Background: some recent studies have suggested that the risks of colon and rectal cancer increase with exposure to higher concentrations of nitrates in drinking water. This study is a meta-analysis of relevant studies.
Methods: literature published up to June 2021 was accessed and final results abstracted. Two cohort studies and seven case-control studies were analysed, and one case-control study not used because of limited data. Mixed effects meta-regression analysis was used to assess trends in colon cancer, rectal cancer, and colon cancer considered together, with nitrate concentrations in drinking water.
Results: The usually accepted exposure upper limit for nitrates is 11.3 mg/l NO3-N. However most studies assess a lower range, with only one study providing data over 8 mg/l. Colorectal cancer risk increased by 2.4% (95% limits 0.4-4.5%) per unit increase in nitrate concentration, over a range from very low values to mid-range values. Extrapolation to higher dosages has insufficient data. The trend for rectal cancer is less than that for colon cancer.
Conclusion: The increase in colorectal cancer risk with increasing nitrate concentration is lower than in some recent studies, and applies only over a small range. Extrapolation of these results to higher nitrate levels is not warranted. The studies vary greatly in their design, the nitrate concentrations assessed, and in their results. This association is weak and inconsistent, and may be influenced by bias and confounding factors. Any association of drinking water nitrates with colorectal cancer risk is small, and is uncertain.
Keywords: Colon cancer; Colorectal cancer; Drinking water; Meta-analysis; Nitrate; Rectal cancer.
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