Ireland began adding fluoride to its public water in 1964 as part of an effort to reduce tooth decay. More than 60 years later, researchers have used that national practice to examine whether lifetime exposure was associated with cognitive decline among older adults.

The findings offered reassurance. Among 2,495 participants in the Irish Longitudinal Study on Ageing, known as TILDA, researchers reported no difference on two cognitive tests between groups with high and low estimated exposure to fluoridated household water.

The practice has roots in observations made at the beginning of the 20th century in Colorado. Dentist Frederick McKay investigated widespread tooth staining and suspected that an element in the water supply might be responsible.

McKay also noticed that affected areas had fewer dental cavities, and that cavities tended to be less severe than in places without the staining. Similar findings were later recorded in the UK, and unusually high levels of naturally occurring fluoride in water were eventually identified as the cause.

Researchers also determined that fluoride protected teeth against cavities. Fluoride in water produces low fluoride levels in saliva, helping reduce the rate at which enamel decays while increasing remineralization during the early stages of cavities.

Governments in countries including Australia, Canada, the UK, and Brazil now add fluoride to water to reduce cavities across their populations. The concentrations used are lower than the levels associated with tooth staining.

“For decades, low-level public water fluoridation has proven to have a dramatic effect on the rates of tooth decay, particularly in children,” UK dentist and dental health writer Ollie Jupes explained to IFLScience.

“Where fluoridation has been introduced, there is invariably a measurable difference in the rates of decay, compared to non-fluoridated areas. Decay rates in fluoridated areas can reduce by up to 25%.”

Although the dental benefits of adding small amounts of fluoride are firmly established, those benefits do not settle broader health questions. Researchers have therefore continued examining whether fluoridated water could have other effects.

Ireland offered an opportunity to study that question because fluoridation began more than six decades ago. That timeline provided a sufficiently aged population for researchers to investigate cognitive decline later in life.

The Trinity College Dublin team examined older adults whose average age was 65. Researchers reviewed where each participant had lived and used national records of fluoride levels to estimate the amount of fluoridated water each person had consumed across a lifetime.

Participants were then assigned to groups ranging from high exposure to low exposure. They completed two cognitive tests used in dementia research, allowing investigators to compare results among the different exposure groups.

No difference emerged between the groups. The result suggested that exposure to fluoridated water was not associated with cognitive decline in the participants studied.

“The present analysis found no evidence that lifetime fluoride exposure from CWF [community water fluoridation] is associated with cognitive function in older Irish adults,” the team wrote.

The researchers added that “no statistical evidence of effect modification by early-life fluoride exposure was found.”

The analysis had limitations. It examined fluoride consumption through household water, although fluoride can also be consumed through foods and drinks including tea, coffee, grapes, and shellfish.

Only around 5% of participants had been exposed to community fluoridation during early childhood. Even with that limitation, the findings provided positive evidence that lifetime exposure through household water did not appear to affect cognition among older adults.

“Fluoridation has been one of the most effective tools we have for reducing tooth decay at a population level, so it matters that we now have long-term Irish evidence that it isn't costing anything in terms of cognitive health,” Dr. Michael Crowe, co-PI of the study, said in a statement.

“Nonetheless, we should continue to check for any adverse effects of water fluoridation in the Irish population on an ongoing basis,” professor Brian O'Connell, another co-author of the research, added.

“This research establishes a way to monitor the health of water consumers but should just be the baseline for future studies.”

The study was published in the Journal of Dental Research.