A new study from Aarhus University has found that levels of nitrate in drinking water that are lower than the current acceptable standard are associated with increased risk of colorectal cancer[i].
Nitrate is a widely used fertilizer, and can be ingested by humans via run-off and accumulation in water supplies. Those that live in agricultural areas are likely to have the highest exposure, particularly those with private wells. Studies have suggested links between waterborne nitrate and colorectal[ii] and other[iii] cancers, and conversely, a 2001 study found that, among women, there was a decreased risk of rectal cancer with higher nitrate levels[iv].
After ingestion, nitrates are transformed in the body into compounds known as N-nitroso compounds. Ingested nitrate that results in formation of N-nitroso compounds is classified as probably carcinogenic to humans by the International Agency for Research on Cancer[v].
In the new study, individual nitrate exposure was calculated for 2.7 million adults based on more than 200,000 samples of drinking water from public waterworks and private wells between 1978 and 2011. For the main analyses, data from the 1.7 million individuals with the highest exposure were included. There were 5,944 new cases of colorectal cancer during the study. Those exposed to the highest nitrate levels (more than 9.3 mg/L) were 16 % more likely to develop colorectal cancer than those with the lowest exposure (less than 1.3 mg/L). There were significantly increased risks at drinking water levels above 3.87 mg/L, which is well below the current standard for drinking water of 50 mg/L. The current standard was devised with the intention of preventing blue baby syndrome, which can occur when infants have high nitrate exposure.
‘The conclusion in our study is in line with the findings of several international studies, which indicates that the drinking water standard ought to be lower in order to protect against chronic health effects and not only acute effects such as Blue Baby Syndrome. With identical results from different studies, this points towards a need for reconsidering the drinking water standard,’ says Professor Torben Sigsgaard from the Department of Public Health at Aarhus University, who has been involved in the research project[vi].
[i] Schullehner, J., Hansen, B., Thygesen, M., Pedersen, C. B. & Sigsgaard, T. Nitrate in drinking water and colorectal cancer risk: A nationwide population-based cohort study. Int. J. Cancer doi:10.1002/ijc.31306 http://onlinelibrary.wiley.com/doi/10.1002/ijc.31306/full (Accessed 23 February 2018)
[ii] Espejo-Herrera, N. et al. Colorectal cancer risk and nitrate exposure through drinking water and diet. Int. J. Cancer 139, 334–346 (2016).
[iii] Weyer, P. J. et al. Municipal Drinking Water Nitrate Level and Cancer Risk in Older Women: The Iowa Women’s Health Study. Epidemiology 12, 327 (2001). https://journals.lww.com/epidem/Abstract/2001/05000/Municipal_Drinking_Water_Nitrate_Level_and_Cancer.13.aspx
[v] Grosse, Y. et al. Carcinogenicity of nitrate, nitrite, and cyanobacterial peptide toxins. The Lancet Oncology 7, 628–629 (2006). http://www.thelancet.com/journals/lanonc/article/PIIS1470204506707896/fulltext (Accessed 23 February 2018)
[vi] Nitrate in drinking water increases the risk of colorectal cancer, study finds. ScienceDaily. https://www.sciencedaily.com/releases/2018/02/180220095019.htm (Accessed 23 February 2018)