CHEMICAL COMPOSITION OF THE BODY 31 



of only as an excretion of the kidneys. The more proteid 

 we burn for fuel, the more of this waste there is, and hence 

 the greater the work of the kidneys. If, therefore, we eat 

 iarge amounts of proteid, the kidneys have extra work to do; 

 indeed, it is believed that some kidney troubles are produced 

 or at least aggravated by overtaxing those organs from the 

 consumption of too much prcteid. 



Nevertheless, the body must continue to burn fuel in 

 order to keep up its life; it can no more live without burn- 

 ing fuel than the locomotive can run without its fires. If 

 it is not wise to burn great amounts of proteid, what can the 

 body use as a source of the necessary heat and power? If 

 there is some kind of food that furnishes the required energy 

 without leaving behind nitrogen waste materials, it will 

 evidently be of much value. There are two such classes 

 of foods: carbohydrates and fats. 



Carbohydrates. Although starches and sugars seem very 

 different, they are really much alike in chemical composition 

 and may be converted, the one into the other. It is especially 

 easy to change starch into sugar. The similarity in their 

 chemical formulae is very noticeable. Fruit sugar is 

 Ce Hi2 Oe; starch is Ce Hio Os. Starches are always turned 

 into sugars and dissolved before they are taken from the 

 intestine into the blood. From the blood vessels the sugars 

 penetrate into all the tissues and are soon very widely 

 distributed. 



Sugars and starches of various sorts are so commonly seen 

 that one should be warned against thinking of these sub- 

 stances, after their assimilation into the body, as having their 

 usual appearance. Both may be present in solid or crystalline 

 form in the cells of plant leaves, stems or roots; but not so in 

 the cells of any kind of animal. The nearest to it is a sub- 

 stance found especially in the cells of the liver and in muscles 

 and called glycogen. The chemical formula of glycogen is the 



