We know about many of the things that can affect our health in negative ways. Smoking increases our risk of lung cancer, for example. Most of us, though, wouldn't have thought the water we drink – which we think of as health promoting - might be one of the baddies.
But that was the alarming out-take from news last week that nitrates in drinking water could be contributing to New Zealand's very high rate of bowel cancer.
Research in Denmark last year, published in the International Journal of Cancer, reported that nitrate in drinking water, even at low levels, was associated with an increased risk for colorectal cancer. The levels that increased cancer risk were much lower than the levels generally accepted to be safe, including the limits we have in New Zealand.
Local scientists, Professor of Public Health Michael Baker at Otago University and senior researcher Mike Joy at Victoria University, have been looking at the local picture, and have raised the alarming point that some water supplies in New Zealand, particularly in rural areas, have nitrate levels above the risky level in the Danish study. Nitrates in drinking water come mostly from fertiliser and animal urine.
Baker and Joy say we currently don't have an accurate picture of nitrate levels across the country – the data has not been collected – so we don't really know how much of a risk nitrate in drinking water poses for Kiwis, or how much of an effect it might be having on our bowel cancer levels. They're calling for measurement of this and a central database to be established as a priority.
In the meantime, what should we do? Stop drinking water? Buy bottled water? Buy a water filter?
Baker told RNZ switching to bottled water would be a "terrible mistake". He said most drinking water in NZ "probably has relatively low levels. Nitrate is a risk factor, but there are many other ways to reduce cancer risk".
That's a useful point. What we know about bowel cancer is that it's a highly preventable disease. Risk factors include obesity, alcohol, physical inactivity, smoking and red and processed meat.
That last one is interesting because processed meat – bacon, ham, salami and smoked meats – contains nitrates, and its association with cancer risk has been known for some time. The advice from the World Cancer Research Fund is to eat "little, if any" processed meat.
But as with all risk factors, this needs to be looked at in context. There's not much point giving up bacon if you're an overweight, sedentary drinker. The same goes for buying a fancy water filter (nitrates can't be removed by regular filtration).
We come back to the big picture once again: water is a healthful beverage; it should be our first choice to drink. Until we know more about our levels of exposure to nitrates in water, there's no reason to give it up. But there is (still) every reason to eat well, drink less alcohol and move our bodies more.
• Niki Bezzant is editor-at-large for Healthy Food Guide