Non-smokers and lung cancer: Clues may be in mouth

With many people with no history of smoking getting the disease, researchers have turned to the microbiome.

Dave Yasvinski 3 minute read December 15, 2020
Lung cancer

A study looked at non-smokers and mouth bacteria. Getty

Researchers made a discovery in their quest to understand why so many people who have never touched a cigarette still develop lung cancer. 

The research, published in the journal Thorax this week, found a link between the type and abundance of bacteria swimming around in the mouths of non-smokers and their chances of developing the deadly disease. With roughly 25 per cent of lung cancer diagnoses occurring in people with no history of smoking or other risk factors — such as family history or exposure to second-hand smoke — researchers have increasingly turned to the bodys microbiome for answers. They may have found some. 

To test their suspicions, the team behind the study relied on two separate pools of participants — the Shanghai Womens Health Study and the Shanghai Mens Health Study — a combined group of more than 135,000 people who had their health monitored periodically between 1996 and 2006. The two groups provided information on their disease risk upon signup, including medical history, lifestyle and diet data, and researchers profiled their resident bacteria using a special rinse. All participants were lifelong non-smokers. 

Over the 10-year period of observation, researchers found 90 of the women and 24 of the men developed lung cancer, typically within seven years. They then matched these participants with another 114 non-smokers from the control group that were of similar age, sex, education and medical history but did not develop cancer. They found key differences between the mouth microbiomes of the two groups that revealed that a wider range of bacterial species was connected to a lower risk of lung cancer. Specifically, large amounts of the Bacteroidetes and Spirochaetes species correlated with a lower cancer risk while an abundance of the Firmicutes species corresponded with an increased risk. 

Researchers noted the limitations of the work, most notably the small sample size and the observational nature of the study, which can detect correlation but not necessarily causation. “While our study provides evidence that variation in the oral microbiome plays a role in lung cancer risk, the interpretation of our study must be done while considering the caveat that our findings are from a single time point in a single geographical location, they said. 

Limitations aside, the research reinforces the findings of other, smaller studies and raises important questions about the role of the mouth’s microbiome in the development of cancer, said David Christiania doctor and professor at Harvard University. “First, how stable is the human oral microbiome over time?,” he said. Second, if the human oral microbiome varies over time, what determines that variability? Third, how does the ambient environment such as exposure to air pollutants, affect the oral (and lung) microbiome? 

More research will be integral to establishing the strength of the connection and determining if there is causation at play. It remains unclear whether the oral microbiome as measured in this (and other) epidemiological studies represents a causative agent or only a marker of disease or immune activity.  

If it is the former, then it will be important to understand whether the oral microbiome actually seeds the lung microbiome and thus acts locally. 

Dave Yasvinski is a writer with 

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