From The June 2001 Issue of Nutrition Science News

Cranberry Ups Kidney Stone Risk

Patients with a history of kidney stones may unknowingly put themselves at risk when they take cranberry (Vaccinium macrocarpon) supplements to prevent urinary tract bacterial infections, according to a study conducted by Martha Terris, M.D., at Stanford University Medical Center in Palo Alto, Calif.

In this study, two women and three men (ages 26 to 37) took two 450 mg tablets of cranberry concentrate daily for a week. Their urine was collected during one 24-hour period before and after cranberry consumption.

Cranberry contains oxalate, a component of calcium oxalate. The average oxalate concentration in participants' urine increased by 43 percent. When urine becomes so saturated with calcium oxalate, it precipitates as kidney stones. Subjects' urine calcium oxalate super-saturation increased from 1.8 mg/day to 2.8 mg/day. One of the five study participants, with a family history of calcium oxalate kidney stones, had the greatest rise in urinary oxalate with supplementation.

When urinary oxalate increases as little as 10 percent above normal, calcium oxalate can precipitate. Therefore, it is possible that dietary oxalates from cranberry can cause kidney stones in susceptible people. However, the daily quantity of cranberry consumed in this study is equivalent to almost three liters of cranberry juice. While this was the quantity recommended on the product label, most cranberry products do not contain such high amounts of cranberry concentrate. Although cranberry products are promoted for urinary tract health, they may not be appropriate for patients at risk for kidney stones.

—Urology 2001 Mar:57:26-9.