How Gut and Oral Health Could Raise Stroke Risk by 60%

Hippocrates is often credited with saying that “all disease begins in the gut.” If he did say it, he was way ahead of his time. We now know that our gut microbiomes do, in fact, play a large role in our health—from more acute illnesses like cold and flu to chronic illnesses, like heart and autoimmune diseases. But did you know that the gut isn’t the only place in your body with a microbiome?

Your skin has its own microbiome. So does your mouth. This makes sense since your digestive system begins in your mouth—digestive enzymes are released into your mouth when you eat to start breaking down your food. Even the act of chewing your food is part of the digestive process. 

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Your mouth’s microbiome is similar to your gut’s, and the two microbiomes influence each other. (That’s called a bi-directional relationship.) When one is imbalanced, there’s a good chance the other is, too.

Researchers have also found a connection between your gut microbiome and inflammation—the chronic kind that can be a culprit in chronic diseases, like heart disease, diabetes and cancer. This means that there is also a connection between your microbiome and these conditions, as well. But there’s another condition that doesn’t seem to garner quite as much attention: stroke. 

Previous research suggests that people who have had strokes also have imbalanced microbiomes. This has led researchers to look at factors beyond the brain—which is where strokes occur—that may contribute to both the stroke event itself and how well people recover from it.

Keep reading to find out what researchers found when they analyzed the connection between the gut and mouth microbiomes and stroke. These preliminary findings are being presented at the American Stroke Association’s International Stroke Conference 2025, so be on the lookout for further research that’s likely to come.

The Microbiome-Stroke Connection

Your microbiome is loaded with trillions of both beneficial and potentially harmful bacteria. This collection of bacteria and other microorganisms in the gut is referred to as the gut microbiota and in the mouth, the oral microbiota. Since you could most likely never completely rid the body of all the potentially harmful organisms, the goal is to maintain an optimal amount of good bacteria, out-numbering the bad bacteria. 

In a previous study by these same researchers, they found a link between a bacteria that causes tooth decay, called Streptococcus mutans, and a higher risk of bleeding in the brain, a cause of stroke.

For this latest study, researchers looked at another harmful bacteria called Streptococcus anginosus. This strain of bacteria has been implicated in conditions like oral abscesses and even abscesses in the brain’s frontal lobes. Researchers have also pointed to Streptococcus anginosus as a cause of tooth decay, as it can break down tooth enamel. 

How Was the Study Conducted and What Did It Find?

There were 250 total Japanese participants in this study with an average age of 70. About 40% of them were female. The 250 participants were split into two groups—the test group and the control group. Researchers compared the microbiomes’ of the mouth and gut in 200 participants who had had a stroke within the past seven days (test group) with the microbiomes of 50 individuals without a history of stroke (control group). While they didn’t have a history of stroke, the control group could have other medical issues, including high blood pressure, diabetes or high cholesterol—all risk factors for stroke. 

Microbiomes were analyzed using stool and saliva samples from both the stroke patients and control group participants between July 2020 and July 2021. Participants were then followed for two years so researchers could observe how many of the stroke patients died or had another cardiovascular event (like a heart attack or another stroke). 

Researchers found that Streptococcus anginosus was significantly more abundant in the saliva and gut of people with a history of stroke than in the control group. Specifically:

• Streptococcus anginosus in the gut was independently associated with 20% higher odds of stroke after controlling for vascular risk factors.
• Anaerostipes hadrus—a gut bacteria associated with beneficial effects—was associated with a drop in stroke risk by 18% and Bacteroides plebeius—a helpful gut bacteria common in the Japanese population—was associated with a drop in risk by 14%.
• Over the two-year follow-up period, stroke survivors with Streptococcus anginosus in the gut had a significantly higher risk of death and major cardiovascular events during the two-year follow-up period.
• Compared to control participants, increased risk of death and poor outcomes were not noted in stroke survivors with Anaerostipes hadrus and Bacteroides plebeius. In other words, those beneficial bacteria seemed to be protective.

How Does This Apply to Real Life?

In a press release about this presentation, the study’s lead author, Shuichi Tonomura, M.D., notes that ideally, in the future we’ll have a quick test to detect harmful bacteria in the mouth and gut (perhaps a mouth swab?) and that it could be used to calculate stroke risk. He feels that targeting the harmful bacteria might be able to prevent strokes.

Until this test is created and made available to the public, each of us needs to do our best to maintain healthy microbiomes—oral and gut. Maintaining healthy oral hygiene habits can help. Brush at least twice a day and floss at least once daily. Drink plenty of water to help keep your mouth clean and avoid beverages with added sugar. Try to get to the dentist for regular cleanings and checkups every six months. 

Beyond oral health, other health habits are also all connected to inflammation and chronic disease. So whether you’re trying to prevent stroke, heart disease, diabetes, cancer or any other chronic illness, your health habits will play a role. This includes your eating habits, how much you move your body, how much quality sleep you’re getting, how well you’re controlling your stress levels and how much quality time you’re spending with loved ones. 

It’s also important to get and maintain healthy gut and oral microbiomes. This involves eating plenty of fermented foods, including yogurt, kefir, kimchi, sauerkraut, miso and tempeh. These foods add probiotics, which are beneficial bacteria, to your mouth and gut. 

Because probiotics are living organisms, they need to eat to thrive and reproduce. This is where prebiotics come in. Prebiotics are fibers that probiotics love noshing on. They’re found in fruits, vegetables, whole grains, nuts, seeds and legumes.  

The good news is that you can pretty quickly improve your gut health. This dietitian-approved weeklong Gut-Healthy Anti-Inflammatory Meal Plan will show you how—and you can get started today.

The Bottom Line

This preliminary study suggests that people with certain harmful bacteria in their mouths and guts had a higher risk of stroke and a higher risk of dying from stroke or suffering another cardiovascular event within two years of the original event. The study authors note that because this study was done exclusively with Japanese participants, it is not yet known if the results would extend to other ethnicities and countries. This is because different areas of the world have different strains of bacteria. 

Until more research is performed, it’s still important to maintain healthy gut and mouth microbiomes, which may help reduce your risk of many diseases. Do this by eating plenty of fermented foods and prebiotic foods. Engaging in healthy habits will also help support your microbiome and reduce inflammation and your risk of disease. This includes eating a variety of foods, moving your body often, getting plenty of quality sleep, spending time with loved ones and reducing your stress levels.