FOOD NUTRIENTS 53 



oxygen only can be substituted for protein. Just as 

 the phosphorus or potassium of a fertilizer cannot re- 

 place nitrogen, so the carbohydrates or the fats can- 

 not replace the protein of a food. While it is true 

 that protein may be substituted for the carbohy- 

 drates and fats, it is to a limited extent, and only 

 for a limited time. Even though the well-being of 

 the animal would permit of this substitution the 

 added expense would be against the practice. 



