December 19, 2021

Bowel Cancer New Zealand Position Statement: Nitrates and drinking water

Nitrates in drinking water are highly unlikely to increase the risk of bowel cancer in New Zealand, according to the current weight of evidence.

Key messages:

• Nitrates are natural and essential for life
• Most of our nitrates come from food; a small amount comes from drinking water
• The weight of evidence strongly suggests that nitrates in drinking water do not cause bowel cancer, and it is not currently understood how dietary nitrates could cause bowel cancer
• Nitrate contamination of water is a serious environmental and public health issue for reasons other than bowel cancer
• People can lower their risk of developing bowel cancer by minimising established risk factors such as enjoying a healthy diet, exercising regularly and minimising alcohol intake

What are nitrates?

Nitrate (NO3) occurs naturally as part of the environmental nitrogen cycle.1 Plants use nitrates as building blocks to make amino acids.1 Nitrates are available to plants because bacteria capture nitrogen from the air and “fix” it in the soil.1

Approximately half of the nitrates in our bodies come from metabolising amino acids, the other half comes from our diet.2 Green vegetables like lettuce, celery, watercress and spinach are rich in nitrates.3,4 In New Zealand, children and adults get less than 10% of their nitrates from drinking water.3

What harm can nitrates do?

Nitrates can enter waterways from animal waste, the use of nitrogen-based fertilisers, animal feed and septic waste units.3 If nitrate levels in waterways are too high, nutrient pollution may result in algal blooms that can deplete the water of oxygen.5 This has harmful effects on aquatic life and can potentially lead to the accumulation of toxins that harm humans.5 Babies who drink water with high levels of nitrates have an increased risk of developing a disorder called methaemoglobinaemia, where the blood’s ability to carry oxygen is lowered (blue baby syndrome).1,3 Very high levels of nitrates in drinking water may also have negative effects on thyroid function.1

To protect human health and specifically to reduce the risk of methaemoglobinaemia, the World Health Organization (WHO) has set a maximum allowable concentration for nitrates in drinking water of 11.3 mg/L.1

Can nitrates cause cancer?

The nitrates in our diet are rapidly absorbed in the small intestine and excreted in the urine, with very little reaching the large intestine (colon) where bowel cancer occurs.1,3,4 Bacteria in our mouths and gut also transform nitrates into other compounds.3 Under certain conditions, some of these molecules can be further transformed into N-nitroso compounds, some of which may be carcinogenic.3 In our bodies this reaction is inhibited by antioxidants present in food and in tea, coffee and fruit juice.1,3 Currently, it is not understood how nitrates in the diet could cause cancer in the bowel.

What do the scientific studies say?

Six large studies have investigated links between nitrate levels in drinking water and bowel cancer with inconsistent results.6–11 Two studies found a statistically significant association between nitrates in drinking water and bowel cancer,7,10 while four studies did not.6,8,9,11 Meta-analyses that have combined the results of multiple studies have also produced mixed results regarding nitrates in drinking water and bowel cancer. One study did report a statistically significant positive association, while the other did not.12,13

The WHO has concluded that:
“…the weight of evidence does not support an association between cancer and exposure to nitrate…”1

The WHO did note that the ingestion of nitrates under conditions that resulted in nitrosation is probably carcinogenic, but not ingestion of nitrate alone.1

A recent report published by New Zealand’s Institute of Environmental Science and Research (ESR) concluded that it is “highly unlikely” that nitrates in drinking water or the diet increases the risk of cancer.3

Association is not causation

A water-quality study published in 2021 assessed nitrate levels in New Zealand drinking water.14 The study found that three-quarters of New Zealanders’ drinking water contained less than 1 mg/L of nitrates.14 The study also found that unregistered water supplies had higher levels of nitrates, which is unsurprising given these supplies are generally in the countryside where farming and fertiliser use is common.14

The higher levels of nitrates in rural drinking water has been suggested as a reason why some rural communities have higher rates of bowel cancer.15 This correlation between nitrate levels in rural drinking water and higher rates of bowel cancer may be an example of correlation not equalling causation. For example, as ice cream sales increase, so do the rates of drowning. Ice creams don’t cause people to drown though. The explanation is that hotter weather is causing more people to go swimming and to buy ice creams. Equally, the higher rates of bowel cancer in rural communities is more likely to be caused by people in these areas eating more red meat and saturated fat, which are known risk factors for bowel cancer.16

The limitations of the water-quality study, in terms of bowel cancer, were acknowledged by one of its authors in an interview: “We’re just pointing out a correlation between nitrates and drinking water from a bunch of studies from around the world and that it’s increasing in our waters and we worked out how many New Zealanders are getting this high level of nitrate.” Mike Joy, freshwater ecologist.17

What can people do to reduce their risk of bowel cancer?

