Dentists have been saying it for years, but now there’s new evidence that brushing your teeth may not be enough to fight gum disease.

A study of twins shows flossing twice a day in addition to brushing reduced gum bleeding — a sign of gum disease — by about 40 percent more than brushing alone in just two weeks.

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"Bad breath and bleeding gums can also occur in people who routinely brush their teeth and gums," said Kenneth A. Krebs, DMD, president of the American Academy of Periodontology, in a news release. "Bleeding gums can be a sign of periodontal disease, and bad breath may be from certain bacteria that have built up in the mouth.”

The results of the study appear in the Journal of Periodontology.

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In the study, researchers compared the effects of twice daily flossing and toothbrushing vs. brushing alone in 51 sets of twins, aged 12 to 21. One twin manually brushed the teeth and tongue twice a day for two weeks and the other twin was given the same instructions in addition to using dental floss twice a day.

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Twin pairs were studied because they each shared the same environmental factors, such as diet, health, and life practices that might contribute to gum disease.

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Before and after the study, researchers examined both groups for evidence of gum bleeding and bad breath. The results showed the group that flossed had 38 percent fewer bleeding gum sites than before the intervention, and overall gum bleeding improved by an average of 42 percent.

No such improvements were found among the brushing-only group. In fact, the number of bleeding gum sites increased by nearly 4 percent during the study among those who didn’t floss.

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Bad breath also improved significantly among both groups, which researchers say was likely due to the recommendation to brush the tongue as well as the teeth. The tongue often harbors odor-causing bacteria. There was no significant difference between the two groups in their improvement of bad breath.

"Gingival [gum] bleeding and halitosis [bad breath are] often the first signs of poor oral hygiene that may eventually lead to further periodontal problems," said researcher Walter A. Bretz, DDS, Ph.D., of New York University College of Dentistry, in a news release. "A good way to prevent periodontal disease and tooth decay is through at-home oral hygiene care and routine dental visits."

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By Jennifer Warner, reviewed by Dr. Louise Chang

SOURCES: Biesbrock, A. Journal of Periodontology, August 2006; vol 77: pp 1386-1391. News Release, American Academy of Periodontology.