A study of tooth decay in Alberta, Canada, has confirmed that adding fluoride to water supplies reduces tooth cavities. The findings are consistent with almost all previous research, but recent changes in local policy made possible an unusually reliable study.

Rocks in certain parts of the world contain fluoride that enters the water supply as they erode. In the 1930s dentist Frederick McKay observed that people who drank water with approximately 1 milligram of fluoride per liter had healthier teeth than people from regions with less natural fluoride. This inspired experiments in adding fluoride to town water supplies, which we now know helps to strengthen teeth.

The reduction in dental cavities, or caries, as a result of this program inspired many other municipalities in north America and around the world to fluoridate their water supplies as well. However, these programs faced considerable resistance which, after dropping away for several decades, has been rising again in recent years, possibly thanks to misinformation spread on the internet.

In May 2011 Calgary became the largest city in the world to cease fluoridation. A team led by Dr Lindsay McLaren of the University of Calgary saw the a perfect opportunity to update research in the area. Not only could they compare tooth decay before and after fluoridation was abandoned, but they had a natural control in the demographically similar city of Edmonton, which chose to continue its fluoridation program.

McLaren's findings, published in Community Dentistry and Oral Epidemiology are unambiguous. Dental caries per child increased for primary (milk) teeth in both cities, possibly indicating worsening diets. However, the increase was almost twice as large in Calgary, 3.8 extra caries to Edmonton's 2.1.

The research was done on Grade 2 students. There was also an increase in decay of the Calgary children's permanent teeth, but it was not statistically significant. No such increase was seen in Edmonton.

“Trends observed for primary teeth were consistent with an adverse effect of fluoridation cessation on children's tooth decay, 2.5-3 years post-cessation,” the paper concludes. “Trends for permanent teeth hinted at early indication of an adverse effect. It is important that future data collection efforts in the two cities be undertaken, to permit continued monitoring of these trends.”

^{If you're a dentist and you like the view, consider moving. You'll have plenty of work. JeniFoto/Shutterstock}

The effects Lindsay measured are anticipated to increase, since future individuals will not have had the same early exposure to fluoride as the 2014 cohort. Calgary's water supply naturally contains up to a third of the recommended dosage, so another city without this background level might have suffered more.

The findings are unsurprising, considering the large number of previous studies finding a benefit fluoridation. However, as the authors noted, “Systematic reviews have highlighted methodological limitations of the evidence base, such as weak study designs.”

The study did not investigate the harm opponents allege fluoridation causes. All available evidence indicates that fluoridation at the 0.5-1.0 milligram per liter concentrations recommended by the World Health Organization have only minor negative effects, primarily tooth discoloration, but the Calgary/Edmonton comparison provides an ideal opportunity for further research.

One of the Calgary councilors who voted to scrap fluoridation has indicated McLaren's work has caused him to rethink his position, while others have remained firm.