CHAPTER XVIII 

 THE ESTIMATION OF METABOLISM 



IT is about fifty years since the first well-equipped 

 laboratory for the quantitative study of human nutrition 

 was opened in Munich. Before that time much had been 

 accomplished in the analysis of foods, the measurement of 

 rations, and the examination of urine, but no satisfactory 

 knowledge of the general metabolism could be had until 

 means should be devised to entrap and measure the gas- 

 eous outgo of the body. This difficult task was accom- 

 plished by the construction of the first respiration chamber, 

 now one of several in various centers of scientific research. 



In the long run there must be a correspondence between 

 the food and the metabolism, the income and the outgo, 

 but on a single day there is no necessary agreement be- 

 tween them. This is radically demonstrated on a day of 

 fasting, when the income is nil and the outgo is consider- 

 able. It is to the excreta that we attend, therefore, when 

 we wish to judge to what extent various materials have 

 been broken down in the body. Studies of the food may 

 be valuable, but in our first discussion we shall limit our- 

 selves to the simple case of the subject without income. 

 A great deal can be learned about the metabolism by deter- 

 mining two chemical elements the carbon and the nitro- 



in of the waste-products. Other facts can be ascertained 

 when the quantity of oxygen consumed is noted. Water 

 excretion is frequently measured also. 



Nitrogen Elimination. The fact is already familiar 

 that the nitrogen leaving the system is found almost 

 wholly in the urine. An additional fraction is in the 

 feces. Concerning this latter item it will be remembered 

 that we cannot easily say how largely it is a residue signi- 



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