An individual who has been treated for type 2 diabetes for the past 24 years had, as part of his annual physical, a 24-hour urine collection. Reduced levels of creatinine were found, which is most likely because of which one of the following?

The patient has diabetes-induced nephropathy. As the kidneys lose function, the ability of creatinine to be absorbed into the urine decreases, and its excretion would be low. Creatine is synthesized from glycine, arginine, and SAM. In muscle, creatine is converted to creatine phosphate, which is nonenzymatically cyclized to form creatinine. The amount of creatinine excreted by the kidneys each day depends on the body muscle mass. Weightlifting and increasing muscle mass would increase the levels of creatinine in the urine. A low-protein diet would not reduce the muscle mass, nor affect the creatinine excretion.