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: control orange juice phase. There was a three-week interval between phases. During each phase, volunteers drank 13 ounces of orange juice 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.