Cranberry juice may be a popular home remedy for urinary tract infections, but new research suggests it’s of little use against another urinary tract woe:  kidney stones.

In fact, researchers found, the beverage may slightly increase a susceptible person’s risk of the most common type of kidney stone.

Their study, of 24 people with and without a history of kidney stones, found that cranberry juice tended to boost urinary levels of calcium and oxalate, two major components of kidney stones.

Indeed, the large majority of stones are composed mainly of calcium.

The juice did reduce substances that contribute to an uncommon type of stone, but on balance, the findings suggest that people should find a different tactic for fighting kidney stones, according to study co-author Dr. Margaret S. Pearle of the University of Texas Southwestern Medical Center in Dallas.

People who tend to be “stone formers” would be better off reaching for orange juice, which does appear to fight the problem, Pearle told Reuters Health. In general, she noted, susceptible people should drink plenty of fluids to help prevent kidney stone formation.

The stones develop when certain dissolved substances in the urine, including calcium, uric acid and oxalate, begin to crystallize and form into hard masses in the kidneys. The stones travel to the bladder and, if they’re small enough, are passed without a problem. In some cases, though, they block the tube that connects the kidneys and bladder, causing symptoms such as pain in the lower abdomen and back, a frequent urge to urinate and pain during urination.

Cranberry juice has long been espoused as a home remedy for recurrent urinary tract infections, and a number of studies have supported that notion. Pearle said many of the kidney stone patients she sees have told her they drink plenty of cranberry juice to help prevent the problem from recurring.

Where they got the idea that this works, according to Pearle, is unclear, so she and her colleagues decided to study the matter.

They had 24 adults drink 1 liter of cranberry juice per day for one week and spend another week drinking a liter of water each day. On the last two days of each week, the researchers collected urine samples for analysis.

Overall, cranberry juice increased participants’ urinary levels of calcium and oxalate, a substance found in foods such as coffee, nuts, chocolate, spinach and wheat bran. Calcium oxalate climbed by 18 percent, on average.

Calcium oxalate stones are the most common subtype of kidney stones, Pearle noted, so the findings suggest that cranberry juice could actually increase a susceptible person’s risk of developing stones.

Cranberry juice contains calcium and vitamin C, which is converted to oxalate in the body, and this may explain the increase in urinary calcium oxalate, according to Pearle.

The beverage did reduce the presence of a substance called brushite in the urine, indicating possible protection against brushite stones. However, Pearle said, these masses are a relatively rare subtype of kidney stone.

In a final strike against the cranberry, the juice did not show the potentially protective effect of increasing urinary citrate, as orange and grapefruit juices have been shown to do.

So if you’re going to turn to juice for kidney stone relief, Pearle said, cranberry juice probably shouldn’t be the choice.

SOURCE: Journal of Urology, August 2005.