Image: Associated Press/Julio Cortez

From a very young age, it’s drilled into us that we need to floss daily to prevent gum disease and cavities. But as a recent investigation by the Associated Press reveals, the benefits of dental floss are largely unproven.

It sounds blasphemous, but flossing may not yield the protective benefits we’ve been told to expect. Since 1979, the federal government in the US has recommended daily flossing, but by law these dietary guidelines, which are updated every five years, have to be supported by scientific evidence. Surprisingly—and without any notice—the federal government dropped flossing from its dietary guidelines this year, telling the Associated Press that “the government acknowledged the effectiveness of flossing had never been researched, as required.”

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That’s a rather monumental revelation—especially considering how often we’re reminded to floss our teeth—and considering that the global market for dental floss is expected to reach almost $2 billion next year (with half of it in the United States).

Flossing is supposed to protect us from gum disease, get rid of plaque, and prevent cavities, but we actually don’t know this to be true. During its investigation, the AP looked at the best research on the subject over the past decade, focusing on 25 studies in particular. The evidence for flossing, concluded the AP, is “weak, very unreliable,” of “very low” quality, and “carries “a moderate to large potential for bias.” AP national writer Jeff Donn reports:

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The two leading professional groups—the American Dental Association and the American Academy of Periodontology, for specialists in gum disease and implants—cited other studies as proof of their claims that flossing prevents buildup of gunk known as plaque, early gum inflammation called gingivitis, and tooth decay. However, most of these studies used outdated methods or tested few people. Some lasted only two weeks, far too brief for a cavity or dental disease to develop. One tested 25 people after only a single use of floss. Such research, like the reviewed studies, focused on warning signs like bleeding and inflammation, barely dealing with gum disease or cavities.

Wayne Aldredge, president of the periodontists’ group, acknowledged the weak scientific evidence and the brief duration of many studies...Still, he urges his patients to floss to help avoid gum disease. “It’s like building a house and not painting two sides of it,” he said. “Ultimately those two sides are going to rot away quicker.”

Part of the problem, says Aldridge, is that many people don’t floss correctly. Instead of moving it in a sawing back-and-forth motion, we’re supposed to move it up and down the sides of our teeth. But flossing can also cause harm when it’s done incorrectly or too vigorously, damaging gums, teeth, and dental work.

The AP also found that manufacturers of dental floss are struggling to provide convincing evidence (which is hilarious considering they actually fund a lot of this research). Johnson & Johnson told the AP that floss helps remove plaque, but when the company was sent a list of contradicting studies, its spokesperson declined to comment. Similarly, Procter & Gamble claimed that flossing fights plaque and gingivitis, but its “proof” for this claim came from a two-week study that was deemed irrelevant in 2011 during a research review.

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All of this isn’t to suggest that flossing isn’t beneficial, but it’s clear that the science is seriously lacking. A dentist with the National Institutes of Health told the AP that it may eventually be appropriate to drop the floss guidelines, but only after more rigorous research is done. Until then, he advises that people should still floss, saying. “It’s low risk, low cost,” and adding “We know there’s a possibility that it works, so we feel comfortable telling people to go ahead and do it.”

[Associated Press]