient digestibility of RD )

Factors Influencing Nutrient Digestibility : Digestibility is influenced by both biological &

environmental factors – however , unlike the influence of biological and environmental factors on metabolic activities such as respiration , that on digestibility is less well defined and little understood.

This is primarily because of the dearth of research on aspects of digestibility.

At times the information available is contradictory. The available information is summarized as follows -

Contd….

Proteins : Fish generally digest proteins with an ADC

exceeding 90 % , a level equal or superior to those observed in terrestrial vertebrates.

Digestibility of protein varies relatively little between fish species & increases slightly with fish size.

It is independent of ingestion rate , temperature & presence of lipids in diet.

Proteins of animal origin are generally more digestible than those of plant origin.

Treatment of plant proteins improves ADC by destroying anti nutritional factors.

Eg: Cooking of whole soya grain leads to an increase in ADC from 70 to 85 %.

Lipids : ADC > 95 % Lipids with low melting points are well utilized

by fish Digestibility of saturated fatty acids decreases

as chain length increases and , for identical chain lengths, it increases with the degree of unsaturation.

Thus lipids with saturated fatty acids with medium chain length have low ADC , when temperature is low.

Eg: In salmonids , the ADC of lard , which has a melting point of 28 – 48 0C changes from 70 – 78 % when the temperature increases from 5 to 15 %.

Carbohydrates : Digestibility of simple sugars like glucose &

sucrose is close to 100 % where as of digestibility of starch is of 70-80 % and can be less than 50 %.

The ADC of starch varies with amylase activity , which is specie specific.

Rise in water temperature improves digestibility of carbohydrates.

Digestibility of starch increases from 25 to 48 % in turbot at 13 to 18 oC and in trout from digestibility increases at 8 to 18 0C.

Starch digestibility depends on its nature i.e., the relative proportion of amylase and amylopectin and as well as size of the grain.

Other Variables :

It is generally assumed that nutrient digestibility is independent of the amount ingested.

In general as temperature increase ADC of protein and energy increases.

In euryhaline species, an increase in salinity leads to a decrease in protein digestibility.

The effects of salinity on the ADC of lipids and carbohydrates are less well known.

Conclusion :

In overall terms , from digestive physiology to nutrient digestibility , which is more difficult to measure than in terrestrial animals, as fish always present unique features. Their remarkable aptitude for digesting proteins contrasts with their low aptitude for digesting carbohydrates.

Progress in the area of nutrition and feeding is necessary for a better knowledge of the physiology of digestion and food digestibility.