Researchers from the University of Texas Southwest Medical Center decided to study the effects of cranberry juice on people susceptible to calcium oxalate kidney stones, after many reported that they consumed it in an effort to prevent a recurrence of the uncomfortable condition.
The symptoms of kidney stones include pain in the lower back and abdomen, painful urination and the urge to urinate frequently. They are formed when minerals that are dissolved in urine, such as calcium and oxalate, crystallize. While small stones may travel to the bladder and be passed with the urine flow, larger formations can become lodged in the tube between the kidneys and the bladder, where they cause an obstruction.
The study group consisted of 12 subjects without kidney stones and 12 subjects with a history of calcium oxalate kidney stones. In the first seven-day phase the subjects consumed a liter of cranberry juice a day, and in the second a liter of deionized water. Throughout both phases they followed a controlled metabolic diet.
The researchers collected urine and blood samples from all the participants on the last two days of each phases. Their findings are published in the August issue of the Journal of Urology (174(2):590-4; quiz 801).
No differences were observed in the results between the two groups.
In both, the cranberry juice increased urine levels of calcium and oxalate.Levels of brushite did decrease, but this substance causes a different type of kidney stones (known as brushite stones), which are comparatively rare.
"Cranberry juice… reduces urinary pH likely by providing an acid load and decreases urinary uric acid perhaps by retarding urate synthesis," wrote the researchers.
The study does not detract from the scientific evidence supporting cranberry juice's use for urinary tract health, but it does indicate that it is not an advisable course of action for those with a tendency towards kidney stones.