Claims linking dairy farming to carcinogenic pollution of water supplies need to be closely examined, writes Dr Jacqueline Rowarth.
Dr Jacqueline Rowarth, assistant professor at the University of Lincoln, is a farmer-elected trustee of DairyNZ and Ravensdown. The analysis and conclusions are his own.
OPINION: Prime Minister Jacinda Ardern made dispelling falsehoods a key theme of her recent Harvard graduation speech. “In the age of disinformation, we must learn to analyze and critique information.”
Even as the Prime Minister made his statements, New Zealand was inundated with yet more claims that I would describe as misinformation – about nitrate in drinking water, cows and cancer.
Despite statements from New Zealand medical professionals and Bowel Cancer NZ, alarmist headlines continue to emerge.
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* 11,000 liters of water to make one liter of milk? New questions about the freshwater impact of New Zealand dairy farming
* Impact of dairy farming on ‘unsustainable’ Canterbury water quality
* Majority of Auckland’s rivers have high levels of E coli, report says
* Tasman council takes action on nitrates at Te Waikoropupū Springs
Ten minutes into Google Scholar will reveal articles that show nitrate in drinking water cannot cause colorectal cancer; neither it nor the nitrogen it contains reaches the large intestine. The World Health Organization (WHO) oversees research and did not find any relationship.
ESR research in New Zealand has shown that less than 10% of nitrates consumed are in water – the rest comes from food. So-called super foods such as kale and beetroot are high in nitrates and their consumption has been associated with decreased cognitive decline, improved athletic performance and better heart health.
Nitrate from food and drinking water is readily absorbed in the upper gastrointestinal tract and distributed throughout the human body. When it reaches the salivary glands it is actively transported from blood to saliva and levels can be up to 20 times higher than in plasma. High salivary nitrate is associated with a significant decrease in systolic blood pressure (the top line), indicating that the body views nitrate as important – it recycles and concentrates it.
Another potential beneficial effect of nitrate is protection against bacterial infections via its reduction to nitrite by enteric bacteria.
Concerns about links between higher nitrate levels in drinking water and an increased risk of premature births (less than 37 weeks gestation) may also be countered by data from New Zealand looking at the broader factors influencing preterm birth.
According The health status of children and young people in New Zealand report in 2015, these factors included maternal age, with significantly higher rates for infants born to mothers under 20 and to mothers over 35.
Interventions aimed at reducing smoking and intimate partner violence, improving access to family planning to reduce closely spaced pregnancies, and providing support to socially disadvantaged women have been suggested to help reduce preterm birth rates.
The research that sounded the alarm abroad did not take into account ethnicity, which has also been shown to affect premature births.
Another concern raised was blue baby syndrome (methemoglobinemia), with mothers of infants being urged to consider water supply and nitrate contamination.
However, a search of the literature found no incidence of methemoglobinemia at nitrate concentrations below 45 mg/L nitrate nitrogen in drinking water for bottle-fed infants under 6 months of age. The the researchers said “most studies have not considered bacterial contamination” in the incidence of methemoglobinemia.
This lack of evidence of a problem and evidence of benefit should reassure anyone living on the Canterbury Plains where campaigners have centered their campaign to try to eradicate dairy cows.
The problem for activists is that nitrate has many sources. Canterbury-based nutrition scientist Dr Graeme Coles has calculated that rural residents of Selwyn inject more than 180 tonnes of nitrogen a year into groundwater via septic tanks.
He also calculated that the crop adds an additional 1166 tons per year. The contribution of gorse and broom has been identified as important (amounting to 2-3 times that from dry and dairy stocks per hectare in the Ruamahanga catchment in the Wairarapa) and underestimated. And the contribution of inherited organic matter turnover is almost always overlooked – but an average annual rate of nitrogen mineralization of around 170 kg per hectare measured on the Canterbury Plains indicates that it can be significant.
These sources explain why some boreholes in Canterbury have been rich in nitrates for decades. The data can be found on the Environment Canterbury website. A borehole at Halswell Junction Rd had a nitrate-nitrogen concentration of 0.4 in 1961, 4.0 in 1977 and 3.7 in 1990. A borehole at Waterholes Rd, Templeton had a nitrate-nitrogen concentration of 8, 2 in 1977 and 8.5 in 2014. Cows have increased on the plains since the 1990s. Increased human activity began before that.
At the time of the media frenzy over colorectal cancer, University of Otago colorectal surgery professor Frank Frizzelle cautioned against overinterpreting research, “especially in the environment in which we find ourselves. let’s find out where we have a big anti-dairy lobby and a clean water lobby who want to throw everything they can at the fire to say it’s causing all this damage”.
Science should be used to put out the flames. Analyzing and critiquing information is the start.