Maternal exposure to nitrate from drinking water and diet and risk for neural tube defects

In this population-based case-control study conducted in California between June 1989 and May 1991, the authors investigated the association between m aternal periconceptional exposure to nitrate from drinking water and diet a nd risk for neural tube defects. The mothers of 538 cases and 539 nonmalfor med controls were interviewed regarding residential history, consumption of tap water at home, and dietary intake during the periconceptional period. Dietary nitrate exposure was not associated with increased risk for neural tube defects. Exposure to nitrate in drinking water at concentrations above the 45 mg/liter maximum contaminant level was associated with increased ri sk for anencephaly (odds ratio (OR) = 4.0, 95% confidence interval (CI): 1. 0, 15.4), but not for spina bifida. Increased risks for anencephaly were ob served at nitrate levels below the maximum contaminant level among groundwa ter drinkers only (OR = 2.1, 95% CI: 1.1, 4.1 for 5-15 mg/liter; OR = 2.3, 95% CI: 1.1, 4.5 for 16-35 mg/liter; and OR = 6.9, 95% CI: 1.9, 24.9 for 36 -67 mg/liter compared with <5 mg/liter). Adjustment for identified risk fac tors for anencephaly did not substantially alter these associations, nor di d control for maternal dietary nitrate, total vitamin C intake, and quantit y of tap water consumed. The lack of an observed elevation in risk for anen cephaly in association with exposure to mixed water containing nitrate at l evels comparable with the concentration in groundwater may indicate that so mething other than nitrate accounts for these findings.