When I heard that there was a new study showing that tooth decay has worsened amongst Calgary’s children since the city removed fluoride from its water supply in 2011, I hastened to have a look at the paper. That’s what I often do when there’s a new piece of research in the news. But this one offered the priceless bonus of an opportunity to poke fun at Calgary.

The city council there seems to be about half clown at the best of times, and when it comes to child health, it’s worth remembering how the Calgary Flames jumped the queue for H1N1 flu vaccine in 2009, absconding with hundreds of doses from a star-struck clinic and having the paperwork fudged because hockey players are too important for socialized medicine. No, I am not one to resist a cheap shot at Calgary.

One reason I am like this, of course, is that Calgary is a nicer city than Edmonton in several obvious ways. And, of course, the tooth decay study from the journal Community Dentistry and Oral Epidemiology turns out to be a disappointment. Edmonton, which has kept the fluoride in its water, serves as the control in the study. The key finding is not that Calgary’s children have worse teeth than Edmonton’s as a consequence of the removal of the fluoride. It’s that their teeth were significantly better before, and they have now merely fallen to the same level as the less affluent, less educated Edmonton. Sigh.

The study is not quite the hammerblow to Calgary’s fluoridation policy that you might have expected, or been encouraged to expect by the headlines. The authors of the study looked at data on the teeth of a cross-section of grade 2 public-school kids in both cities. There was a “pre” sample of kids observed in the school year 2004-05, and a “post” sample taken in 2013-14, when the Calgarian children would have had up to three years of exposure to unfluoridated water. Hygienists went around to schools and basically counted decayed, filled, and extracted teeth.

The big change was that Calgary children had a lot less damage to their “primary” or baby teeth in ’04-’05. The two cities are now even; both got worse, but Calgary got … more worse. One notices, however, that the Calgary ’04-’05 sample is a lot smaller than the Edmonton one. The surveys were taken by hygienists hired by the old regional health boards, and Calgary’s approach was less aggressive. Only some schools were included in the sample, and the participation rate of the students in those schools was an unimpressive 60 per cent.

In Edmonton the rate was 89 per cent, and all elementary schools were sampled. Might be a problem; might not. It’s a possible hint at bias in the results, and of course there was no way to “blind” the hygienists to whether the kids were in the experimental group (Calgary) or the control (Edmonton).

There are other odd elements to the study. Some of the children in the surveys had gotten permanent teeth, and, for some reason, it was Edmonton’s children who had significantly better permanent teeth in the “pre” surveys. By this measure, Calgary’s kids actually improved marginally, relative to Edmonton, after fluoride was dropped.

When recording the “post”-cessation data, the hygienists were able to check which children were lifelong local residents and drank mostly tap water at home. The hope of the researchers here must have been to zero in on the subset of children for whom fluoridated tap water would have the strongest effect. But that table in the paper is pretty boring, because the cities come out about the same. If fluoride helps, why would this be so?

When you consider all the possible confounders at work in this study, its inferential power does not look overwhelming. This is some of the first careful research of its kind. The basis for the original introduction of fluoride was not too strong by present-day standards, and there are not yet a lot of “natural experiments” using communities that dared get rid of it.

It may be important that the authors found one strong signal — the primary teeth of Calgary children got worse over time, relative to a neighbouring city of similar size and circumstances. But they are cagey with their language, saying things like “our findings were consistent with an adverse effect of fluoridation cessation.” They do not prove that there is such a long-term effect on permanent teeth; they guess (“one might expect”) that it should be so, and it is probably the right way to bet, but it will have to be demonstrated. (They also say the results are “robust to adjustment for” the sizable social and economic differences between the cities, but don’t show their work.)

And meanwhile, why the hell are the primary teeth of children getting worse in both cities, even as sugar consumption trends downward ?

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