“Poor oral hygiene may increase risk of cancer death,” The Daily Telegraph has reported. The newspaper said that Swedish research has linked a higher levels of dental plaque to premature cancer death.
The research examined the association between oral hygiene and death due to cancer in a group of volunteers over a 24-year period. The participants underwent a dental check at the beginning of the study where the amount of plaque on their teeth was determined. The researchers then examined national death registries in the years that followed to see how many participants had died and what their cause of death was. They found that those who had died tended to have had more plaque at the beginning of the study compared with those who were still alive. While the association between plaque levels and death due to cancer was significant, it was less strong than the associations that age and gender had with death.
This study suggests that oral hygiene may be linked with death due to cancer, but its design means it cannot prove any cause-and-effect relationship between the two. For example, it is possible that oral hygiene is associated with other factors that affect health and cancer risk, such as social and economic circumstances, and therefore does not in itself trigger cancer.
Where did the story come from?
The study was carried out by researchers from the University of Helsinki and the Karolinska Institute in Sweden. It was funded by the Swedish Ministry of Health and Social Affairs, and the Karolinksa Institute.
The study was published in the peer-reviewed journal BMJ Open.
The media accurately covered this research, with The Daily Telegraph and Daily Mail correctly reporting that the researchers found that poor oral hygiene was associated with an increased risk of dying of cancer, but that this couldn’t be proved to be a cause-and-effect relationship. Given that it is only an association, caution should be exercised when interpreting the study’s results.
What kind of research was this?
This was a prospective cohort study that aimed to determine whether or not poor oral hygiene is associated with an increased risk of dying of cancer over time. Dental plaque, a film of bacteria that forms on the teeth and along the gum line, is involved in the development of oral diseases. Some theories propose that these oral diseases, which often involve inflammation of the mouth’s tissues, may play a role in the development of other diseases through the spread of bacteria and the inflammation they cause in the body. As some cancers are thought to be triggered by infections and inflammation, the researchers behind this particular study thought that there could be an association between plaque levels and eventual cancer mortality.
Prospective cohort studies are useful for determining the association between two factors. However, they can’t generally establish on their own whether or not a given association represents a cause-and-effect relationship.
What did the research involve?
In 1985, the researchers invited 3,273 randomly selected individuals to participate in the study. Approximately half (51.2%) accepted the invitation to participate and underwent an initial (baseline) dental exam and filled out a health questionnaire. The questionnaire asked about regular dental visits, tobacco use and other health-related subjects. The researchers excluded participants who had signs of periodontal disease (disease of the tissues surrounding the teeth, including the gums and bones).
After making these exclusions, the final research group was comprised of 1,390 participants aged between 30 and 40 years old with good oral health. The researchers recorded several measures of oral health for these remaining participants, including overall oral hygiene status, which was defined by the amount of plaque present. A higher plaque index score indicates the presence of more of the bacterial film and was interpreted as poorer oral hygiene.
The researchers then followed up participants for 24 years, examining national death registries to determine how many of the 1,390 participants died during that time. They then compared those who died during the follow-up period and those who were still alive in the context of several baseline variables, including age, gender, education, smoking, income, plaque levels and other dental hygiene factors.
The researchers also recorded the cause of death of the participants who had died, and further analysis was conducted based on deaths due to cancer. Splitting the group into those who had died of cancer during the follow-up period and those who were still alive, the researchers compared the amount of plaque at the beginning of the study between the two groups. During this analysis, they also controlled for multiple other factors, including age, gender, dental visits, education level, income, socioeconomic status and smoking status.
What were the basic results?
A total of 58 participants (4% of the research group) died during the follow-up period, and 35 of these deaths were due to cancer: 21 women and 14 men died from cancer.
Compared with those participants who were still alive, the participants who had died during the follow-up period were more likely to:
- be male
- be older at the start of the study (baseline)
- have completed fewer years of school
- smoke more
- have higher plaque, gum inflammation and tartar levels
Further data analysis of only cancer deaths revealed that (when controlling for other factors) age, gender and the amount of dental plaque at baseline were all associated with increased odds of dying of cancer. Regular dental visits, education level, income, socio-economic status and smoking were not significantly associated with cancer deaths. More specifically:
- Greater levels of dental plaque were associated with a 79% increase in odds of dying of cancer over the 24 years follow-up (OR 1.79, 95% CI 1.01 to 3.19).
- Age was associated with a 98% increase in the odds of dying of cancer (OR 1.98, 95% CI 1.11 to 3.54).
- Being a male was associated with a 91% increase in cancer death odds (OR 1.91, 95% CI 1.05 to 3.46).
How did the researchers interpret the results?
The researchers concluded that the amount of dental plaque was associated with an increased risk of dying of cancer over 24 years.
This long-running cohort study suggests that oral hygiene during our 30s is associated with an increased risk of dying of cancer over nearly a quarter of a century.
As emphasised in the media, this study can’t prove that 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 study had both strengths and limitations. On the strengths’ side, it was a long-running study that randomly selected participants for involvement. However, it is important to keep in mind several factors when interpreting the results:
- Only half of the randomly selected participants elected to be involved in the study after they were told of the study aims. This may 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.
- The researchers adjusted for some factors known to be linked to cancer (such as smoking and socioeconomic status) but it is uncertain how completely these adjustments could be made using the single measurements taken at the start of the study. Smoking can lead to dental plaque too and so is a particularly important factor in making these adjustments as accurate as possible.
- Only a small number of people in the group studied went on to die of cancer. A larger study that included a larger number of deaths could increase the confidence in the results.
- Data on oral health was only collected at the beginning of the study. It is possible that the participants’ dental habits changed over the intervening 24 years, potentially biasing the results.
- Dental plaque levels at baseline were used as an indicator of likely future dental infections. The researchers did not, however, confirm whether their subjects went on to develop these infections.
- The study also excluded participants with certain types of gum and mouth problems at the start of the study, and therefore the population in the study may not fully mirror the general population.
Finally, it is important to remember that while the odds of dying of cancer may have increased by 1.78-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 oral health may be associated with an increased risk of dying. However, more research is needed to confirm this finding and examine whether or not this link is causal.
Poor teeth brushing linked to cancer. The Daily Telegraph, June 12 2012
Poor brushing of teeth linked to premature cancer deaths as bacteria increase risk by up to 80%. Daily Mail, June 12 2012
Söder B, Yakob M, Meurman JH, Andersson LC, Söder PO. The association of dental plaque with cancer mortality in Sweden. A longitudinal study. BMJ Open, June 12 2012