The objectives of this study were to (1) examine the relationship between
nitrate levels in public water supplies and risk of death from
gastric cancer and (2) determine whether
calcium (Ca) and
magnesium (Mg) levels in
drinking water might modify the effects of
nitrate on the risk of
gastric cancer development. A matched
cancer case-control study was used to investigate the relationship between the risk of death attributed to
gastric cancer and exposure to
nitrate in
drinking water in Taiwan. All deaths due to
gastric cancer in Taiwan residents from 2006 through 2010 were obtained from the Bureau of Vital Statistics of the Taiwan Provincial Department of Health. Deaths from other causes served as controls and were pair-matched to
cancer cases by gender, year of birth, and year of death. Information on the levels of
nitrate-
nitrogen (NO(3)-N), Ca, and Mg in
drinking water were collected from Taiwan Water Supply Corporation (TWSC). The municipality of residence for
cancer cases and controls was presumed to be the source of the subject's NO(3)-N, Ca, and Mg exposure via
drinking water. Relative to individuals whose NO(3)-N exposure levels were <0.38 ppm, the adjusted odds ratio (OR) and 95% confidence interval (CI) for
gastric cancer occurrence was 1.16 (1.05-1.29) for individuals who resided in municipalities served by
drinking water with a NO(3)-N exposure ≥ 0.38 ppm. There was apparent evidence of an interaction between
drinking water NO(3)-N levels and low Ca and Mg intake via
drinking water. Our findings showed that the correlation between NO(3)-N exposure and risk of
gastric cancer development was influenced by Ca and Mg levels in
drinking water. This is the first study to report effects modification by Ca and Mg intake from
drinking water on the relationship between NO(3)-N exposure and risk of
gastric cancer occurrence. Increased knowledge of the mechanistic interactions between Ca, Mg, and NO(3)-N in reducing risk of
gastric cancer development will aid in public policy decisions and setting threshold standards.