Nitrate concentrations in drinking-water even below the current regulatory limits for household tap water are associated with an increased risk of fetal growth restriction. A researcher says that these limits may need to be reconsidered.
Almost all countries have established limits for the concentration of nitrate in drinking-water, but new research shows that even lower concentrations are associated with an increased risk of small- for-gestational-age.
Previous studies also found that nitrate concentrations in drinking water are associated with increased risk of preterm birth or fetal growth restriction.
According to a researcher behind the study, more studies are needed to confirm any association, but if it is confirmed, the public health authorities should examine whether the limit should be reduced.
“If our results represent a real association, which we can only confirm through more studies, they imply that reducing the limit for nitrate in drinking-water can improve global public health. Harmful birth outcomes, such as small-for-gestational-age, can potentially increase the risk of several diseases later in life,” explains Anja Søndergaard Jensen, a researcher at the National Centre for Register-based Research, Aarhus University.
The research has been published in Environment International.
Data from more than 1 million children in Denmark
Limits for nitrate in drinking-water were originally established because fetuses can develop methaemoglobinaemia, also called blue-baby syndrome, which reduces the transport of oxygen throughout the body.
The European Union has therefore set a limit of 50 mg of nitrate per litre and the United States has a limit of 44 mg/litre.
In the study, the researchers investigated the association between nitrate concentrations in drinking-water and a harmful birth outcome, specifically small-for-gestational-age, taking into account the child’s sex and the length of pregnancy. The small-for-gestational-age measure is defined as newborns with a birthweight in the lowest 10%.
The researchers used data for 1,078,892 children in Denmark, both data from birth and data on the nitrate concentration in the drinking-water in the area where the mother lived.
The researchers also considered the child’s sex, the calendar year, birth order, the mother’s age, education and income level, her employment status and whether she was registered as smoking during pregnancy.
The researchers examined nitrate concentrations above and below the European Union limit of 50 mg/litre.
More nitrate associated with lower birthweight
The results show that the nitrate concentration in the drinking-water where the mother lived was associated with the child’s birthweight.
The children born to mothers exposed to nitrate concentrations of 2–5 mg/litre had a 4% increased risk of small-for-gestational-age compared with those born to mothers exposed to less than 2 mg/litre.
This also applied at higher nitrate concentrations, but the association disappeared at the highest nitrate concentration.
“We were surprised to find a lower estimated risk at the very highest nitrate concentration, but other studies have also found this,” says Anja Søndergaard Jensen.
Reduces oxygen transport
According to Anja Søndergaard Jensen, the possible association between nitrate concentrations in drinking-water and birthweight may result from the digestive system converting nitrate to nitrite, which through oxidation converts haemoglobin to methaemoglobin.
Haemoglobin transports oxygen but methaemoglobin cannot, and thus excessive nitrate can reduce the blood’s ability to transport oxygen.
This is especially problematic for infants because, unlike adults, they have little methaemoglobin reductase, an enzyme that converts methaemoglobin back into haemoglobin.
“Previous research has associated a lack of haemoglobin in the blood with pregnancy complications and an increased risk of low birthweight,” explains Anja Søndergaard Jensen.
Implications for public health
Anja Søndergaard Jensen thinks that if the researchers have found a real association, it could affect public health globally, which can be improved by reducing the limit for nitrate in drinking water.
This is primarily because babies born small-for-gestational-age may have an increased risk of ischaemic heart disease, hypertension and stroke later in life.
Denmark mostly has low concentrations of nitrate in drinking-water, so most people are not exposed to excessive nitrate.
However, the researchers found a slight increase in the risk of small for gestational age of 2% for each 10-fold increase in the concentration of nitrate in drinking water.
“Two percent is not much for each individual, but this may be relevant at the population level. Since nitrate is present in drinking-water worldwide, and since nitrate in drinking-water is not monitored and regulated everywhere, improving the global regulation of nitrate concentrations can improve global public health,” explains Anja Søndergaard Jensen.
She says that nitrate is present naturally in the environment, but the main source of elevated nitrate concentration in groundwater is runoff from nitrogen fertiliser used in agriculture.