Oxalate Intake and the Risk for Nephrolithiasis

Most kidney stones consist of calcium oxalate, and higher urinary oxalate increases the risk for calcium oxalate nephrolithiasis. However, the relation between dietary oxalate and stone risk is unclear. This study prospectively examined the relation between oxalate intake and incident nephrolithiasis in the Health Professionals Follow-up Study (n = 45,985 men), the Nurses' Health Study I (n = 92,872 older women), and the Nurses' Health Study II (n = 101,824 younger women). Food frequency questionnaires were used to assess oxalate intake every 4 yr. Cox proportional hazards regression was used to adjust for age, body mass index, thiazide use, and dietary factors. A total of 4605 incident kidney stones were documented over a combined 44 yr of follow-up. Mean oxalate intakes were 214 mg/d in men, 185 mg/d in older women, and 183 mg/d in younger women and were similar in stone formers and non-stone formers. Spinach accounted for >40% of oxalate intake. For participants in the highest compared with lowest quintile of dietary oxalate, the relative risks for stones were 1.22 (95% confidence interval CI 1.03 to 1.45; P = 0.01 for trend) for men and 1.21 (95% CI 1.01 to 1.44; P = 0.05 for trend) for older women. Risk was higher in men with lower dietary calcium (P = 0.08 for interaction). The relative risks for participants who ate eight or more servings of spinach per month compared with fewer than 1 serving per month were 1.30 (95% CI 1.08 to 1.58) for men and 1.34 (95% CI 1.10 to 1.64) for older women. Oxalate intake and spinach were not associated with risk in younger women. These data do not implicate dietary oxalate as a major risk factor for nephrolithiasis.