Poor oral hygiene may be linked to cancer
There is no doubt that good oral hygiene is important; if dental plaque build up is left for a long time, it can lead to periodontal disease and tooth loss. This long-running cohort study suggests that poor oral hygiene during our 30s is associated with an increased risk of dying of cancer over nearly a quarter of a century.
This study cannot prove that dental plaque levels either directly or indirectly cause cancer or contribute to death due to cancer. As the researchers said, further studies are required to determine whether or not oral hygiene plays a causal role in either the development of cancer or the likelihood of dying from cancer.
This prospective cohort study has both strengths and limitations. On the strengths' side, it is a long-running study that participants were selected at random. However, only half of the randomly selected participants chose to take part in the study after they were told of the purpose of the study. This might have introduced a bias, and people who decided to participate may have been characteristically different from those who decided not to participate. If the two groups differed in terms of key factors (such as oral hygiene or risk of developing cancer), this could have influenced the results.
Cancer has been proven to be linked to many risk factors, including age and smoking. Although some of these have been taken into account in the study, other key risk factors such as family history of cancer, diet and alcohol consumption were not included. Furthermore, the study was carried out over a 24-year period based on one dental assessment at the beginning of the study. A lot can happen in people's lives and their mouths over 24 years. It is possible that the participants' dental habits changed over the intervening 24 years, potentially confounding the results.
Finally, while the odds of dying of cancer may have increased by 1.79 -fold among people with poor oral health, this is a relative measure. In absolute terms, this may not represent a very big increase in the number of cancer deaths.
Overall, this study indicates that poor oral hygiene may be associated with an increased risk of dying. More research, however, is needed to confirm this finding and to examine whether or not this link is causal.
Conflict of Interest:
Low serum 25-hydroxyvitamin D concentrations may explain the association between dental plaque and cancer mortality
The finding that dental plaque is directly associated with 1.79 times the odds ratio of cancer mortality rates  is interesting. However, the conclusion that the bacterial load on tooth surfaces and gingival pockets may play a role in carcinogenesis is most likely incorrect. It is not clear how oral bacteria would cause cancer.
An alternative explanation for the association is that high dental plaque is a marker for low serum 25-hydroxyvitamin D [25(OH)D] concentrations as well as for increased risk of cancer incidence and mortality rates. As mentioned, dental plaque results from oral bacteria. The active metabolite of vitamin D, 1,25-dihydroxyvitamin D, induces production of cathelicidin , which has antibacterial and antiendotoxin effects . Higher serum 25(OH)D concentrations have been linked to reduced risk of dental caries  and periodontal disease . Interestingly, the first paper reporting that vitamin D supplementation reduced the risk of dental caries noted that the bacteria in the caries were dead . At that time it was thought that the effect of vitamin D was to increase the calcium content of teeth.
There is also very good evidence that vitamin D reduces the risk of cancer. While the evidence is strongest from ecological studies , there is also good evidence from observational studies for breast and colorectal cancer . Serum 25(OH)D concentration at time of cancer diagnosis has been found inversely correlated with survival rates for about six or seven types of cancer . Interestingly, a recent study found significant inverse correlations between outdoor occupation and 15 types of cancer in Nordic countries including seven types listed in Table 4 lf : bladder, breast, colon, lung, pancreatic, prostate, and rectal cancer, as well as weakly inversely correlated with melanoma .
Thus, the paper by Soder et al.  provides additional support for the UVB-vitamin D-dental health hypothesis. There are two important consequences as a result. One is that dentists and periodontists who find that patients have dental caries, dental plaque, and/or periodontal disease should tell the patient that he/she may have vitamin D deficiency and should consider increasing serum 25(OH)D concentrations through increased oral vitamin D intake or production from natural or artificial UVB. The optimal serum 25(OH)D concentration for reducing the risk of many adverse health outcomes is above 75 nmol/l  and likely over 100 nmol/l . To reach these concentrations takes 1000-5000 IU/d (25-125 mcg/d) vitamin D3 in the absence of UVB irradiance. However, since there is considerable individual variability between oral vitamin D intake and serum 25(OH)D concentration, testing serum 25(OH)D concentration might be advisable .
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Conflict of Interest:
I receive funding from the UV Foundation (McLean, VA), Bio-Tech Pharmacal (Fayetteville, AR), the Vitamin D Council (San Luis Obispo, CA), the Vitamin D Society (Canada), and the Sunlight Research Forum (Veldhoven).