OBJECTIVES: Cranberry juice has been recommended for patients with recurrent urinary tract infections. However, cranberry juice has a moderately high concentration of oxalate, a common component of kidney stones, and should be limited in patients with a history of nephrolithiasis. Cranberry concentrate tablets are currently available at nutrition stores and are sold as promoters of urinary tract health. After one of our patients with a distant history of calcium oxalate nephrolithiasis developed recurrent stones following self-administration of cranberry concentrate tablets, we sought to investigate the potential lithogenic properties of cranberry supplements.

METHODS: Five healthy volunteers on a normal diet provided 24-hour urine collection for pH, volume, creatinine, oxalate, calcium, phosphate, uric acid, sodium, citrate, magnesium, and potassium. Cranberry tablets were administered to these volunteers at the manufacturer's recommended dosage for 7 days. On the seventh day, a second 24-hour urine collection was obtained.

RESULTS: The urinary oxalate levels in the volunteers significantly increased (P = 0.01) by an average of 43.4% while receiving cranberry tablets. The excretion of potential lithogenic ions calcium, phosphate, and sodium also increased. However, inhibitors of stone formation, magnesium and potassium, rose as well.

CONCLUSIONS: Cranberry concentrate tablets are marketed for urinary tract ailments. Physicians and manufacturers of cranberry products should make an effort to educate patients at risk for nephrolithiasis against ingestion of these dietary supplements.