168 PHYSIOLOGICAL CHEMISTRY 



occurring in the urine. Creatinine is the anhydride of creatine. 

 It occurs in the urine of adults in amounts ranging from 1-2 

 grams a day. On a diet which contains no creatinine, the amount 

 is quite independent of the amount of protein in the food. It is 

 thus evident that it is not an end product of the metabolism of 

 food proteins. If creatinine is present in the food, however, (it 

 is found in meats, meat extracts, etc. ) , almost all of the amount 

 ingested reappears in the urine, and thus evidently is not de- 

 stroyed in the body. On a creatinine free diet, the amount of 

 creatinine in the urine is remarkably constant for each individ- 

 ual, about 7-11 mg. of creatinine nitrogen per kilo body weight 

 per day being excreted. This value is called the creatinine coef- 

 ficient. 



Creatine is found in the urine of children, of women during 

 pregnancy, menstruation and after childbirth, and in the urine 

 of adult men and women during fasting. At this time the 

 amount of creatinine decreases, the total of creatine + creat- 

 inine remaining about constant. 



If creatine is taken by mouth, little or no increase in urinary 

 creatinine or creatine occurs, a very singular fact. Creatine oc- 

 curs as a constituent of most organs, and of voluntary muscle. 

 There are about 120 grams of creatine in the body of an average 

 adult. Creatinine also may be obtained from muscles. 



The fact that the creatinine excretion is independent of the 

 protein and creatine intake, that it varies with age and sex, and 

 sfiows a fairly constant value in each individual has led to the 

 conclusion of Folin that creatinine is a product and index of the 

 endogenous metabolism of the tissues. The source of the urinary 

 creatinine is probably the creatine of the tissues. The forma- 

 tion of creatinine appears to be independent of muscular work, 

 as no increase is observed after exercise. Just what governs the 

 formation of creatinine is still uncertain. The excretion of 

 creatinine is greater during the day than at night, so that it has 

 been suggested its formation may depend in some way on the 

 maintenance of muscle tone, but this point is as yet undecided. 



The whole subject of the role and formation of the creatine of 



