Health Stream Literature Summary - Issue 31 September 2003

Nitrate in Public Water Supplies and Risk of Bladder Cancer.
Ward M H, Cantor K P, Riley D, Merkle S, Lynch C F. Epidemiology (2003) 14 (2):183-90.

A population-based case-control study was conducted on risk of bladder cancer and nitrate in public water supplies in Iowa. Iowa has had elevated levels of nitrate in public supplies for several decades attributed mainly to high-nitrogen fertilizer use.

Bladder cancer cases were identified through the Iowa State Health Registry. Cases were residents of Iowa who were 40-85 years of age and newly diagnosed with histologically confirmed bladder cancer between 1986-1989, who did not have a previous diagnosis of a malignant neoplasm except for basal or squamous cell carcinoma of the skin. Controls less than 65 years of age were selected from state driver's license records and those older than 65 years were selected from US Health Care Financing Administrations records.

Participants were posted a questionnaire about demographic factors, smoking, occupations, lifetime residences and drinking water sources. A food frequency questionnaire was also completed to gather information about intake of food and beverages. Questions about the frequency of intake of beverages and soup made with tap water consumed at and away from home were also asked. Nitrate measurements for Iowa public water supplies were collected. As there were few measurements before the 1960s, only those who used public supplies with nitrate data for 70% or more of their person-years since 1960 were included (808 cases, 1259 controls) in order to reduce misclassification.

No increased risk of bladder cancer with increasing average nitrate levels was found. Rather those in the highest quartile of exposure had significantly lower risk - the odds ratio for women was 0.8 (95% CI = 0.4-0.8) and for men 0.5 (0.4-0.8). No association with risk of bladder cancer was seen after stratification by median intake of vitamin C, ever having a bladder or kidney infection, or smoking status. Daily median intake of nitrate in tap water among men was 2.6 mg/day and among women it was 2.4 mg/day. Results were not modified by amount of tap water consumed. Dietary nitrate and nitrite intake was not found to be associated with risk of bladder cancer.

The authors conclude that even though it is biologically possible that drinking water nitrate intake is risk factor for bladder cancer, no evidence was found here that nitrate levels under around 5 mg/litre were associated with risk. The study could not assess exposures more than 30 years in the past due to lack of water testing for earlier times, and some studies of risk factors for bladder cancer suggest very long latency times may occur.