A new study has suggested it is highly unlikely that nitrates in New Zealand’s drinking water present an increased risk of getting bowel cancer.
The study by the Institute of Environmental Science and Research (ESR) assessed the levels of nitrates New Zealanders are consuming from both water and food.
It found that on average, the nitrates New Zealanders take in from food and water add up to about one-quarter of the internationally agreed acceptable daily limit.
This study comes after a Danish research study in 2018 reported an association between the level of nitrate in drinking water and the occurrence of bowel cancer and a subsequent NZ study also concluded it was a risk factor.
The NZ Food Safety Science & Research Centre commissioned its partner the ESR to carry out the study.
The Ministry of Business, Innovation and Employment (MBIE) and Fonterra co-funded the research, which was undertaken by the ESR’s Peter Cressey and Dr Belinda Cridge.
Cressey outlined the study in an online webinar.
Its conclusion of a low cancer risk was calculated using data from the Ministry of Health’s national nutrition survey of 8000 New Zealanders to calculate the mean exposure of nitrates in food and water.
Using that data, it broke down where the nitrate exposure was coming from in food and water among adults and children using a dataset of nitrate and water values and randomly assigned those values to those surveyed people.
That data set had values ranging from 0 -130mg/l of nitrate. This covered the range of nitrate levels seen in NZ.
It found that the major source of nitrates consumed by these New Zealanders came from lettuce and potatoes.
Just under 50% of nitrates also came from other foods, which are a variety of different fruits and vegetables and processed meat.
Drinking water contributed 9.1% of nitrate consumption. Of that, 1.9% and 2.6% came from drinking water consumed with and without food respectively, coffee and tea made up 1.6% and 1.9% and the balance came from fruit juice and other beverages.
Among children, 42% of nitrates consumed came from potatoes, 13.9% came from lettuce, 36.6% came from other foods and 7.5% came from drinking water.
Of that 7.5%, 3.7% came from water consumed with food, 1.8% came from fruit juices and the balance came from other beverages, tea, coffee and water consumed without a meal.
“The exposure analysis shows that a low overall percentage of nitrates come from drinking water consumption and a very small proportion is from consumption of drinking water as a beverage,” the study said.
Cressey says it was important to look at the nitrate intake from both food and water, as less than 10% of nitrates consumed came from water – the rest is from food, especially green vegetables, which contain antioxidants that cancel out nitrates.
Tea, coffee and fruit drinks, for example, are rich in these antioxidants.
The link between nitrates and cancer comes from the human gut’s reaction to nitrites which form nitrosamines. Some of these nitrosamine compounds in the gut can mutate and while they can generally be absorbed by the small intestine, he says they also form the crux of the link between nitrates and cancer.
“Even if they’re formed down there, we’re still not sure how they’ll make their way to the colon to initiate cancer,” Cressey said.
It is also why the study treated nitrates in food and water as separate entities. Nitrates in food come with all of the other nutrients and compounds when it is consumed, whereas nitrates in water do not.
“There’s so much of the nitrates from drinking water that is in a food-like form associated with antioxidants or is being consumed at the same time as the food and will hit the digestive system at the same time,” he said.
However, Dr Tim Chambers, a senior research fellow from Otago University’s department of public health, criticised the report.
He says it would have been more informative to have undertaken a detailed analysis of the 800,000 New Zealanders who are potentially at risk from high nitrate levels in drinking water, as determined in a study completed by Victoria and Otago universities earlier this year.
“The authors also assume that any nitrate from water consumed with a meal, or as another type of beverage poses zero risk to human health, regardless of the potential imbalance between nitrate and antioxidants,” Chambers said.
“To my knowledge, there is very limited research investigating the level of antioxidants required to nullify the effect of any given level of nitrate in drinking water on endogenous nitrosation.
“It is great to see additional attention dedicated to this important health issue. However, the research aims and analyses within this report were never designed to support the definitive conclusions used in public statements.”
