Comment: Dr Jacqueline Rowarth wants to reassure people that nitrate in drinking water does not cause colorectal cancer.

Aucklanders can rest assured that drinking water sourced from the Waikato River will not result in any increased risk of suffering from colorectal cancer or any other health issues.

The World Health Organization (WHO) can find no link. Nor can the International Association of Research on Cancer (IARC).

Each time an alarm is raised about nitrate in drinking water, scientists try and point out the reality – that drinking water provides a very small proportion of nitrate consumed in a day, that green vegetables and beetroot contain far more nitrate than is found in water, and that intake of dietary fibre (found in whole grains, pulses, fruit and vegetables) is linked to reduction in colorectal cancer - yet the alarms continue.


Reassurance should be in the fact that WHO reviewed the available literature in 2017 and reported no clear association between nitrate or nitrite in drinking water and risk of cancer of the gastrointestinal tract, non-Hodgkin lymphoma, tumours of the central nervous system, urinary tract tumours, thyroid cancer, breast cancer or pancreatic cancer.

IARC came to the same conclusion in 2010, stating that "there is inadequate evidence in humans for the carcinogenicity of nitrate per se from exposure in food or in drinking-water".

Cancer Therapy Advisor's fact sheet states that nitrate in drinking water is not associated with risk of colon or rectal cancers. The fact sheet also indicated that there is no consistent association between high intakes of nitrate or nitrite and other types of cancers.

These reports from august bodies are all focused on health and should calm concerns, yet the scaremongering continues.

One piece of work published in 2018 and referred to as the Danish Study, is usually cited as "emerging evidence".

The implication is clearly that because it is "emerging" the precautionary principle should be employed while further research is done.

A different explanation is that a piece of research published in 2018 gained no traction globally.

This is because the results of the study showed no dose related response – that is, incidence of colorectal cancer did not increase with exposure to nitrate in drinking water.


In addition, the results were confused with lifestyle factors such as diet, weight and exercise, all of which have been shown to be implicated in incidence of colorectal cancer.

The World Cancer Research Fund International (WCRFI) has helpful material on its website for staying healthy and avoiding all sorts of cancers.

The message is simple:

• Be as lean as possible within the normal range of body weight.

• Be physically active as part of everyday life.

• Limit consumption of energy-dense foods. Avoid sugary drinks.


• Eat mostly foods of plant origin.

• Limit intake of red meat and avoid processed meat.

• Limit alcoholic drinks.

• Limit consumption of salt. Avoid mouldy grains and pulses.

• Aim to meet nutritional needs through diet alone (avoid supplements).

The message boils down to "eat sensibly, manage weight and keep fit".


Following this advice would also have beneficial effects in reducing other health issues connected with obesity and inactivity.

Promoting healthy lifestyles with sensible food and exercise, as the government did during the Covid-19 lockdown, would seem to be far more sensible than creating angst about unproven research.

Of course keeping hydrated is part of sensible exercise, and water is now more available in Auckland than it was because of the increased intake from the Waikato River at Tuakau.

The nitrate concentration at the Tuakau bridge is approximately 5 per cent of the WHO health limit for drinking water.

- Dr Jacqueline Rowarth is an agricultural scientist and a farmer-elected director on the Boards of DairyNZ and Ravensdown. The analysis and conclusions above are her own.

The research:


The Danish Cancer Society published a report in the same year as the drinking water study indicating that a modest reduction in daily consumption of approximately 25g in both red meat and processed meat could result in a 9.1 per cent reduction in colorectal cancer.

The Danish equivalent of the Ministry of Health has been urging reduced consumption of meat and more consumption of fibre, fruit and vegetables for several decades.

Consumption of red meat in the 2018 study was estimated at approximately 104g per day, and processed meat (sausages, bacon, salami) at approximately 58g per day for males. (Note pork and chicken consumption are not included). Eliminating red and processed meat was associated with a 19 per cent reduction in colorectal cancer.

Further research from Sweden indicated a reduction of almost 10 per cent in all cancers if obesity and overweight was eliminated in Nordic countries, with the greatest absolute reduction observed in colon cancer. (Only 20 per cent of the Danish population is considered obese, in comparison with over 30 per cent of New Zealand's population.)

Colon cancer is also related to lack of physical activity. A reduction by 19-21 per cent is associated with increased exercise to ≥8,000 Metabolic Equivalents of Task-minutes/week (approximately 12 hours of running or 17 hours of bike riding on the flat… or 27 hours of brisk walking depending on body mass and speed).

A 6 per cent reduction was associated with 600 Metabolic equivalents of task minutes/week, which is approximately 150 minutes/week of brisk walking or 75 minutes/week of running.