We make and swallow up to a litre and a half of saliva a day, and with it a huge number of bacteria from the mouth. So it's no surprise that researchers are looking at how the mouth and the gut affect each other.
What the research shows
-Bowel cancer. A mouth bacterium, Fusobacterium nucleatum, has repeatedly been found in higher amounts in bowel (colorectal) tumours than in healthy tissue. In 2024, a study in Nature identified a particular subtype that appears especially associated with these tumours.
-Gut health more widely. Mouth bacteria have been found in the gut in some people with inflammatory bowel disease and other conditions, and researchers are studying whether they play a role.
What it doesn't show
Finding a bacterium in a tumour doesn't prove it caused the tumour. It may prefer the environment a tumour provides. This is active, early research, and it isn't a reason to worry about the normal bacteria in your mouth.
Why it's interesting
It reminds us that the mouth isn't a separate system. It's the front door to the digestive tract, and what happens there may travel further than we used to think.
What this means for you
-Look after your gums. Healthy gums mean fewer unwanted bacteria.
-Eat plenty of fibre and whole foods. They're good for both ends of the system. Nutrition and your teeth.
-Take part in bowel cancer screening when invited, and see your GP about any change in bowel habit.
Where to next
Back to the mouth–body connection · The book
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General information only, not medical advice. Please speak to your GP about your general health, and don't stop or change any treatment because of anything you read here.
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