Laivs of Nutrition 185 



nance requiremeut. Milk formation maj" sometimes 

 go on at the expense of the body substance, but with 

 proper feeding, milk, flesh or muscular work are pro- 

 duced at the expense of food supplied in excess of that 

 needed for maintenance. 



8. Regard must be had to the supply of particular 

 nutrients as well as of total food. Even with an ani- 

 mal doing no work and giving no milk a certain 

 amount of protein will be broken up constantly into 

 urea and similar compounds, an amount which will be 

 withdrawn from the body tissues to the extent that it 

 is not supplied by the food. In addition to this, a milch 

 cow, for instance, must have protein for the formation 

 of the nitrogen compounds of the milk, or a steer for 

 the growth of flesh in a quantity proportional to the 

 production, and food must supply it. There is, there- 

 fore, a minimum supply of protein, which, in a par- 

 ticular case, is necessary for maintenance and for 

 constructive purposes, less than which ultimately dimin- 

 ishes production to the extent of the deficiency, or else 

 requires the use of body tissue. 



9. The different classes of nutrients are to some 

 extent interchangeable in their functions. That is to 

 say, all the organic nutrients may be burned to supply 

 energy. Protein may be so used even to withdrawing 

 it from the purposes to which it is necessary unless 

 the carbohydrates or fats are sufficient to protect it from 

 being consumed as fuel. A proper supply of the non- 

 nitrogenous nutrients is required, therefore, to insure 

 the application of the necessary minimum of food pro- 

 tein to its peculiar uses. 



