678 PHYSIOLOGY 



for instance that, from every mucous surface, dead cells are being continually 

 cast off and that a constant disintegration of red blood corpuscles goes on, 

 resulting in the production of the bile pigments ; and we are warranted in 

 extending the operation of these changes of which we have ocular evidence 

 to the case of other cells, such as those of the liver and of the muscles, where 

 direct proof of destruction of tissue during normal metabolism is more 

 difficult to obtain. It is certain that some portion of the nitrogen excreted 

 during complete starvation must come from this source, and that one of 

 the functions of protein food is the replacement of tissue which has been 

 lost in this way. When however we are feeding an animal on a pure 

 protein diet, by far the larger portion of the food is utilised for meeting 

 the energy requirements of the body. In this function protein food, apart 

 from accidents of digestibility and structural adaptation of the animal's 

 digestive arrangements to its habits of life, presents no apparent advantages 

 over the other two classes of foodstuffs. Its value to the animal is repre- 

 sented by its physiological heat value. It may be represented therefore 

 numerically as 4-1, and is equivalent to the value of carbohydrate 1 and is 

 far inferior to the value of fats with a heat equivalent .of 9-3. If, instead of 

 giving to the starving animal a pure protein diet, we administer a mixed diet 

 containing a sufficient quantity of fat or carbohydrate, or of both substances, 

 to meet the normal energy requirements of the body, we can restrict the 

 utilisation of protein more nearly to the replacement of tissue waste in the 

 body, and are therefore able to attain nitrogenous equilibrium with a much 

 smaller proportion of protein than is possible when this substance furnishes 

 the whole diet. In omnivora, such as man, it is easy to attain nitrogenous 

 equilibrium on a mixed diet with a smaller nitrogen turnover than is found 

 during starvation. In the experiment given on p. 672 the average nitrogen 

 output during starvation was about 12 grm. of nitrogen. In Succi, the 

 fasting man, the nitrogen output varied from 11-19 grm. on the fifth day 

 to 2-82 grm. on the twenty-first day. 



DAILY NITROGEN EXCRETION OF Succi IN STARVATION 



Lay N. 



1 ... 17-0 



2 ... 11-2 



3 ... 10-55 



4 ... 10-8 

 6 ... 11-19 



6 ... 11-01 



7 8-79 



Day N. 



8 ... 9-74 



9 ... 10-05 



10 ... 7-12 



11 . . . 6-23 



12 . . . 6-84 



13 . . . 5-14 



14 ... 4-66 



Day N. 



15 . . . 5-05 



16 .' . . 4-32 



17 ... 5-4 



18 ... 3-6 



19 ... 5-7 



20 ... 3-3 



21 2-82 



Chittenden has shown that in man a perfectly normal nutrition may 

 be maintained on a mixed diet containing about 7 grm. of nitrogen daily. 

 In the cases investigated by Chittenden the energy output of the men could 

 be regarded as normal, corresponding to 32-35 Calories per kilo body weight. 

 If the amount of fat and carbohydrate be very largely increased it is possible 

 to maintain nitrogenous equilibrium on even smaller quantities of protein. 



1 This may be expressed by saying that protein is isodynamic with an equal weight 

 of carbohydrate. 



