Creatinine

Creatinine (see Creatine) is contained in muscles in very small quantities (e.g., according to Meyer and Fine, 0.002-0.007% in human muscles) and is a normal constituent of human urine and that of other animals. The excretion of C. does not depend on the amount of protein in food and consequently is not in direct proportion to the total content of nitrogenous substances in urine. The excretion of C. represents a constant value for any given person; only the presence of meat in food can increase the excretion of C., since the creatine contained in meat is partially converted to C. in the animal body (exogenous), which is excreted in urine along with the always excreted endogenous C. With a diet poor in meat, men excrete 1.3-2 g of C. per day, women 0.8-1 g; thus the proportion of nitrogen from C. accounts for 2.5% to 6.9% of all urine nitrogen. The excretion of endogenous C. increases in a number of pathological conditions.