The Journey to Your Gut Starts in Your Mouth
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By Sally Chase — co-founder of Arbor. Sally writes about the oral microbiome, gut health, and the everyday habits that shape long-term wellbeing.
Sometime in the next minute, you will swallow. You will not plan it or notice it. We do it hundreds of times a day, between sentences, between bites, in our sleep. It is one of those things the body handles so quietly that we forget it is happening at all.
But every one of those swallows is the start of a journey. And the journey does not begin where most of us imagine.
The journey starts higher than we think
When we picture digestion, we picture the stomach. The gut is where the real work seems to happen, so that is where our attention goes. Probiotics, fibre, fermented foods, all of it aimed downstream.
The digestive system does not start there, though. It starts in the mouth. The first thing any meal meets is your saliva, your tongue, and the hundreds of species of bacteria that live there. Everything you swallow passes through that crowd on the way down, and it picks up passengers as it goes.
Those passengers are the part we tend to overlook. With every swallow, bacteria from your mouth travel into your gut. Not occasionally. Constantly, all day long, a steady drift of whatever is living in your mouth heading toward whatever is living below.
It is easy to assume they simply dissolve along the way, the way we assume a lot of things about the body we cannot see. They do not.
What goes down does not vanish
Researchers have followed those bacteria, and what they are finding is quietly reshaping how we think about both ends of the system.
Studies that track microbes through the body have shown that oral bacteria regularly reach the gut and take up residence there. The mouth and gut are not sealed off from one another. Scientists increasingly describe them as one connected ecosystem rather than two separate organs, and they have a name for the link, the oral-gut axis. It helps to picture it less as two destinations and more as a single stream, with the mouth at the source.
Most of the time this is unremarkable. A balanced mouth sends mostly cooperative bacteria downstream, and the gut carries on. The interesting part is what happens when it does not. When the balance in the mouth tips, less friendly species make the same trip, and some have been shown to settle in the gut and stir up inflammation that was not there before.
Sit with that for a second. The state of your mouth is not only about your mouth. It is one of the small, daily things shaping the very system so many of us are busy trying to look after.
Once you see it, it sits underneath a lot of questions people already ask. Why does bad breath keep coming back, however carefully you clean? Why has a good gut routine quietly stalled? Why does a stronger mouthwash never seem to be the answer? On their own, they look like separate problems. Put side by side, they start to look like the same one.
We spend so much time thinking about what we put into the gut. Maybe the more interesting question is where the journey starts.
If this is new to you, my piece on why your gut probiotic might not be working the way you think follows the same idea from the other direction.
References
- Schmidt TSB, et al. Extensive transmission of microbes along the gastrointestinal tract. eLife, 2019.
- Atarashi K, et al. Ectopic colonization of oral bacteria in the intestine drives Th1 cell induction and inflammation. Science, 2017.
- Kitamoto S, et al. The intermucosal connection between the mouth and gut in commensal pathobiont-driven colitis. Cell, 2020.
- Olsen I, Yamazaki K. Can oral bacteria affect the microbiome of the gut? Journal of Oral Microbiology, 2019.