According to the National Kidney Foundation, the kidneys are responsible for removing waste products that would otherwise build up within the body, controlling the production of red blood cells and regulating both the body's fluid pressure and the release of certain hormones. Cranberry juice consumption is thought to help promote normal kidney function and to prevent the development of a number of kidney-related health disorders. Creatinine, a waste product produced in the blood from the metabolism of creatine during muscle use, is filtered out by the kidneys and eliminated from the body in urine. Supplementation with creatinine's precursor, creatine, is thought to not be beneficial for kidney function.
Kidney Infection Prevention
Both the Aurora Health Care and World's Healthiest Food sites report that cranberry juice consumption has been linked with a decreased risk of developing kidney infections. Cranberries are thought to contain a particular type of tannin that is capable of preventing common infection-causing bacteria, such as E coli, from binding to and reproducing on the inner tissue walls of the bladder, resulting in a bladder infection. Untreated bladder infections are one of the most common causes of serious kidney infections, and drinking cranberry juice can prevent either from developing.
Kidney Stone Prevention
Both consuming cranberries and drinking cranberry juice regularly have been linked to a lower chance of developing kidney stones. According to the World's Healthiest Foods site, cranberries contain an acidic compound called quinic acid, along with a high amount of citric acid; both are believed to prevent excess amounts of phosphate and calcium ions in the urine from binding together to form kidney stones. The 3 Fat Chicks on a Diet site also reports that many of the vitamins and minerals contained in cranberries and cranberry juice -- magnesium, Vitamin B6, potassium and Vitamin E -- are thought to also prevent kidney stone formation.
Kidney Disease Development
According to the Mayo Cinic, supplementing with additional creatine--the precursor compound for the metabolic waste product creatinine -- is thought to be detrimental to the kidneys, particularly when taken in excessive amounts or if you suffer from kidney disease. The Mayo Clinic cautions that studies indicate taking additional creatine is not as damaging to the kidneys as once thought, but that cases of interstitial nephritis have been reported. Interstitial nephritis occurs when the tissue between the kidneys' inner tubules becomes inflamed, decreasing the kidneys' ability to filter toxins and waste products out of the blood.