374 HUMAN PHYSIOLOGY 



and warm and supplied with water, a fast of thirty days may 

 in some cases be borne without injury. 



3. Effect of Feeding. 



When food is given to a fasting animal or man, the first 

 effect is to increase the rate of wasting by calling into action 

 the muscles and glands concerned in digestion. The result 

 is an immediate increase in the excretion of nitrogen and 

 carbon, indicating an increased breaking down of proteids 

 and fats. Zuntz and Magnus Levy found that a diet of white 

 bread and butter increased the metabolic processes by an 

 amount equivalent to about 10 per cent, of the energy value 

 of the diet. For this reason, to give an animal which is fast- 

 ing a diet containing just the amount of nitrogen and of 

 carbon which the animal is excreting, will not at once stop 

 the loss of weight. Suppose, for instance, that to a fasting 

 animal using daily 30 grms. of the proteids and 160 grms. of 

 the fats of his body, a diet containing these amounts is given, 

 the disintegration of proteids and of fats will at once rise, say, 

 to 50 grms. of proteid and 280 of fat. Thus the result will 

 be that, instead of his losing 30 grms. of proteid, he will lose 

 only 20 grms. per diem, and instead of 160 grms. of fat, only 

 120 grms. But, if the diet is sufficient to supply the energy 

 required, in a few days the intake and output will balance, 

 and the individual is then said to be in metabolic equili- 

 brium, and he neither gains nor loses weight. The following 

 table gives an idea of how this adjustment of the metabolism 

 is reached : 



If the amount of food be further increased, a small pro- 

 portion of the proteids and a larger proportion of the fats are 



