to floss or not to floss?

The federal government has recommended flossing since 1979, first in a surgeon general's report and later in the Dietary Guidelines for Americans issued every five years. The guidelines must be based on scientific evidence, under the law.

Last year, the Associated Press asked the departments of Health and Human Services and Agriculture for their evidence, and followed up with written requests under the Freedom of Information Act.

When the federal government issued its latest dietary guidelines this year, the flossing recommendation had been removed, without notice. In a letter to the AP, the government acknowledged the effectiveness of flossing had never been researched, as required.

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However, most of these studies used outdated methods or tested few people.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.

When the ADA was asked for proof of its claim that flossing helps prevent early gum disease and cavities, the group cited the 2011 review and a 2008 two-week study that measured bacteria and did not even consider gum disease.

In a later statement to the AP, the ADA said flossing "removes plaque" and "is proven to help remove" debris from between teeth. A video on its website proclaims that flossing "helps prevent gum disease." When pressed, Matthew J. Messina, a practicing dentist and spokesman for the dental association, acknowledged weak evidence, but he blamed research participants who didn't floss correctly.

National Institutes of Health dentist Tim Iafolla acknowledged that if the highest standards of science were applied in keeping with the flossing reviews of the past decade, "then it would be appropriate to drop the floss guidelines."

Regardless, he added, Americans should still floss.

"It's low risk, low cost," he said. "We know there's a possibility that it works, so we feel comfortable telling people to go ahead and do it."