Nitrate Risks For Babies Need Further Investigation, Liggins Researchers Say

A review of the international scientific literature by the University of Auckland’s Liggins Institute found no conclusive evidence that nitrate in drinking water causes adverse birth outcomes such as preterm birth or birth defects.

However, ongoing monitoring of nitrate in Aotearoa New Zealand’s drinking water is needed, as is regular review of the emerging scientific research on the topic, the research institute says in a report published on its website on 17 November.

The Liggins Institute, New Zealand’s authority on pregnancy and baby health, was asked by the government to review the global evidence on nitrate in drinking water and birth outcomes.

Researchers conducted a meta-analysis, using statistical methods to assess a decade’s worth of relevant scientific papers.

“The current evidence is uncertain and findings from different studies are inconsistent,” said Dr Luling Lin, the postdoctoral fellow who led the research, supported by colleagues including Professors Jane Harding, Caroline Crowther and Frank Bloomfield.

“We need more evidence, and this country should expand the monitoring and reporting of levels of nitrate in drinking water to help to gather that evidence,” said Dr Lin. Maximum acceptable levels of nitrates in drinking water should be reviewed regularly as more evidence becomes available, she said.

New Zealanders are generally exposed to low levels in drinking water, especially if their water comes from reticulated supplies in cities and towns. However, private rural wells can be vulnerable to accumulating nitrate from sources such as fertilisers and animal waste from farming.

Internationally, concerns about the potential for links between nitrate in drinking water and adverse birth outcomes have been triggered by studies showing statistical associations.

No causal connections were established by the Liggins researchers.

The studies under review included research showing a statistical association between nitrate in drinking water and an increased risk of giving birth to a baby with a limb deficiency. (A limb deficiency is when a baby's leg, arm, hand, finger, foot or toe is crooked, uneven, partially formed or even missing because part or all of a child's limb does not completely form during pregnancy.)

However, as with other adverse birth outcomes, including cleft palates, neural tube defects, preterm births, and reduced lengths and weights of babies, the evidence was not strong and any causal link remains uncertain.

Aotearoa New Zealand deems 50 milligrams per litre the maximum acceptable level, which is the same as the World Health Organisation guideline and similar to a US federal maximum, although there are concerns about chronic exposure at lower levels.

According to estimates from a 2020 study, more than 60 percent of our population were exposed to less than 2 milligrams per litre, 8.2 percent to more than 5 milligrams, 2.2 percent to more than 10 milligrams, and only 0.1 percent to more than 25 milligrams.

To help journalists and the public to understand the findings, Dr Lin and her colleagues answer some questions:

Q: How concerned should pregnant women be about nitrate in drinking water?
A: Women drinking water from a registered water supply have no cause to worry based on the evidence available. Only a tiny proportion of birth defects are caused by exposure during pregnancy to chemicals and it’s still uncertain whether nitrate in drinking water could lead to birth defects. Less than 10 percent of total nitrate intake is from drinking water, with most of the remainder coming from the diet, and the nitrate levels in our registered water supplies are low. Women living on farms using unregistered water supplies might consider getting their water tested, since nitrate from fertilisers and animal waste can leach into groundwater.

Q: What potentially preventable risks for birth defects should pregnant women be more concerned about?
A: For most birth defects, the causes are unknown, but likely contributors can include: not getting enough of certain nutrients, for example, folic acid before and during pregnancy; exposure to harmful substances such as alcohol, cigarettes, illicit drugs or certain medicines; and infections during pregnancy, such as rubella.

Q: Why should Aotearoa New Zealand step up nitrate monitoring? 
A: Right now, we don’t do a lot. Routine monitoring is not required if nitrate levels in a water supply have previously been below 25 milligrams per litre, which is half of the maximum acceptable level. In 2019, nitrate monitoring was only required for water supplies servicing 53,900 people or 1.1 percent of the population. In the US, all public water systems are required to be monitored at least annually. Our limited monitoring means it would be very difficult to discover any statistical association between nitrate in drinking water and pregnancy-related problems here.

Q: What’s the difference between a statistical association and causation?
A: An association between two things merely implies that knowing the value of one provides some information about the value of the other. It does not necessarily imply that one causes the other.

Q: How could a causal connection between nitrate in water and adverse birth outcomes be shown in the scientific studies undertaken around the world?
A: In the ideal scenario, this would require a randomised trial, where participants were assigned at random to be in an exposure group or a control group. In this type of trial, other factors such as diet or genetics would tend to even out between the exposed group and the control group, making it easier to spot any potential nitrate effect. In the absence of this type of evidence, more precise evidence of levels of nitrate intakes, and also of other potential contaminants of drinking water, in large numbers of people and how these relate to birth outcomes would provide more robust evidence. For example, water that contains nitrate may also contain other compounds and, if these are not also measured, it is difficult to know if any association is due to nitrate or some other unmeasured substance.

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