ANIMAL FOODS AND FEEDING 289 



living animal, that is, the relation between the food eaten and the 

 result produced in the form of energy. In the case of domestic 

 animals, however, with the exception of the horse and other 

 draft animals, the ultimate production of energy is not the 

 only result to be obtained by feeding. In all young growing 

 animals a part of the food is used for the building of body 

 substance, and this is also true of sheep, cows and poultry or 

 other animals producing wool, milk, eggs, etc. In the case of 

 fattening animals, also, part of the food is used for the produc- 

 tion of stored body fat. Whether the food, however, is used 

 entirely for the production of energy or partly for building 

 body protein or storing body fat, the value of the food is always 

 measured in terms of energy. 



Digestibility of Food Constituents 



In considering the relation of food to the energy produced, 

 i.e. the value of the food, the first factor to be taken account of 

 is the digestibility, or the amount of the different food materials 

 actually digested in the animal body. If we consider pure food 

 constituents, that is, pure carbohydrates, pure fats or pure 

 proteins, it might seem natural to suppose that the animal 

 body would completely digest and absorb each constituent. 

 Yet, it will not be surprising to know that, even under the most 

 favorable conditions, the animal body is not thus able to digest 

 all of any pure food .constituent. In all cases the amount 

 digested is something less than 100 per cent. 



Coefficient of Digestibility. — Thus we have for each food 

 constituent what is termed the coefficient of digestibility , which 

 expresses the percentage amount of the food constituent which 

 the animal does actually digest and absorb. For example, 

 the human body, when fed carbohydrate food in the form of 

 starch in cooked cereal grains or as bread, is able to digest ap- 

 proximately 98 per cent of it. This figure, then, is the coefficient 

 of digestibility for carbohydrate food in this form for man. 



This coefficient of digestibility is determined directly, by 



