Clinical Abstracts

With Comments by Adriane Fugh-Berman, MD

Cranberry and Urinary Oxalate

July 2001; Volume 3; 56

Source: Terris MK, et al. Dietary supplementation with cranberry concentrate tablets may increase the risk of nephrolithiasis. Urology 2001;57:26-29.

Design and Setting: Uncontrolled trial in five healthy volunteers (two women and three men, age 26-37) with no history of renal stones, consuming usual diet.

Treatment Dose/Route/Duration: Cranberry tablets administered according to manufacturers recommended dosage for seven days.

Outcome Measures: 24-hour urine collection for pH, volume, creatinine, oxalate, calcium, phosphate, uric acid, sodium, citrate, magnesium, and potassium.

Results: Urinary oxalate levels increased significantly (P = 0.01) by an average of 43.4%. All volunteers experience elevation of urinary oxalate levels. Urinary sodium, magnesium, and potassium all increased significantly, as did calcium oxalate supersaturation; uric acid supersaturation did not increase. Urinary calcium, phosphate, uric acid, and citrate did not differ significantly from baseline.

Funding: Not stated.

Comments: Oxalate content of the cranberry preparation was not specifically analyzed, but the authors estimate that commercially available high-dose cranberry concentrate tablets taken in a manufacturer-recommended dose could increase dietary intake of oxalate by 142% (363 mg). Urinary oxalate concentration is very important in stimulating stone formation; the authors note that as little as 10% increase in urinary oxalate can cause calcium oxalate crystallization. The clinical implications of cranberry concentrate consumption are somewhat unclear as the preparation increased magnesium and potassium (which inhibit stone formation). Also, the authors note that their volunteers had relatively high urine output and urinary calcium levels and may not be representative of the general population or stone-forming individuals.

This is a small and imperfect study. It would have been interesting to include stone formers and it would have been helpful to analyze oxalate levels in the preparation used. Still, it raises an important issue. Cranberry juice is a popular home treatment or prophylactic against urinary tract infections. (See page 51). While it is difficult to increase oxalate intake substantially with cranberry juice (which contains 1.89 mg oxalate/30 ml juice), cranberry concentrate tablets may contain significant amounts of oxalate. Patients should be advised to drink juice rather than take tablets. Alternatively calcium, which binds oxalate in the gut, can be administered concurrently with concentrated cranberry products.