Nitrate in drinking water and colorectal cancer - a nationwide population-based follow-up study

Importance of work and objectives Studies have suggested that nitrate in drinking water increased the risk of colorectal cancer. However, often exposure estimations and study size were insufficient to yield unequivocal... [ view full abstract ]

Importance of work and objectives
Studies have suggested that nitrate in drinking water increased the risk of colorectal cancer. However, often exposure estimations and study size were insufficient to yield unequivocal results. We addressed these challenges by conducting a detailed exposure assessment of the entire Danish population.

Methodologies
GIS methods were used to assign nitrate concentrations at the waterworks to the 2,779 water supply areas and 55,752 private wells. Annual nitrate concentrations were assigned to each resident of Denmark from 1978-2012, based on their exact address in the Civil Registration System. For each person the individual adult exposure (age 20-35) was calculated. Information on colon and rectal cancer diagnoses was obtained from the national Cancer Registry. Cox proportional hazard models using age as time scale were fit to assess the risk within exposure deciles. Possible confounding co-variates, such as sex, education, region and previous cancer diagnoses were included and sensitivity analyses conducted taking the uncertainty of the exposure estimate into account.

Main results
2,833,825 individuals were enrolled, totaling a follow-up time of approx. 32 million person-years. Preliminary results will be presented, indicating an increased risk for colon cancer at concentrations far below the drinking water standard (50 mg/l). Results for rectal cancer did not show the same consistent pattern.

Conclusions
This nationwide population-based study addresses previous studies’ challenges of poor exposure assessment and insufficient study population sizes. It adds to the increasing body of evidence of negative chronic health effects associated with increased levels of nitrate in drinking water.