Nitrate contamination in New Zealand's drinking water could cause up to 100 cases of bowel cancer each year and lead to around 40 deaths, new research has estimated.
The collaborative research, conducted by the Universities of Otago, Loughborough, Auckland and Victoria, found 800,000 people are exposed to nitrate via New Zealand's drinking water - in concentrations above the levels identified as posing a risk.
The estimate is based on international studies linking nitrate in drinking water to the disease, combined with an assessment of the levels of nitrate in New Zealand's drinking water. The study is the first to assess the levels of nitrate exposure in New Zealand and collate a national dataset of drinking water quality.
Dr Tim Chambers, a senior research fellow in the Department of Public Health at the University of Otago, Wellington, says the study found a substantial number of New Zealanders are drinking from water supplies contaminated with nitrate, which could increase their risk of bowel cancer - the second highest cause of cancer-related death in New Zealand.
"This research highlights the potential health burden of nitrate contamination, particularly if further evidence on the link between nitrate and bowel cancer reinforces existing studies. The results support the need to take a precautionary approach towards nitrate contamination in New Zealand," Dr Chambers said on Monday.
However, the research did not directly assess the link between nitrate exposure and bowel cancer as some overseas epidemiological studies have done, he noted.
The study, published in the international journal Environmental Research, is a preliminary step to assessing the potential public health impacts of water quality in New Zealand. It follows the 2016 campylobacter outbreak in Havelock North, which caused an estimated 8320 people to fall ill, 42 hospitalisations and four deaths.
Campylobacter infection is a type of gastroenteritis, or food poisoning, that is mainly spread to humans by eating undercooked, contaminated meat. It is the most common cause of foodborne illness in New Zealand. People and animals can carry and spread the infection, even if they don't have symptoms.
"The results also highlight the need for the Three Waters Reform in New Zealand. Our recent Official Information Act requests to district councils found that some councils have not tested for key contaminants in drinking water for years as the current legislation does not require them to do so," Dr Chambers said.
The researchers collated data from all 67 district and regional councils, plus a sample of private water supplies, to generate a national nitrate exposure dataset. The researchers then applied the estimated risk of bowel cancer from a meta-analysis of eight studies investigating the link between nitrate in drinking water and the disease.
"This work on bowel cancer also sits alongside a wider set of evidence linking nitrate in drinking water to preterm births, low birthweight and congenital abnormalities which are a growing concern. Other environmental research also shows the negative impact of nitrate contamination on freshwater ecosystems and the wider environment," Dr Chambers said.
"While more research is required in this area, particularly around the biological mechanism and additional epidemiological studies, the potential health and environmental impacts of nitrate contamination provide a compelling case for intervention."
Nitrate contamination in New Zealand is mainly caused by nitrate leaching from urine patches used for pastoral farming, specifically from intensive dairy farming. Nitrate leaching has increased substantially since 1990 and many groundwater sites continue to degrade.
The likely biological mechanism for nitrate increasing the risk of bowel cancer is complicated, the research says, but it is influenced by dietary factors - particularly the balance of red meat, fruit and vegetables, and gut microflora. The interplay of these factors determines the amount of N-nitroso compounds, a class of cancer-causing chemicals, that are formed in the gut - a process known as endogenous nitrosation.
The International Agency for Research on Cancer concluded that "ingested nitrate or nitrite under conditions that result in endogenous nitrosation is probably carcinogenic to humans (group 2A)".
New Zealand experiences one of the highest rates of bowel cancer in the world (35.3 age-standardised cases per 100,000), with an average of 3000 cases per year. It is our second biggest cancer killer, causing an average of 1200 deaths each year. Obesity, alcohol consumption, physical inactivity, smoking, and consumption of red and processed meat can put people at risk of developing the disease.
