The mouth is different than the rest of the alimentary canal (aka “gut”). We use it to express emotions when we smile or grimace with it. We express our thoughts and feelings with words that come out of it. And we put food into it. We experience the sensory features of what we eat thanks to the taste receptors on our tongues and elsewhere in our mouth. This is the first place in the gut where food influences brain functions (such as mood), and where digestion of food begins. Chewing food prepares it for the stomach, which breaks food down so that it can be absorbed in the small intestine. The more we chew our food, the easier things are for the stomach. Enzymes in saliva begin digestion of carbohydrates.
So, the alimentary canal, aka “gut,” begins with the mouth, but even people who study the “oral cavity” may not think of it as part of the gut. But the mouth is directly connected to the rest, and it can have marked effects on it.
As a dentist once pointed out, the mouth is the gateway to the inside of the body. What is happening in the mouth can directly influence the rest of the gut, and as new research is showing, oral health influences a wide variety of other bodily functioning, from the cardiovascular system to the nervous system (Sedghi 2021, Kunath 2024, Xi 2024, Adil 2025). Immune cells in the mouth, as in the rest of the gut, can influence local and systemic immune function. The link between oral health and conditions in the rest of the body is now considered to be carried out via inflammation and oral microbial dysbiosis (Kunath 2024, Senel 2021).
Historically, factors recognized to influence periodontal inflammation and oral dysbiosis have centered on oral hygiene. But now it is clear that other environmental factors, in particular psychological stress and diet, are interacting in several ways to improve or exacerbate oral health (Sedghi 2021).
Oral microbes in health and disease (gingivitis, periodontitis)
Each section of the gut (mouth, esophagus, stomach, small and large intestines, rectum, and anus) has a unique set of microbial species. These populations make up whole ecosystems, in which each species inhabits specific habitats or niches. In the mouth, these include the tongue, mucosa (part of the gums), or plaque.
As in other parts of the gut, the populations of these microbes need to be balanced so that none overgrow and cause dysbiosis. Indeed, dysbiosis underlies the main dental conditions: dental caries, periodontal disease, and oral candidiasis (Sedghi 2021, Senel 2021).
What are the key factors that influence oral microbe populations? Diet is important, unsurprisingly (Sedghi 2021). In general, diets high in sugar and starchy carbohydrates, and low in fiber and antioxidants (such as those found in vegetables and fruits), contribute to both periodontal disease and caries (tooth decay). Other factors include environmental exposures, gender, age, and lifestyle habits (Zhu 2025). Some authors have reported genetic contributions as well, but these are not well understood and may be expressed as genetic variation of immune responses to the microbes.
Perhaps more surprisingly, psychological stress is associated with oral dysbiosis and dental problems (Sedghi 2021). One way this happens is that stress dysregulates hormones, including cortisol, leading to increased growth and virulence of pathogenic bacteria. Chronic stress, especially psychological stress, also dysregulates the immune system, leading to inflammation (e.g., Sedghi 2021; there’s lots on this effect). All of this causes and exacerbates dysbiosis and its oral and systemic consequences.
Implications for the rest of the gut: IBD and cancer
Oral dysbiosis can also contribute to disease conditions farther along in the gut (Lam 2023, Zheng 2025), particularly inflammatory bowel disease (IBD) and gastrointestinal (especially colorectal) cancer. How can this happen? Microbes from the mouth can move farther along the gut, carried by saliva or food (Xi 2024). The idea is that overgrown “pathobionts,” which are microbes that are not known to be beneficial but that don’t cause disease unless they overgrow, end up in the rest of the gut where, as in the mouth, they induce inflammation. This inflammation can be chronic, driving further inflammation in IBD, and can cause DNA damage to gut cells that leads to cancer (Lam 2023, Wang 2025).
Oral dysbiosis and inflammation can influence the brain
Poor oral health can influence brain health and function both directly, via systemic inflammation or malnutrition, or indirectly as a function of personal and social experiences such as pain, difficulty eating, and embarrassment over missing teeth or wearing dentures (Zhang 2025).
Depression (in particular) and poor oral health (such as periodontitis and tooth loss) seem to be common companions. Dental disorders can drive systemic inflammation, which is now a well-known contributor to depression. But difficulty chewing, for instance because of tooth loss or pain, can reduce “diet diversity” (especially harder-to-chew fibers), which can lead to malnutrition (also associated with depression) and gut bacterial dysbiosis (Huang 2025, Zhang 2025), as well as constipation. Gut bacterial dysbiosis is also associated with depression, most likely due to gut barrier disruption and increased systemic inflammation that goes along with dysbiosis. But this relationship likely goes both ways. Self-care practices, such as tooth brushing and flossing, can be neglected during mood disorders, further driving oral health problems.
Oral dysbiosis may also contribute to neurodegenerative diseases, particularly Alzheimer’s disease (Adil 2025, Mo 2025, Rozenblum 2025, Sedghi 2021, Shawkaßtova 2025). I find this possibility intriguing, so Journal Club this month will feature the Rozenblum paper.