A mathematical model of creatine metabolism in normal males--comparison between theory and experiment

Based upon knowledge that the body creatine pool in normal males undergoes a constant fractional conversion rate to creatinine and that 24-hr urinary creatinine excretion is dependent upon creatine and protein intakes, we developed a mathematical model with feedback which describes the creatine pool size and, consequently, the 24-hr urinary creatinine excretion as a function of time after change in diet. Validity of the model was tested by comparing calculated with experimental changes in 24-hr urinary creatinine excretion rates of male volunteers who were participating in studies of effects of "high" and "low" protein diets on mineral requirements. The model was further verified by comparing published changes in creatinine excretion rates with changes predicted by the model. It is concluded that the effects of changes in creatine and protein intakes upon the body creatine pool size in healthy males can be described mathematically by a model with a feedback component. It is also cautioned that 24-hr urinary creatinine excretion may not be a good reference for quantifying other urinary substances under circumstances where the creatine or protein intakes are not constant.