Perhaps you’ve heard of the gut microbiome and that it plays a major role in the health of your digestive system. But did you know it also can impact your overall health — and even possibly your behavior?
Here’s a primer on what the gut microbiome is, how it connects to your brain and the rest of your body, and what researchers are learning about its importance.
The “second brain”
The microbiome is made up of colonies of bacteria and other microbes in the human body. Most of them live on our skin and in our intestines; the latter make up the gut microbiome.
In fact, there are more microbes living in our bodies than human cells: Humans play host to roughly 38 trillion bacteria and microbes compared to 30 trillion cells that make up our bodies. And research shows they may influence our behavior.
For Dr. Christopher Damman, a gastroenterologist who sees patients at the Digestive Health Center at UW Medical Center – Montlake, the significance of those numbers is perhaps best illustrated in an artist sticker he saw once while walking in the city. It said, “meat puppet.”
“I sometimes have a one-track mind and it immediately went to the microbiome. It’s somewhat grotesque and yet it fits how the microbiome sees us,” he says.
Along with trillions of microbes, our guts also play host to neurons. Yep, you read that right: The gut contains a mesh-like structure of neurons called the enteric nervous system, or ENS. It communicates with our brain via the vagus nerve, part of the gut-brain axis, which stretches from the brain all the way down to the colon. (As well as via hormones and metabolites, and possibly the immune system, according to Damman.)
The ENS is often referred to as the “second brain” because it operates independently and, with the help of the vagus nerve, allows for direct, two-way communication between the neurons in our brain and the neurons and microbes in our gut.
Microbial puppet masters
Going back to the whole meat puppet idea: Nature is full of microbes and other small creatures that puppeteer the actions of their host, and the gut microbiome is no exception.
For example, take the trope in zombie movies and TV shows that a virus turns its host into a member of the walking dead. That concept has a basis in the real world — though thankfully not (yet) in humans.
Google “zombie mice” or “zombie ants” and you’ll find eerie videos of the creatures intentionally yet unknowingly putting themselves in danger because the parasitic microbes in their bodies want them to.
“There are also well-known examples of toxoplasma gondii, it infects mice and changes their behavior so they’re more likely to be caught by a cat. Zombie ants infected by fungi go to a place where the fungi can grow and then it sprouts from their head. There are plenty of examples in nature of how microbes can quite literally control behavior of higher order organisms,” Damman explains.
So, what do zombie parasites have to do with the gut microbiome? There’s emerging research that suggests the gut microbiome can, in fact, puppet our behavior (though, thankfully, in less gruesome ways).
The gut microbiome and your mental health
One way they could do this is by affecting mental health, though this research, too, is still in very early stages. There’s a long history of connection between mood disorders like anxiety or depression with stomach problems such as irritable bowel syndrome (IBS).
Selective serotonin reuptake inhibitors (SSRIs) are a common medication used to treat mood disorders. Researchers don’t know exactly how they work but think they prevent the brain from reabsorbing the neurotransmitter serotonin. Interestingly, at least 90% of someone’s serotonin supply is manufactured by their gut bacteria.
“This is highly theoretical, but it may be that the SSRIs are also exerting an effect in the gut on the nervous system,” Damman says.
Recent studies in mice are supporting this idea.
Damman also cites the example of vagus nerve stimulation, a therapy used for treatment-resistant depression. A neurosurgeon implants a small device that sends electrical pulses through the vagus nerve, kind of like a pacemaker.
Additionally, the gut microbiome may play a role in the development of brain diseases such as Parkinson’s and Alzheimer’s, and research shows that it could also impact temperament in infants, Damman says.
The gut microbiome and your palate
So, why do they do this? One theory is that types of bacteria that feed off of a certain type of biomolecule — say, sugar — manipulate our eating habits by telling us to eat more sugar so they can get more of the substance they need to thrive.
If this is true, it means that some of your food cravings could be caused by the collection of microbes living in your gut. Talk about taking “you are what you eat” to a whole new level.
Some researchers even speculate that unhealthy eating habits, which are so often hard to break, could be due to the gut microbiome’s preferences rather than, say, a lack of willpower. Changing the gut microbiome, then, could be a way to encourage healthy eating.
The gut microbiome and your health
The microbes in your gut don’t just impact the health of your digestive system. Turns out, they play a pretty big role in your overall health.
When the balance of microbes in your gut is upset, it can lead to health issues. This imbalance, called dysbiosis, happens when the number of helpful microbes decreases or the number of unhelpful microbes increases.
A major diet change, antibiotics, chronic stress or drinking a lot of alcohol can all lead to dysbiosis. Sometimes it’s a short-term problem that can be fixed with medication or a change in diet.
Research suggests that dysbiosis is also linked to many chronic and serious health problems, however, including heart disease, colon cancer, diabetes, kidney disease, obesity, autoimmune diseases and neurological conditions — and potentially even developmental disorders such as autism, Damman says.
There is still a lot researchers need to learn about the connection between dysbiosis and chronic disease. And just because dysbiosis is linked with different diseases doesn’t mean it causes them or that fixing the dysbiosis will help cure the disease.
However, there are some therapies that involve the microbiome that can already make a difference for certain conditions.
Fecal transplant, also called bacteriotherapy, is used to treat people who have too many C. diff bacteria in their gut. It may sound gross, but it can be a way to reintroduce helpful bacteria into someone’s system when other treatment options have failed.
There have been documented cases of fecal transplant that show it can sometimes transmit infections to the transplant recipient; however, several companies are working on developing bacteriotherapies that remove this risk, Damman says.
How behavior shapes the microbiome
Remember how the vagus nerve offers two-way communication between the brain and gut? That means that, not only does the gut influence the brain, but the brain — and the choices it makes — influences the gut.
“The foods we eat influence the microbiome in a profound way,” Damman says.
Prebiotic and probiotic foods can also help increase the number of beneficial bacteria in your gut. Prebiotic foods include things like bananas, seaweed, onions and garlic, while probiotic foods include things like fermented foods, tempeh, yogurt and miso.
Though there are lots of supplements out there that claim to be full of pre- and probiotics, the most helpful way to get these nutrients is through whole foods.
Since built-up stress can wreak havoc on your gut, finding ways to reduce stress can help prevent gut problems, too. Meditation and deep breathing can be helpful, as is exercising, whether that’s taking walks or weightlifting.
The bottom line
While there’s still a lot science has to learn about the gut-brain connection, one thing is clear: The future of gut microbiome research will have important implications for our health.
“If you look back 100-plus years to the foundation of understanding of infectious etiology of disease, the breakthrough was learning that microbes cause disease. In the next 50 years I hope we’ll look back at how we started using microbes to treat disease,” Damman says.