The gut microbiome gets most of the attention in discussions of bacterial health. But the oral cavity is where the digestive tract begins, and the connection between oral and gut microbial health is increasingly apparent and clinically relevant.
How bacteria travel
The oral cavity and gut are connected by the digestive tract, and bacteria from the mouth are continuously swallowed — estimates suggest we swallow around 100 million bacteria per milliliter of saliva, amounting to billions per day. In a healthy person with a healthy gut, these oral bacteria are largely kept in check by stomach acid, the gut immune system, and the established gut microbial community. But in people with reduced stomach acid (common in those taking proton pump inhibitors), or with compromised gut immunity, or with small intestinal bacterial overgrowth (SIBO), oral bacteria can colonize parts of the gastrointestinal tract they normally wouldn’t reach.
Inflammatory bowel disease and oral health
The connection between Crohn’s disease, ulcerative colitis, and oral health runs in both directions. IBD patients have higher rates of certain oral manifestations including mouth ulcers, angular cheilitis, and a specific type of gum overgrowth (pyostomatitis vegetans). Conversely, some researchers have proposed that oral dysbiosis might contribute to gut inflammation by seeding the gut with inflammatory bacteria. A 2017 study in Cell Host & Microbe found that the oral microbiome of Crohn’s disease patients was significantly different from healthy controls, with specific enrichment of oral bacteria in the gut microbiome — suggesting that impaired gut barrier function was allowing oral bacteria to persist in the intestine.
The probiotic angle
An interesting implication: oral probiotics — particularly when taken as chewable tablets that dissolve in the mouth — may have effects beyond the oral cavity. Some L. reuteri strains used in oral health supplements (particularly DSM 17938) are also well-studied for gut health effects. It’s plausible that bacteria colonizing the mouth and being continuously swallowed have downstream effects on the gut microbiome — though this isn’t fully established in clinical research. Our reviews of ProDentim and GumAktiv cover the specific strains used.
Nutritionally, the nutrients most directly relevant to both systems overlap: vitamin D (essential for gut immune function and oral immunity), omega-3 fatty acids (anti-inflammatory in both contexts), and probiotic bacteria (beneficial to both microbiomes). Our dental health supplement guide covers the evidence for these.
Educational content. If you have IBD or significant gut symptoms, consult a gastroenterologist.