METABOLISM. 477 



requirement. When proteids break up in metabolism sugar is pro- 

 duced in considerable quantity, which gives heat but not energy to the 

 cells. Proteids in the body are most readily metabolized. Fats are 

 the most difficult to burn. Outside the body fats are more easily burnt 

 than either carbohydrates or proteids. 



Proteid Metabolism. 



Pfliiger's dog fed upon flesh a long time and working hard, 

 excreted somewhat more nitrogen than ingested, so that some proteid is 

 used in work. But when fats and carbohydrates are abundant they 

 are the main source of energy in muscular work. One hundred to one 

 hundred and twenty grams of proteid should be allowed daily per 

 adult. 



Man can live only when the chemical elements are arranged in 

 a certain manner with others in the form of foods, for the proportion 

 of carbon to nitrogen in foods is not that required in a diet. If 

 sufficient proteid was used to supply carbon, the diet would contain 

 four times more nitrogen than the body needs. If he received enough 

 proteid to furnish the nitrogen required, then he would be deficient in 

 carbon. Hence it is plain that man's diet should be made up of 

 proteids, fats, carbohydrates, water and salts. 



Each gram of nitrogen corresponds to 6.25 grams of proteid; and 

 since meat contains on an average 34 per cent, of nitrogen, each gram 

 of the latter will represent 30 grams of muscle. 



Living proteid is not destroyed either in metabolism of proteid or 

 in that of the nonnitrogenous materials, but is comparatively stable. 



Folin holds that there are two kinds of proteid metabolism, one 

 constant, the other variable. The variable form, exogenous metab- 

 olism, has urea as its chief product. Urea represents the products of 

 foods which have undergone hydrolysis by trypsin and erepsin, the 

 nitrogen part going to liver to form urea, whilst the carbonaceous 

 residue is converted into carbohydrate or oxidized. The constant 

 form, endogenous or tissue metabolism is represented chiefly by 

 creatinin and by uric acid. Creatinin is a constant quantity, no matter 

 how much variation in the food-proteid, provided meat is not used, as 

 it contains creatinin. Hence creatinin is an index of tissue proteid 

 metabolism. 



Storage of Proteids. The storage of proteids is denied by some 

 physiologists. It certainly is not so readily stored as fats and carbo- 

 hydrates. 



