The differences between the two graphs don’t leap out at the viewer. Nonfluoridated nations such as Belgium, Luxembourg, and Denmark actually have better dental health by this measure than the United States, one of the world’s fluoridation champions. Finland, Germany, Japan, the Netherlands, Sweden, and Switzerland tried fluoridation, abandoned it years later—and saw no rise in tooth decay. What’s going on?

One of the lesser-known advantages of government-run health-care systems, such as Britain’s National Health Service, is the fact that because taxpayers are funding everything, the government occasionally tries to determine whether the money is being spent usefully. In 1999, the government asked the NHS to “carry out an up-to-date expert scientific review of fluoride and health.” A research team based at the University of York evaluated every study of fluoridation it could find—about 3,200 of them. The team’s conclusion was, it said, “surprising.” Despite the long fight over fluoridation, few of the thousands of studies counted as “high-quality research.” The implication was that Britain had been tinkering with its water supply with little empirical support. Trevor Sheldon, the head of the York review’s advisory board, was blunt: “There’s really hardly any evidence” that fluoridation works, he told Newsweek. “And if anything there may be some evidence the other way.” These findings were respectfully ignored.

In 2015, the Cochrane organization waded into the debate. Founded in 1993, Cochrane is a London-based global network of about 30,000 medical researchers in multiple countries that provides systematic analyses of medical issues. The goal is to produce painstaking, rigorous assessments of what research has—and hasn’t—established about a given subject. Cochrane has a fiercely guarded reputation for impartiality and thoroughness. Its verdicts have global impact. Which may be why the pushback on its fluoridation work was so strong.

To evaluate the efficacy of water fluoridation, the Cochrane researchers wanted to select properly conducted scientific research, discarding studies that were badly designed (too few participants to produce sound data, for example) or incompetently executed (for instance, the researchers didn’t follow their own protocols). To evaluate the studies, the team used two simple but strict criteria: They needed to have two large groups of subjects, one with fluoride (the intervention group) and one without (the control group), and each group had to be examined at least two times. Moreover, the studies needed to be prospective (meaning the scientists announced beforehand what they were looking for, then measured it) as opposed to retrospective (meaning the scientists sifted through historical data looking for patterns). Scrutinizing medical databases, the Cochrane team found 4,677 fluoridation studies. All but 155 of them—20 that focused on tooth decay, and 135 that focused on dental fluorosis—failed to meet the two criteria. Worse, all of the tooth-decay studies and all but a handful of the fluorosis studies were, in the jargon, “at high risk of bias”—for example, variables such as age and income hadn’t been properly taken into account.

The Grand Rapids study is an example of these problems. Not only was it cut short when the control city, Muskegon, started fluoridating its water, but the experimenters had not established whether the two populations had similar incomes or ethnic backgrounds. Nor did the researchers evaluate people’s teeth blindly, by taking X-rays to be examined by technicians who did not know which group a patient belonged to. Instead the study dentists simply looked into patients’ mouths and subjectively reported what they saw—a recipe for what is called “confirmation bias,” in which people tend to interpret what they see in ways that reinforce their prior beliefs.

The Grand Rapids researchers cannot be much faulted for these lapses, according to the Cochrane spokesperson Anne-Marie Glenny, a researcher at the University of Manchester School of Dentistry. In the late ’40s and early ’50s, the proper procedures for clinical trials were just being established. Few scientists understood how small imbalances between the intervention and control groups could compromise an entire trial. And the researchers definitely cannot be blamed for the unhappy fact that their experiment—indeed, all of the original fluoride research—occurred before the introduction of Crest, the first fluoride toothpaste, in 1956. Today, given that almost all toothpaste contains fluoride, and that most people brush their teeth, assessing the impact of fluoridated water remains highly problematic.

“It’s a really difficult area to evaluate,” Glenny told me. “You can’t really do the ideal experimental study,” because it is next to impossible to assemble two large, similar groups of people, one of which is not drinking fluoridated water or brushing their teeth. On top of that, “measuring the confounders—sugar consumption, socioeconomic status, and so on—is really tricky.” How much, I asked, of the improved dental health of the ’60s and ’70s was due to water fluoridation? How much was due to the soaring popularity of fluoride toothpaste and mouthwash? And how much was due to rising affluence, which generally translates into more visits to the dentist? “I’m not sure you can answer that question,” Glenny said.

The Cochrane group reported its work carefully. The evidence, it said, is poor and sparse, but what little there is “indicates” that the fluoridation of water reduces cavities in children. But, the group said, “these results are based predominantly on old studies”—from before 1975—“and may not be applicable today.” For adults, there is “insufficient evidence,” old or new, to determine whether fluoridation is effective. The report did not support or attack fluoridation; it only asked for more research.

Nonetheless, it set off an uproar. A blog post on the Cochrane website attracted so many vitriolic comments from anti-fluoridation zealots that the organization eventually removed it. When a writer for Harvard Public Health magazine used the Cochrane report to ask “Is Fluoridated Drinking Water Safe?,” the heads of the American Dental Association, the American Public Health Association, the American Dental Education Association, the American Association of Public Health Dentistry, the American Association for Dental Research, and the Harvard School of Dental Medicine demanded that the article be amended or taken down. (The story included earlier versions of the two charts in this article.) Fluoride, Glenny told me, is “the only topic that I’ve been involved in that has created so much angst and controversy.” The responses also critiqued the Cochrane report itself. The president of the American Dental Association said that it was “shaped by its unusually narrow inclusion criteria, excluding 97 percent of the more than 4,000 relevant studies that it identified.” In a joint letter, the president of the American Dental Education Association and the executive directors of the American Dental Association and the American Association for Dental Research concurred, scoffing at Cochrane’s “rigid inclusion criteria.” But the inclusion criteria were not “unusually narrow” or “rigid”—they were based on those in a standard textbook, now in its fourth edition. The implication of the dental experts’ critique seemed to be that if only statistical analysts would lower their standards, everything would look good.

… Alas, epidemiologists have been complaining about the safety studies for decades, according to Sander Greenland, an emeritus professor of epidemiology and statistics at UCLA. Greenland, who is a co-author of the standard textbook Modern Epidemiology, began his own fluoridation work in the ’70s by examining a “typical crap ecological study” supposedly showing that fluoride caused cancer. “But then I got into the literature, just because I wanted to do a thorough job, and I noticed there was really no safety information. They didn’t have any good rationale for the dose.” The current U.S. recommendation is 0.7 milligrams per liter.

… I asked Anne-Marie Glenny whether there were other ways of reaching poor children who can’t go to dentists—training them to brush their teeth in school, for instance. Or providing free dental care in impoverished communities. She said she was unaware of any research that compared the outcomes of fluoridation with these alternatives.

Given all the uncertainties, I asked, can we really say that fluoridation works? “There’s no argument that fluoridation doesn’t work,” Glenny said. “The question is whether it is still the right way forward.”

*Read the full article online at https://www.theatlantic.com/magazine/archive/2020/04/why-fluoride-water/606784/