The findings indicate that although many people assume that all citrus fruit juices help prevent the formation of kidney stones, not all have the same effect. The study is available online and is scheduled to be published in the Oct. 26 issue of the Clinical Journal of the American Society of Nephrology.

Medically managing recurrent kidney stones requires dietary and lifestyle changes as well as treatment such as the addition of potassium citrate, which has been shown to lower the rate of new stone formation in patients with kidney stones.

But some patients can't tolerate potassium citrate because of gastrointestinal side effects, said Dr. Clarita Odvina, assistant professor of internal medicine at the Charles and Jane Pak Center for Mineral Metabolism and Clinical Research and the study's lead author. In those cases, dietary sources of citrate - such as orange juice - may be considered as an alternative to pharmacological drugs.

"Orange juice could potentially play an important role in the management of kidney stone disease and may be considered an option for patients who are intolerant of potassium citrate," Dr. Odvina said.

All citrus juices contain citrate, a negatively charged form of citric acid that gives a sour taste to citrus fruits. Researchers compared orange juice and lemonade - juices with comparable citrate contents - and found that the components that accompany the citrate can alter the effectiveness of the juice in decreasing the risk of developing new kidney stones.

Kidney stones develop when the urine is too concentrated, causing minerals and other chemicals in the urine to bind together. Over time, these crystals combine and grow into a stone.

In the UT Southwestern study, 13 volunteers - some with a history of kidney stones and some without - underwent three phases, each lasting one week. Chosen in random order, the phases included: a distilled water or control phase; an orange juice phase; and a lemonade phase. There was a three-week interval between phases.

During each phase, volunteers drank 13 ounces of orange juice, lemonade or distilled water three times a day with meals. They also maintained a low-calcium, low-oxalate diet. Urine and blood samples were taken at intervals during each phase. The study was done at UT Southwestern's General Clinical Research Center.

Orange juice, researchers found, boosted the levels of citrate in the urine and reduced the crystallization of uric acid and calcium oxalate - the most frequently found ingredient in kidney stones.

But lemonade did not increase the levels of citrate, an important acid neutralizer and inhibitor of kidney stone formation.

"One reason might be the different constituents of various beverages," Dr. Odvina said.

For instance, the citrate in orange and grapefruit juice is accompanied by a potassium ion while the citrate in lemonade and cranberry juice is accompanied by a hydrogen ion. Ions of hydrogen, but not potassium, counteract the beneficial effects of the high citrate content.

"There is an absolute need to consider the accompanying positive charge [of hydrogen ions] whenever one assesses the citrate content of a diet," Dr. Odvina said.

utsouthwestern