GENERAL CHEMISTRY OF MILK 77 



Colostrum fat, however, is nearly identical with body fat, so that 

 in early milk secretion the formation of milk-fat depends upon 

 body fat, while later fat formation is influenced also by the char- 

 acter of the food. The composition of milk-fat may fluctuate, 

 therefore, according to which one of the two sources is predomi- 

 nant. However, the influence of the source of milk-fat is limited, 

 since it is known that each species of mammal produces milk-fat 

 of characteristic composition, so we must assume that there is a 

 standard composition of fat derived from the milk of a certain 

 species. This standard is variable only within narrow limits. 



The source of milk-fat has been investigated by Jordan and 

 Jenter in a carefully controlled experiment. Recognizing that 

 many previous experiments did not cover a sufficiently long period 

 of time, these authors fed a cow for ninety-five days on a ration 

 from which the fat was nearly all extracted. The cow "continued 

 to secrete milk similar to that produced when fed on the same 

 kind of hay and grain in their normal condition. The yield of 

 milk-fat during the ninety-five days was 62.9 pounds. The food 

 fat eaten during this time was 11.6 pounds, only 5.7 pounds 

 of which was digested. Consequently at least 57.2 pounds of 

 milk-fat must have had some source other than the food fat. 

 The milk-fat could not have come from previously stored body 

 fat, because, 1, The cow's body could have contained scarcely 

 more than 60 pounds of fat at the beginning of the experiment; 

 2, she gained 47 pounds in body weight during this period of time 

 with no increase of body nitrogen, and was judged to be a much 

 fatter cow at the end; 3, the formation of this quantity of milk- 

 fat from the body fat would have caused a marked condition of 

 emaciation which, because of an increase in the body weight, 

 would have required the improbable increase in the body of 104 

 pounds of water and intestinal contents. During fifty-nine con- 

 secutive days 38.8 pounds of milk-fat were secreted and the urine 

 nitrogen was equivalent to 33.3 pounds of protein. According to 

 any accepted method of interpretation not over 17 pounds of fat 

 could have been produced from this amount of metabolized pro- 

 tein. The quantity of milk sohds secreted bore a definite relation 

 neither to the digestible protein eaten nor to the extent of the 

 protein metaboUsm. In view of these facts it is suggested that 

 the well-known effect upon milk secretion of a narrow nutritive 

 ratio is due in part to a stimulative, and not wholly to a con- 

 structive, function of the protein." The authors also state that 

 "the composition of the milk bore no definite relation to the 

 amount and kind of food," and that "the changes in the propor- 

 tion of milk soUds was due almost whoUy to changes in the per- 

 centage of fat." 



