186 THE FEEDING OF ANIMALS 



of foods have certain functions in common. For instance, 

 the proteins, carbohydrates, and fats are all oxidized 

 wholly or in part to supply the necessary energy for 

 muscular activity. The proteins then serve both construc- 

 tive and fuel purposes. Carbohydrates and fats are 

 alike in being sources of energy through oxidation, and 

 in being utilized for the deposition of animal fat. In 

 view of these facts, the question arises whether the physi- 

 cal welfare of the animal requires the mixture of nutrients 

 that commonly exist in the average ration and that is 

 enforced in the standards that are recommended by 

 students of animal nutrition. It is certain that some 

 species of animals may exist wholly on a flesh diet which 

 is practically devoid of carbohydrates, but this is not 

 true of farm animals. 



269. Protein physiologically necessary. The neces- 

 sity of protein in the ration is abundantly demonstrated. 

 Many investigations have shown that when the food 

 contains no protein, the waste of body nitrogen continues, 

 no matter how abundant is the supply of carbohydrates 

 and fats. In other words, a continuous protein cleavage 

 is demanded by the animal organism, and no other 

 nutrients can serve as a substitute for protein in meet- 

 ing this demand. If the food contains no protein, 

 body tissue will be depleted. It cannot be said that 

 either carbohydrates or fats are an essential part of the 

 diet in the sense protein is, because it is possible to sub- 

 stitute the one for the other as energy-producers and pro- 

 tein for both. 



270. Carbohydrates physiologically economical. In 

 spite of these facts, it is safe to assert that the welfare of 

 the animal organism demands a food carrying a mixture 

 of the three classes of nutrients. The larger part of the 



