18 



MAINTENANCE RATIONS OF FARM ANIMALS. 



to the organism. (Compare pp. 78 to 82.) This nonnitrogenous 

 residue can be substituted for body fat, as is illustrated in an experi- 

 ment by Kubner in which extracted lean meat was given to a fasting 

 animal, with the result tabulated below: 



Replacement of body fat by protein — Rubner. 





Nitrogen 

 of food. 



Nitrogen 

 katabo- 

 lized. 



Fat katab- 

 olized. 



Fasting 



Grams. 

 



Grams. 

 5. 25 

 26. 37 



Grams. 

 84. 39 

 28. 37 



Fed 



35.22 





Difference 





+21.12 



-56. 02 







FAT OR CARBOHYDRATES AND PROTEIN. 



A certain minimum of protein is essential to the maintenance of 

 the protein tissues of the body, but feed protein in excess of this 

 amount undergoes rapid katabolism and serves substantially as a 

 source of energy. Such an excess of protein in the feed can be re- 

 placed by nonnitrogenous nutrients, particularly the carbohydrates. 

 This effect of fat or carbohydrates as a substitute for protein may be 

 illustrated by the following tabulation of the average results of a 

 number of Pettenkofer and Voit's experiments : 



Replacement of feed protein by fat or carbohydrates — Pettenkofer and Voit. 



Rations. 



Protein only: 



Series I 



Average of all (22 experiments) 

 Protein and fat: 



1(10 grams fat (1 experiment) . . , 



200 grams fat (5 experiments).. 

 Protein and carbohydrates: 



Starch (8 experiments) 



Grape sugar (3 experiments) . . . 



Feed per day. 



Gain or loss by body 



Meat. 



Grams. 

 1,500 

 1,500 



500 

 500 



500 

 500 



Fat. 



Grams. 



100 

 200 



5.3 



Starch. 



Grams. 



200 



Grape 

 sugar. 



Grams. 



Nitrogen. 



Carbon. 



Grams. 



Grams. 







+ 3.3 



+0.6 



+ 8.7 



+ .3 



+27.1 



- .6 



+67.3 



-1.8 



+ 9.0 



-1.3 



+ 7.2 



It appears, then, that all the principal nutrients may serve to supply 

 energy to the body, and the facts just considered show a remarkable 

 degree of flexibility on the part of the animal organism as regards 

 the nature of the material which can be utilized for its metabolism. 

 Aside from the small minimum of protein required, the metabolic 

 activities of the body may be supported now at the expense of the 

 stored body fat, now by the body protein, and again by the protein, 

 the fats, or the carbohydrates of the feed. Whatever may be true 

 economically, physiologically the welfare of the mature animal is not 

 conditioned upon any fixed relation between the classes of nutrients 

 in its feed supply apart from the minimum requirement for protein. 



