Could bacteria in your mouth play a role in obesity?

Obesity isn't just about excess weight. It comes with an increased risk of a range of other metabolic conditions, including type 2 diabetes.

As it stands, around 26% of adults in the United Kingdom have obesity. And in the United States, that figure is close to 42%.

Pinning down the direct causes of obesity and its related metabolic conditions is challenging, but many factors appear to be involved. And the causes likely vary from person to person.

In recent years, some researchers have focused on the gut microbiome's role in these conditions. And evidence is mounting that it’s an important part of the puzzle.

However, the oral microbiome — the bacteria in your mouth — has received much less attention. 

Some research suggests that the community of microorganisms in your mouth may play a role in obesity and metabolic disease. Below, we’ll take a look at this theory.

An intro to the oral microbiome

Your oral microbiome is the second largest microbial community in your body, after your gut. 

And each part of your mouth — like your gums, tongue, cheeks, and teeth — provides a unique environment for bacteria to thrive in.

Researchers have identified around 700 species of bacteria in the oral microbiome.

Some of the most common species in a healthy mouth include: 

  • Streptococcus

  • Actinomyces

  • Veillonella

  • Fusobacterium

Like the gut microbiome, the oral microbiome varies from person to person.

Its makeup is influenced by various factors, including your oral hygiene, your diet, whether you smoke, whether you drink alcohol, where you live, and your socioeconomic status. 

Other factors — like genetics, obesity, age, and pregnancy — can also impact your oral microbiome. So, what does all this have to do with obesity? 

Oral microbiome and health

Already, there’s some good evidence that oral bacteria are linked to cardiovascular disease. 

And there’s a growing suspicion that these microbes might also be important in digestive system diseases and cancer.

They also seem to increase the risk of poorer outcomes during pregnancy, like preterm birth.

Scientists are only starting to investigate whether the oral microbiome is linked to obesity, but there’s good reason to suspect that a link exists. 

We’ll dig into this fascinating relationship in the rest of this feature. 

Mouth bacteria and weight gain

Several studies have found associations between certain oral bacteria and weight gain

Also, people with obesity tend to have more oral bacteria associated with gum disease — like Porphyromonas gingivalis — than people without obesity.

Some experts believe that this relationship runs in two directions:

People with obesity are more likely to have gum disease than people who don’t have obesity. And gum disease may increase the risk of obesity and obesity-related health conditions.

But how? Well, we don’t know for sure. But scientists have some theories. And inflammation seems to be key.

Inflammation and obesity

Obesity is associated with long-term, low-level systemic inflammation. “Systemic” means that the inflammation happens throughout your body.

This inflammation seems to be a major reason why obesity increases the risk of metabolic disorders like diabetes.

The oral bacteria associated with gum disease also cause inflammation. Then, the inflammatory compounds produced during gum disease can spread to other parts of your body through your blood. 

Studies have shown that when a rat has gum disease, inflammation increases in fat tissue and the liver. Importantly, the researchers only saw increased inflammation in obese animals. 

Other animal studies have reached similar conclusions — gum disease causes inflammation in fat tissue and promotes insulin resistance, the forerunner to type 2 diabetes.

In this way, the inflammation associated with gum disease may amplify the inflammation that comes with obesity.

And because inflammation drives metabolic changes with obesity, gum disease could increase the risk of developing metabolic issues.

Although there's a lack of research in humans, scientists have shown that people with obesity and type 2 diabetes have distinct oral microbiomes. Again, this doesn’t prove causation, but it’s part of the puzzle.

How else might oral bacteria influence obesity and metabolic health? There are a few theories.

The oral-gut axis

Your saliva contains billions of bacteria. Each time you swallow, they’re washed down into your stomach.

Once there, your stomach acid kills most of these bacteria, preventing them from reaching further into your gut. But, as powerful as your stomach acid is, some bacteria do sneak through

Large studies suggest that your oral microbiome plays a significant role in shaping your gut microbiome.

So, the theory goes like this: Bacteria associated with gum disease, like P. gingivalis, make it through to your gut. Once there, they cause dysbiosis, which is an imbalance in your gut microbiome. 

This, in turn, interferes with metabolic pathways and increases inflammation. And, eventually, this helps drive the metabolic conditions associated with obesity.

We don’t have evidence from human studies, though some animal research provides hints. 

For instance, in one study, scientists fed mice P. gingivalis. Introducing these bacteria altered the animals’ gut microbiomes and induced changes in their immune systems and metabolism.

The oral-blood axis

We’ve established that “bad” oral bacteria may influence the makeup of your gut microbiome. But they might play a role in obesity and other metabolic conditions via a different route, too.

There’s evidence that oral bacteria can, in some situations, make it into your blood. For instance, researchers have identified oral bacteria associated with gum disease — including P. gingivalis — in people’s blood vessels.

Bacteria aren’t supposed to be in your blood, so when this happens, your immune system mounts an inflammatory response. This might drive the negative changes to your metabolism that come with inflammation.

Also, as we touched on before, inflammatory compounds from your mouth can reach your blood vessels and travel throughout your body.

Scientists are still debating how important the oral-blood axis is for driving inflammation. But it’s likely to play some part.

Taste perception

Moving away from inflammation, let’s talk about flavor. Perhaps surprisingly, the bacteria in your mouth play an important part in your sense of taste.

For instance, bacteria can form a physical barrier on the tongue, which prevents flavor molecules from reaching your taste buds.

Bacteria also generate compounds as they feed on your food. And these compounds can influence flavor by activating taste receptors.

Oral bacteria might also break down flavor compounds, thereby reducing the flavor of the food.

And animal studies have shown that gut bacteria can influence the number of taste receptors in your gut. Yes, it's weird that you’ve got taste receptors in your gut, and no, we don’t know why they’re there.

Anyway, although little research has investigated whether oral bacteria can influence taste receptor numbers in human mouths, it is at least plausible. 

According to the authors of a review, we can “speculate” that oral microbes might influence your taste perception and, therefore, what you eat.

In that way, they could hold some sway over your risk of developing obesity.

And one study in mice showed that inflammation related to obesity reduces the number of taste buds you have. So, that might play a part, too.

The study's authors explain that taste receptors aren’t just a way of detecting flavor; they’re also important for generating a sense of reward from eating delicious food. 

So, the authors theorize, if your taste buds aren’t providing that rewarding hit, you might overeat to attain it.

The roundup

The picture is complex, and inflammation, among other factors, likely plays a pivotal role.

In brief, people with obesity already have low-level, ongoing inflammation. And the oral bacteria involved in gum disease can increase the inflammation throughout your body in a number of ways.

So, because scientists think that inflammation causes metabolic conditions, oral bacteria ramping up inflammation may further increase the risk.

So, what should you do? While scientists probe the links between oral health and obesity, we should look after our teeth.

After all, you only get two sets in life, and if you’re reading this, you’re probably on your last (real) set.

And because links between oral health and cardiovascular disease are growing stronger, having a good oral hygiene routine is a sensible route to take.


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