MAINTENANCE THE ENERGY REQUIREMENTS 271 



may be supported, now at the expense of the stored body fat, 

 now by the body protein, and again by the proteins, the fats, 

 the carbohydrates, or the organic acids, of the feed. What- 

 ever 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 requirements for protein and ash. But while the 

 body may draw its energy from the most varied feed materials, 

 it by no means follows that the gross energy of these materials 

 is of equal value for the functions of the organism. On the 

 contrary it has been shown that there are wide differences in this 

 respect. 



i. NET ENERGY VALUES FOR MAINTENANCE 



363. Method of determination. The value of any nutrient 

 or feeding stuff as a source of energy for maintenance is obviously 

 measured by the extent to which it can diminish the loss of 

 energy which the body would otherwise suffer. Suppose, for 

 example, that a fasting dog was found to produce 600 Cals. 

 of heat per day by the katabolism of his own tissues. If, in a 

 subsequent experiment, fat be fed, this loss from the body will 

 be diminished, more or less feed fat being virtually katabolized 

 in place of body tissue. If fifty grams of fat are fed, and if a 

 balance experiment shows that the loss of energy from the 

 body is reduced from 600 Cals. to 200 Cals., it is plain that 

 each gram of fat has reduced the loss by 400 -i- 50 = 8 Cals. 

 and the latter number shows the value of this particular fat 

 for the maintenance of this particular animal. 



364. Comparison with metabolizable energy. As already 

 defined (322), metabolizable energy is that portion of the 

 gross energy of the feed which is not carried off as chemical 

 energy in the excreta but is capable of transformation in the 

 body. It was natural to suppose, therefore, that the metab- 

 olizable energy of a substance would represent its value for 

 maintenance and this was long believed to. be true, but later 

 investigations have shown that such is not the case. 



For example, in balance experiments by Armsby and Fries a 

 steer received in successive periods two different amounts of 

 timothy hay, both insufficient for maintenance. The metab- 