People who are concerned about their risk of bowel cancer can take steps that are known to reduce their risk:

• Enjoy a healthy diet rich in vegetables, legumes, fruits and cereals, and low in saturated fat, red and processed meats
• Exercise regularly
• Limit alcohol intake
• Quit smoking
• Talk to your doctor if you experience symptoms that might be bowel cancer, e.g. bleeding from the bottom
• Participate in the National Bowel Screening Programme if you are aged 60-74 years

The take-home message

Water quality is a serious issue that is central to everybody’s well-being. There are potentially many reasons to be concerned about the level of nitrates in New Zealand’s waterways and the effects they may be having on ecosystems and our health. However, the evidence does not currently support the hypothesis that nitrates in drinking water increase our risk of bowel cancer. If future research identifies that dietary nitrates are causing bowel cancer, this position statement will be updated.

N.B. This position statement was created independently by Bowel Cancer New Zealand and was not funded, or supported by, any external organisations.

References

1. World Health Organization (WHO). Guidelines for drinking-water quality: Fourth Edition. Published online 2017. www.who.int/publications/i/item/9789241549950

2. Hord NG, Tang Y, Bryan NS. Food sources of nitrates and nitrites: the physiologic context for potential health benefits. Am J Clin Nutr. 2009;90(1):1-10. doi:10.3945/ajcn.2008.27131

3. Cressey P, Cridge B, Risk Assessment and Social Systems Group. Nitrate in food and water. Published online 2021. www.nzfssrc.org.nz/node/207

4. National Research Council (US) Subcommittee on Nitrate and Nitrite in Drinking Water. Exposure assessment. In: Nitrate and Nitrite in Drinking Water. Washington: National Associated Press; 1995.

5. United States Environmental Protection Agency. The Issue. www.epa.gov/nutrientpollution/issue

6. De Roos AJ, Ward MH, Lynch, C. F, Cantor K. Nitrate in public water supplies and the risk of colon and rectum cancers. Epidemiology. 2003;14(6):640-649. doi:10.1097/01.ede.0000091605.01334.d3

7. Espejo-Herrera N, Gràcia-Lavedan E, Boldo E, Aragonés N, Pérez-Gómez B, et al. Colorectal cancer risk and nitrate exposure through drinking water and diet. Int J Cancer. 2016;139(2):334-346. doi:10.1002/ijc.30083

8. Jones RR, DellaValle CT, Weyer P. J, Robien K, Cantor K., et al. Ingested nitrate, disinfection by-products, and risk of colon and rectal cancers in the Iowa Women’s Health Study cohort. Environ Int. 2019;126:242-251. doi:10.1016/j.envint.2019.02.010

9. McElroy JA, Trentham-Dietz A, Gangnon R, E, Hampton J. M, Bersch A. J, et al. Nitrogen-nitrate exposure from drinking water and colorectal cancer risk for rural women in Wisconsin, USA. J Water Health. 2008;6(3):399-409. doi:10.2166/wh.2008.048

10. 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. 2018;143(1):73-79. doi:10.1002/ijc.31306

11. Weyer PJ, Cerhan JR, Kross B. C, Hallberg G. R, Kantamneni J, et al. Municipal drinking water nitrate level and cancer risk in older women: the Iowa Women’s Health Study. Epidemiology. 2001;12(3):327-338. doi:10.1097/00001648-200105000-00013

12. Essien EE, Said Abasse K, Côté A, Mohamed KS, Baig MMFA, Habib M. Drinking-water nitrate and cancer risk: A systematic review and meta-analysis. Arch Environ Occup Health. Published online November 3, 2020:1-17. doi:10.1080/19338244.2020.1842313

13. Hosseini F, Majdi M, et al. Nitrate-nitrite exposure through drinking water and diet and risk of colorectal cancer: A systematic review and meta-analysis of observational studies. Clin Nutr Edinb Scotl. 2021;40(5):3073-3081. doi:10.1016/j.clnu.2020.11.010

14. Richards J, Chambers T, Hales S, Joy M, Radu T, et al. Nitrate contamination in drinking water and colorectal cancer: Exposure assessment and estimated health burden in New Zealand. Environ Res. Published online November 2, 2021:112322. doi:10.1016/j.envres.2021.112322

15. Mike Houlahan. Nitrate levels may be factor in cancer rates: research. Otago Daily Times. www.odt.co.nz/news/dunedin/health/nitrate-levels-may-be-factor-cancer-rates-research. Published 2021.

16. Frizelle F, Keenan J. Hype gets in the way of cause and effect. Ground Eff. 2021;(13):18-19.

17. Kaysha Brownlie. Bowel Cancer NZ medical advisor claims research linking nitrates to bowel cancer is misleading. www.newshub.co.nz/home/rural/2021/02/bowel-cancer-nz-medical-advisor-claims-research-linking-nitrates-to-bowel-cancer-is-misleading.html. Published 2021.