Experiments on Use of Nutrients 195 



Practically the same plan has been followed in study- 

 ing the source of milk fat. Several cows were fed on 

 carefully weighed and analyzed rations extremely poor 

 in fat, and the amount and composition of the feces, 

 urine and milk were ascertained during sixty to ninety 

 days. The fat digested from the food and the theo- 

 retical fat equivalent of the decomposed protein as 

 measured by the urine nitrogen were charged up 

 against the milk fat, and a large quantity of the lat- 

 ter could be accounted for only as having had its 

 source in carbohydrates. 



Another method of investigating fat formation has 

 been used with dogs. It is well known that when an 

 animal is deprived of food the expenditure of energy 

 by the body is maintained at the expense of body sub- 

 stance. Both muscular tissues and fatty substance are 

 broken down and used in this way, the latter being 

 regarded as furnishing the most natural and available 

 supply of fuel. It was found in the case of dogs that 

 after a certain number of days of- starvation there oc- 

 curred a sudden and large increase in the waste of 

 nitrogen compounds as shown by the urine excretion, 

 the explanation for this being that the body fat had 

 become exhausted and a demand was at once made upon 

 the proteid tissues for the necessary supply of energy. 

 As soon as this rise of nitrogen waste appeared, then the 

 dog was allowed to eat, and whatever fat was found in 

 the body at the end of the feeding period was regarded 

 as having been formed from the food taken after the 

 starvation period. If, for instance, the ration was 

 wholly protein and fat was found to have become de- 



