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However, they would not be an explanation in themselves for the region’s high rates of bowel and colorectal cancer, University of Otago Wellington researcher Tim Chambers cautioned.
‘‘It could potentially be exacerbating the effect of other risk factors and it could be contributing a small part by itself, but it could in no way by itself be an explanation for the huge disparity [in bowel cancer figures] in the Southern District Health Board.’’
In Otago and Southland, on an age-adjusted per capita basis, bowel cancer rates are the highest in the country.
Nitrate levels in the region’s water, especially in private water supplies, have long been suspected as a factor contributing to those statistics.
Dr Chambers and several other academics, including epidemiologist Michael Baker, have just published an article in the journal Environmental Research in which the authors suggest nitrate contamination of drinking water could be responsible for about 100 cases of bowel cancer nationally each year, leading to 40 deaths.
‘‘Around 3.7% of all southern colorectal cancer or bowel cancer cases could be attributable to nitrate,’’ Dr Chambers said.
‘‘The 2013 figure we have is 1354 cases in the Southern DHB and that converts to around 13 per year.’’
The article accepts there is still debate about whether there is a threshold effect of nitrate exposure, and also that there are other acknowledged nitrate risk factors such as high red meat consumption, smoking, drinking, poor fitness and eating too much processed meat.
However, examination of council-provided drinking water and samples from 119 private water supplies suggested there could be a connection between nitrate levels and cancer.
‘‘It could be one factor in a complex range of factors,’’ Dr Chambers said.
‘‘The rates of bowel cancer in the Southern DHB compared to the rest of the country really are quite huge ... and nitrates can accelerate the rate of bowel cancer if a person has other risk factors as well.
Nitrate levels were ‘‘a very small part of the picture’’.
However, there was enough evidence in their study to suggest there was something to be concerned about, Dr Chambers said.
Queenstown-based water engineer Jayne Richards contributed to the paper and her work had identified high nitrate rates in Canterbury and Southland water.
The researchers said New Zealand’s rate of drinking water nitrate was lower than many European countries and also the United States.
However, New Zealand’s rate was increasing, probably because of the scaling up of dairy farming in recent years.
Although nitrate levels remained relatively low, their variability meant drinking water standards needed to be urgently reviewed.