Is flossing an important component of a healthy diet?

Last summer you probably heard that we all can skip flossing, or at the very least, resume our habit of regular flossing one week before dental appointments. 

But before you toss your flossers, let's pause and consider the evidence the Associated Press used to make the case against flossing. The story evolved from the AP's FOIA request to the departments of Agriculture and Health and Human Services for the evidence they used to develop the flossing guidelines that have been in place since 1979. In response, those departments acknowledged that no definitive research on flossing effectiveness had been done. Because a stipulation of the guidelines is including recommendations supported by scientific research, the two departments removed flossing from the guidelines.  

Before you start yelling "the government is LYING to us!," it's important to review two key details left out of the AP's flossing story. 

The "guidelines" referred to are Dietary Guidelines for Americans.

The AP piece weaves a story about the surreptitious removal of flossing from the 2015-2020 Dietary Guidelines for Americans, from which "the flossing recommendation had been removed, without notice." That's both true and misleading.

Although it sounds generic, "Dietary Guidelines for Americans" is a proper noun, and refers to nutrition guidelines published every five years by the USDA and HHS. These documents are primarily, as their names would suggest, about eating.

In the 2010-2015 Dietary Guidelines, the "flossing" section whose removal led to the AP story is two paragraphs, contained in a sidebar about fluoridated water, in a section about caloric content of beverages, in a chapter titled "Healthy Eating Patterns." The seemingly-suspicious removal of flossing is less suspicious when viewed in this context. 

All oral health guidelines were removed.

The AP story also fails to mention that it's not just flossing that gets the brush: it's all of dental health. Here are the two paragraphs that were removed from the 2010-2015 guidelines: 

Drinking fluoridated water and/or using fluoride-containing dental products helps reduce the risk of dental caries. Most bottled water is not fluoridated. With the increase in consumption of bottled water, Americans may not be getting enough fluoride to maintain oral health. 
During the time that sugars and starches are in contact with teeth, they also contribute to dental caries. A combined approach of reducing the amount of time sugars and starches are in the mouth, drinking fluoridated water, and brushing and flossing teeth, is the most effective way to reduce dental caries.

It's not as though flossing was removed and brushing was left in. Both of these items, as well as fluoridated water, were left out of the 2015-2020 Dietary Guidelines. It would be more appropriate to state that oral health was removed from the Dietary Guidelines, not that flossing was removed.

What counts as evidence?

Does this mean we should all stop flossing? That's what the AP article argues, based on the lack of studies in scientific literature. In a response to the AP's article, "Flossing and the Art of Scientific Investigation," the New York Times' Jamie Holmes argues differently.

The "missing" scientific literature in the AP article was randomized controlled trials, "the so-called gold standard for scientific research." But, as Holmes explains, that absence is hardly surprising, as the kind of trial necessary to evaluate flossing would be expensive and impractical, and perhaps even unethical because it would require participants not to floss for many years, because it's unethical to conduct a randomized controlled trial "without genuine uncertainty among experts" about a particular health intervention. 

Holmes offers the excellent example of parachutes, which haven't undergone the rigors of randomized controlled testing. Even despite this evidence, experts would likely agree to their effectiveness. Although not flossing does not present quite the same immediate danger as leaping from a plane without a parachute, experts in the dental health field do not have widespread disagreement about flossing. Therefore, a randomized controlled trial could be considered unethical. 

One of the many dangers of our brave new world of alternative facts is how science itself is being invoked as an appeal to credibility. Headlines like "5 science-backed ways to get your kids to brush" and phrases like "because science" are used to close arguments rather than begin them. "Science" invites us to consider countless types of evidence, all of which can be used in bad ways. A researcher might, for example, use a two-paragraph section on dental health as evidence of a crisis of research in the dental field. 

So, should we stop flossing? Even if a randomized controlled trial was the best possible evidence, not having any such trials is not the same as having a trial that recommends against flossing. "No evidence" means just that: nothing in favor or against. 

Now, if a 3-year enormous study of flossing is announced any time soon, that might be a cause for suspicion, as it would suggest that researchers had reason to believe flossing was either harmful or unhelpful. Until then, I'll keep flossing. 


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