Comment: There is a weight of evidence supporting that nitrate in drinking water does not cause cancer, yet the scaremongering continues, writes Dr Jacqueline Rowarth.

Nitrates, drinking water and bowel cancer ... the headlines implying a link persist despite evidence to the contrary.

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Global bodies such as the World Health Organisation (WHO) review the research regularly and can find no link to suggest that nitrate in drinking water causes health problems.

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The WHO's 2017 report, based on a review of available scientific studies, stated that "although numerous epidemiological studies have investigated the relationship between exposure to nitrate or nitrite in drinking-water and cancer occurrence, the weight of evidence does not clearly support an association between cancer and exposure to nitrate or nitrite per se".

The statements by WHO should allay fears, yet the scaremongering continues.

Headlines have come from lobby groups and environmentalists, particularly those concerned about the effect of agriculture on the environment.

In New Zealand the focus has been the impact of dairy cows on the Canterbury Plains.

A flurry of headlines mid-2019 has now resulted in funding to investigate whether (or not) nitrate in drinking water is responsible for colorectal cancer.

The justification for the research funding appears to be a study in Denmark that was published in 2018. The implications of the study have been hyped up, and in some cases simply misrepresented.

Contrary to statements made in the media, the Danish researchers stated that they were NOT able to correct for lifestyle and diet factors.

The World Cancer Research Fund International has reported that a diet high in fruit and vegetables (which tend to have high fibre and nitrate) is associated with low risk of colorectal cancer (CRC), whereas a diet high in processed meat, red meat and alcohol, and obesity in general, is associated with "convincing increased risk".

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The Danish diet during the 80s and 90s did not meet the 5+ a day recommendation and the equivalent of the Danish Ministry for Health campaigned actively for improvements. (In fact, for the sake of a good slogan, the Danish Greengrocers campaigned for six a day: "sex om dagen".)

Dr Jacqueline Rowarth. Photo / Supplied
Dr Jacqueline Rowarth. Photo / Supplied

Denmark does have high CRC, ranking sixth in the world for incidence (40 cases per 100,000 people); Australia is 11th (36.9 per 100,000) and New Zealand 14th (35.3 per 100,000). And CRC is the third most common cancer, behind lung and breast cancer, and ahead of prostate cancer.

Although for lung cancer causation is relatively easy to diagnose, with the other cancers it is difficult to prove.

The Danish CRC study involved 2.7 million adults and calculated drinking water nitrate exposure in public water works and private wells between 1978 and 2011. The likelihood of any person drinking from one water source for several decades is slim.

In the late 1990s, research showed a reduced rate of gastric and intestinal cancer in groups with a high vegetable-derived nitrate intake.

The authors suggested that there was evidence that dietary nitrate had a beneficial role and stated that "healthy adults are able to consume large quantities of nitrate in drinking water with little effect".

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More recent research (published in 2007) indicated a vaso-protective effect of vegetables high in nitrate – hence the superfood status of beetroot and kale.

The fact that the Danish dietary intake of nitrate is low by world standards points to ongoing lack of fruit and vegetables in the diet.

Critically, research published in the 80s and 90s indicated that the amount of dietary nitrate reaching the colorectal part of the gut is negligible. Dietary nitrate is mostly excreted in the urine within 48 hours of ingestion; it is not excreted in faeces.

Given the weight of evidence supporting that nitrate in drinking water does not cause cancer, it is disappointing that New Zealand officials have invested in research about this rather than, for instance, investing in screening programmes that have been shown to be effective through early detection of cancer.

Listen to Jamie Mackay's interview with Dr Jacqueline Rowarth on The Country:

The New Zealand research funding appears to be another example of decision made on emotion over evidence; media articles often follow suit. Funds directed at early screening of CRC and at education about the importance of a diet rich in vegetables and fruit would be more effective in reducing incidence.

For those still concerned about nitrate in drinking water, Canterbury-based Dr Graeme Coles, who lived in Denmark during the late 70s, has crunched the numbers from the Danish research:

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Of the 2.7 million people in the study, at least 1133 people would have to be using a particular drinking well exclusively before there was a worse than 50 per cent chance of being diagnosed with CRC. This number would only rise to 1292 people using a "clean" water supply.

And everybody trying to avoid CRC should follow the simple advice from the Ministry:

• Maintain a healthy weight

• Be physically active

• Eat plenty of fruit and vegetables

• Choose wholegrain

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• Choose foods low in salt, sugar and fat

• Drink alcohol only in moderation

- Dr Jacqueline Rowarth CNZM CRSNZ HFNZIAHS has a PhD in Soil Science. The analysis and conclusions above are her own and should not be attributed to any of the organisations with which she is affiliated. She can be contacted on jsrowarth@gmail.com