Fluoride occurs naturally in drinking water, especially well water, but the concentrations are generally low in public water supplies. In some countries, such as the US, Canada, UK, Australia and Ireland, fluoride is commonly added to the public water supply at around 0.7mg per litre to prevent tooth decay. The World Health Organization guideline for fluoride in drinking water is 1.5mg per litre .
Author
- Maria Kippler
Associate Professor, Institute of Environmental Medicine, Karolinska Institutet
Given the concern that fluoride in drinking water might affect children's intelligence, the addition of this mineral to drinking water has become controversial. Consensus among researchers about the precise nature of the link between fluoridation and intelligence is lacking and the existing evidence is widely debated .
The US National Toxicology Program's, part of the Department of Health and Human Services, most recent evaluation states with moderate confidence that higher fluoride exposure (above the World Health Organization guideline) is consistently associated with decreased child intelligence, while they conclude that more research is needed to understand the effects at lower fluoride exposure levels.
A new study my colleagues and I conducted found that relatively low exposure to fluoride during the foetal stage (as a result of the mother's exposure to fluoride) or in the child's early years may affect their intelligence.
For the study, which was published in Environmental Health Perspectives, we followed 500 mothers and their children in rural Bangladesh, where fluoride occurs naturally in the drinking water, to investigate the link between early life exposure to fluoride and children's intelligence.
Psychologists evaluated the children's cognitive abilities at five and ten years of age, using standard IQ tests. The exposure to fluoride in the mothers during pregnancy and children at five and ten years of age was determined by measuring the concentrations in urine samples. Urine samples reflect the continuing exposure from all sources, such as drinking water, food and dental products (such as toothpaste and mouthwash). Urine samples are the most accurate way of determining fluoride exposure in people.
Increasing urinary concentrations of fluoride in pregnant women were linked to decreasing intelligence in their children at five and ten. Even the lowest fluoride concentrations were associated with decreases in the children's cognition. The average maternal urinary fluoride concentration was 0.63mg per litre, with the vast majority of concentrations falling between 0.26 and 1.4mg per litre.
The children's average urinary fluoride concentrations at five and ten years of age (0.62 and 0.66mg per litre, respectively) were similar to those of their mothers during pregnancy.
Among children who had more than 0.72mg per litre of fluoride in their urine by age ten, increasing urinary fluoride concentrations were associated with lower intelligence. In children with less fluoride in their urine, there were no consistent associations with their intelligence. So childhood exposure seemed to be less detrimental than the exposure during early foetal development.
Out of the cognitive abilities measured, associations of both maternal and child urinary fluoride concentrations were most pronounced with nonverbal reasoning and verbal abilities. There were no consistent differences between boys and girls.
We didn't find a link between fluoride concentrations in the urine of the five-year-olds and their intelligence. This could be due to the shorter exposure time or that urinary fluoride concentrations aren't as reliable in younger children owing to greater variations in how much fluoride is taken up and stored in the body, particularly in the bones.
As well as the children's urinary fluoride concentration, the fluoride concentrations in drinking water were measured at the age of ten for a random subset of the studied children. The average was 0.20mg per litre, which is well below the WHO guideline value for fluoride in drinking water.
The concentrations in drinking water tracked with the concentrations in urine, confirming that water is a main source of exposure. Still, we couldn't exclude the possibility that there were contributions from other sources. Fluoride in toothpaste is important for preventing tooth decay, but it's important to encourage small children not to swallow the toothpaste during brushing.
Limitations
A limitation of our study is that we measured fluoride only in one urine sample at each time point. As a large fraction of the absorbed fluoride is excreted in some hours, one measurement may give uncertain levels for the individual. However, as the exposure largely comes from water it can be assumed that the intake is rather constant over time.
Another limitation is that the intelligence tests that were used have not been standardised for the Bangladeshi population. As a result, we did not convert the results to IQ scores (with an average of 100) that can be compared across populations.
Our findings support previous well-designed studies from Canada and Mexico , where exposure levels obtained below the existing WHO guideline for fluoride in drinking water were associated with impaired cognitive development.
Similar findings were recently provided when combining multiple studies from several countries . It was noted that at low exposure levels, findings with cognitive development were more conclusive among studies estimating fluoride exposure via urine than among studies that relied on concentrations in drinking water only. This highlights that imprecise estimation of the exposure can lead to difficulties in assessing the true impact on cognitive development.
Taken together, the concern about the effect of fluoride on children's intelligence at low exposure levels is further strengthened by our study. In particular exposure during foetal development, but also prolonged childhood exposure seems to be of concern.
Still, as this is an observational study, no firm conclusions can be drawn about causalities. There is still a need for more well-designed research studies on low-level fluoride exposure and cognitive development, in combination with experimental studies to determine the possible molecular mechanisms driving it. Collectively, this will create a robust basis for reviewing fluoride health risks and thresholds for drinking water, foods, and dental care products, especially for children.
Maria Kippler receives funding from Swedish Research Council and the Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning.
