FOODS. 143 



of reserved food the body contains. The excretion of urea declines 

 very rapidly during the first two days a fact which has been attrib- 

 uted to a rapid consumption of the surplus proteid food. After 

 this period, when the tissues begin to metabolize their own proteid, 

 the excretion remains fairly constant until toward the close, when the 

 amount eliminated falls very rapidly. As proteids contain about 16 

 per cent, of nitrogen, i part of nitrogen equals 6.25 parts of proteid. 

 Hence, for every i gram of nitrogen or 2.14 grams urea excreted, 

 it may be assumed that 6.25 grams of proteid or, according to Voit, 30 

 grams of flesh have been metabolized. The daily excretion of urea, 

 therefore, indicates the extent of the proteid metabolism. It has 

 been observed also that there is a steady diminution in the excretion of 

 carbon dioxid, though this is greatest in the last few days. As fat 

 contains about 76 per cent, of carbon, i part of carbon equals 1.31 

 parts of fat. Hence, for every i gram of carbon or 3.66 grams carbon 

 dioxid excreted it may be assumed that 1.31 grams of fat have been 

 metabolized. The daily excretion of carbon, therefore, indicates the 

 extent of fat metabolism. The carbohydrates are here left out of 

 consideration, as they constitute only about i per cent, of the body- 

 weight. It must be borne in mind, however, that in the metabolism 

 of proteid a certain quantity of fat is produced which also undergoes 

 oxidation. The amount of the carbon or the fat that the proteid 

 would give rise to, as previously determined, must therefore be sub- 

 tracted from that eliminated by the lungs, etc., in order to determine 

 the amount of body-fat metabolized. Observations of human beings 

 in the fasting condition show that for a period of ten days there is a 

 daily excretion of about 21 grams of urea, equivalent to about 70 

 grams of proteid. This amount, however, may be reduced to from 

 50 to 60 per cent, if the individual has a surplus of body- fat. Human 

 beings under similar circumstances may lose during the first few days 

 200 grams of fat daily. 



The following table shows the excretion of nitrogen and 

 carbon and the calculated amounts of proteid and fat metabolized 

 from an experiment made by Ranke on himself during a fast of 

 twenty- four hours, beginning twenty- four hours after the last meal : 



