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According to Life Tips:

Regular flossing, long considered an essential part of daily dental cleansing, has been found to have a positive effect on your life expectancy, increasing it by up to 6 years.

Is this true? How would flossing increase life-span? If so, how can anyone prove that it does?

Other articles:

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    Correlation does not imply causation. People in developed nations, who have a longer life expectancy, are more likely to floss than those in developing nations with short life expectancy. By that same standard eating at "In-n-out" increases your life expectancy. People in sub-saharan Africa who don't eat In-n-out have lower life expectancy than those in California who do. This looks like one of those cases.
    – Coomie
    Commented Sep 6, 2012 at 1:37
  • possible dupe? skeptics.stackexchange.com/questions/8702/is-flossing-helpful
    – mmr
    Commented Sep 6, 2012 at 3:41
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    @MMR: Related but not a dupe. That question asks whether flossing improves oral health. This one asks if the improvement in oral health leads to longer lifespan.
    – Oddthinking ♦
    Commented Sep 6, 2012 at 4:18
  • @Oddthinking-- I respectfully disagree. That question is very broad, and asks if flossing is 'helpful'-- the second answer attempts to address this question. The title can be adjusted to reflect any perceived difference you have, of course.
    – mmr
    Commented Sep 6, 2012 at 15:35
  • @mmr: I think I see your point. Just checking I understand: The title of your "Is flossing helpful?" question is broad (and could conceivably incorporate this question too), but the cited claim inside is limited to oral health. Someone actually answered that question with this claim, but it was deleted by a mod as being a new question, rather than an answer. You are proposing to edit the title of that question to limit it to the claim in the body? Sounds good.
    – Oddthinking ♦
    Commented Sep 7, 2012 at 2:21

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This was a fun journey.

The first couple of links didn't have any references, but the third one did. Not only did it propose a mechanism, but it ascribed it to "Dr. Perls" and "Dr. Roizen".

Picking the second doctor (with the biggest claim) at random. His name is Michael F. Roizen, MD. of the Cleveland Clinic. His claims are somewhat controversial - popular, but attacked by academics.

He wrote a popular book RealAge. The relevant section was quoted in more detail by this dentist:

"These studies show that the presence of periodontal diseases, diseases most common in people with tooth loss, actually affects longevity. The best of these studies done at Emory University with the Centers for Disease Control, indicated that people with gingivitis and periodontitis have a mortality rate that is 23 percent to 46 percent higher than those who don't... why? They are linked to increased rates of cardiovascular disease and stroke, as well as to an increase in mortality from other causes, such as infections."

So, I wandered over to the Center for Disease Control's Oral Health site, and looked for some references.

This seemed to be the most direct reference they provided:

which referenced an earlier paper by a similar team:

The 1998 paper makes some very tentative associations between periodontitus and coronary heart disease.

We conclude that the available evidence does allow an interpretation of periodontitis being a risk factor for atherosclerosis/CHD. This conclusion, however. is made with some qualifications. While the associations found across a wide variety of subjects are remarkably consistent, for the most part they are represented by incidence odds ratios around 2.0. While this level of association would result in oral conditions contributing to a large number of CHD cases, it is possible that associations of this magnitude are due to bias in the study designs. In addition, some studies report that periodontitis is associated with all-cause mortality and low birth weight infants. These multiple associations detract from the credibility of periodontitis as a risk factor, as specificity of association is more often related to causality. However, all-cause mortality may largely be driven by mortality from cardiovascular events: and some exposures, such as smoking. are indeed risk factors for multiple conditions. On the other hand, current findings regarding the associations between oral conditions and atherosclerosis/CHD imply that the criteria for causality may be met in the not-too-distant future.

The 2005 paper shows they are still working establishing this association (e.g. by continuing to try to remove cofounders, such as smoking, and work out the best measures of periodontal disease) and reviews some of the criticism it has received.

Case-control, cross-sectional, and longitudinal studies have found that periodontitis is associated with coronary heart disease (CHD) and cerebrovascular disease, even after adjustment for a variety of potential confounders of these associations. However, other studies have found either nonsignificant positive trends or no association after adjustment for variables considered to be confounders. Concerns about the validity of the periodontitis–cardiovascular disease associations have been expressed.

They go on to defend their hypothesis, but admit that standard clinical measurements of periodontal disease show no associations. (Instead, they recommend measuring antibody responses.) Again, their claims are heavily qualified.

Conclusion

There has been some interesting research tentatively suggesting that periodontal disease is closely correlated to coronary heart disease, with a mechanism proposed. That research has been criticised and (as of 2005), the science hadn't reached a consensus.

Roizen published a popular health book in 2000 (which spawned a series of books and a web-site business) that claimed periodontal disease could cause an elevated risk of coronary heart disease, without considering the confounding factors. He made the leap that flossing would therefore reduce the risk. His book was promoted on Oprah and gained widespread attention from dental web-sites, but it was widely criticized by scientists as being exaggerated.

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    Very nice answer. Surprising how these sort of things happen so often. This is mentioned in detail in the book "Damned Lies and Statistics" Thanks for the answer!
    – dukevin
    Commented Sep 6, 2012 at 6:42
  • I think its more indicative of the type of person by correlation rather than causation. (not backed by science but could be a tentative hypothesis) Brushing your teeth is a regular habit that is commonly considered a healthy and hygienic practice. If people are partaking in more healthy practices than not, it would seem they would also be healthier in other areas of their life such as eating habits and regular exercise.
    – King Friday
    Commented Sep 6, 2012 at 16:44
  • @kitgui: I absolutely understand your point about confounding factors and correlation not being causation. In this case, the actual science wasn't about flossing, but about periodontal disease, so it isn't even clear that flossing itself was relevant.
    – Oddthinking ♦
    Commented Sep 7, 2012 at 2:16

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