University of Otago (Wellington) Department of Public Health researcher Dr Tim Chambers said the formation of cancer-causing chemicals in the gut could be at the heart of understanding how high levels of nitrate in our drinking water contribute to increasing the risk of bowel (colorectal) cancer.
Writing in the latest Australian and New Zealand Journal of Public Health, researchers from the Universities of Otago, Auckland and Massey said the likely process was complicated but gaining an understanding of it was key to assessing the potential risks high levels of nitrate in drinking water posed to human health.
Dr Chambers said nitrate levels in source water had increased as a result of agricultural intensification, particularly the rise in livestock numbers, leading to increased nitrate contamination in drinking water sources.
When nitrate was ingested, he said, the degree to which it was modified in the body was dependent on the levels of nitrate-reducing bacteria, the amount of acid in the stomach, antioxidants such as vitamin C, and compounds called amines, which were derived from proteins such as those in red meat.
An oversupply of nitrate, particularly in the absence of antioxidants, could cause formation of n-nitroso compounds (NOC) — many of which were proven causes of cancer, he said.
Last year, Dr Chambers and fellow researchers estimated nitrate contamination of drinking water could be causing up to 100 cases of bowel cancer in New Zealand each year, resulting in 40 deaths.
That study was the first to assess the levels of nitrate exposure and drinking water quality nationally.
It also found up to 800,000 New Zealanders (17%) could be drinking water containing levels of nitrate which international evidence suggested may put them at increased risk of bowel cancer.
In the peer-reviewed Australian and New Zealand Journal of Public Health article published recently, the researchers said some recent debate had focused on the fact that not all research found a statistically significant link between nitrate levels and colorectal cancer.
However, they noted the combination of positive and negative results from these research studies would be consistent with the highly variable biological processes at work.
They said studies needed to account for other factors affecting bowel cancer risks, such as physical activity levels, obesity and the intake of red and processed meat.
Even though the science was still evolving, the researchers said policymakers needed to consider this evidence for a risk to health from nitrates when setting acceptable levels of nitrates in drinking water and regulating to protect sources of drinking water.
