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Everything You Need to Know About How the Gut-Brain Connection Works

And what it means for you as a runner.

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We know the brain and gut are connected (remember those pre-race nerves that sent you rushing to the porta-potty?). But there’s more to that communication system than you might realize. Gut bacteria and digestive system function play an important role in your mental health, affecting everything from anxiety and mood to energy and focus. When you appreciate the role the gut plays in your overall health, you can start healing it from the inside out.

In the eyes of modern medicine, the brain-gut axis, or the system that allows for communication between the brain and the gut, is a relatively new field of research. It has long been touted in naturopathic medicine, but in traditional scientific research, it has remained a theory for a long time. However, automatic physiological reactions like the fight-or-flight response pointed to the fact that it likely existed. “When you’re running away from a bear, that’s not the time to be hungry,” says Dr. Ellen Stein, associate professor of medicine at the Johns Hopkins Center for Neurogastroenterology. “Your digestion is going to slow down—that’s not when you’re going to think of cheesecake.”

Outside of those stress-related reactions, the brain-gut axis can also be “triggered,” or activated, by viruses and their resulting inflammation, like with COVID-19. “Scientists had been looking to really prove that the gut-brain axis was there, and I think nothing is doing that better than COVID,” Stein says.

Some scientists have noticed a link between the severity of the disease and its neurological effects—including loss of taste, fatigue, depression, and anxiety—when the virus is found in the gut. There are still more questions than answers (e.g., did the person contract the virus through the digestive tract or respiratory tract? Did the coronavirus disrupt the balance of their gut bacteria, or did the patient already have low microbiota diversity?). But if nothing else, COVID-19 is giving scientists more opportunity to explore this unique brain-gut relationship within the human body.

Large grants through the National Institutes of Health have funded new research in this field the last 10–15 years, Stein says. And the giant leaps in technology, specifically micro-computing power, over the last decade have allowed researchers to more accurately analyze what’s in the microbiome. “They’ve been able to figure out a lot more a lot faster,” Stein says.

Before you can improve your gut health, though, you first need to understand the gut’s role and how the brain-gut connection works.

The Brain-Gut Basics

Let’s flash back to 10th grade for a quick anatomy review of the digestive system: Starting at the top—your mouth—saliva contains enzymes that help digest the carbs in your food. Your food then travels down the esophagus to your stomach, where stomach acid digests and grinds up food into smaller and smaller pieces. Then hormonal and nervous system triggers signal the stomach to empty into the small bowel, or the duodenum.

In the duodenum, the food mixes with several fluids, including bile salts, which are produced by the liver. Bile salts help digest fats and absorb fat-soluble vitamins, like A, D, E, and K. Digestive enzymes made by the pancreas also break down carbohydrates, fat, and protein. As the food travels through your small intestine, this is where almost all of the nutrients are absorbed through the wall of the intestine. If your food isn’t broken down properly to this point, you won’t reap as many benefits of your food—even nutrient-rich food—because the intestine won’t be able to absorb them.

After that comes the large intestine, or colon. By this point, essentially only water, electrolytes, and waste remain. The colon is a “thirsty organ,” says Stein, so a lot of your fluids are drawn back into the body through your colon before the waste leaves your body. This process can take anywhere between eight and 24 hours, depending on the person. (Take note: This digestive timeline is why it’s crucial to be hydrated a full 24 hours before hard runs.)

Within that digestive tract reside two critical body systems. The first is your body’s largest concentration of microorganisms (bacteria, fungi, and viruses). There are an estimated 38 trillion bacterial cells in your body, which outnumbers your more than 30 trillion human cells. And there are up to 1,000 bacterial species in the human gut microbiome, which each play a different role in your body. Altogether, your gut microbiome weighs 2–5 pounds and functions as an extra organ in your body, sometimes referred to as a “second brain.”

RELATED: Your Brain on Running: A Deep Dive on the Latest Science and How to Become Smarter, Happier, and More Creative Through Running

Secondly, your digestive system contains the enteric nervous system (ENS), a tangled web of neurons embedded into the gut itself. The ENS is made up of more than 100 million nerve cells lining your gastrointestinal tract from the lower third of your esophagus to your rectum.

When we’re referring to that communication pathway between the brain and gut, that includes the central nervous system (brain and spinal cord), the autonomic nervous system (the control system that regulates body functions), and the ENS. The brain-gut connection also involves certain hormones, immune cells, and various other signaling molecules. We understood that the brain can influence the gut, like in the running-from-the-bear scenario, but now we know that the gut affects the brain—as in, that pathway is a two-way street.

“Nerve fibers connecting the gut and brain go both ways,” says Patrick Wilson, Ph.D., a sports nutrition scientist and author of The Athlete’s Gut. One of those nerves that has been the focus of research is called the vagus or vagal nerve. “By some estimates, 80–90 percent of the fibers in the vagal nerve go upstream, from the gut to the brain,” he says.

In addition to the physical nerve connection, the gut can affect the brain chemically. “The gut releases a host of hormones into your bloodstream, and the amounts secreted vary depending on time of day, when you’ve last eaten, what you ate, and your sleep patterns,” Wilson says. “Some of these hormones target the brain, influencing things like mood, satiety, and appetite.”

And on top of all of that, the gut plays an important role in your immune regulation and host defense. According to the Johns Hopkins School of Medicine, some stomach lining cells excrete massive quantities of antibodies into the gut. “Although it’s hard to pin down an exact number,” Wilson says, “a significant proportion of the immune system resides in the gut.”

Because the gut is incredibly complex and human microbiomes are so unique, scientists know they still have a lot to explore. “We’re starting to learn that the gut microbiota influences many different functions in the body, from metabolism to mood to cognition to appetite,” Wilson says. “What we currently don’t know well, however, is how to best take advantage of these links on an individual level.”

While experts can’t yet offer specific advice for taking advantage of the brain-gut axis, we do know that a healthy gut can aid a healthy brain. Multiple studies have shown that certain bacteria help produce chemicals in the brain called neurotransmitters, such as serotonin, an antidepressant. And studies have linked certain gut bacteria to mental health conditions, such as depression and anxiety.

We also know that stress and your gut are connected. While not many studies have been done on brain-gut health in athletes, one study done on runners found a correlation between perceived life stress or anxiety and GI distress.

Gastrointestinal issues are more common than you might think, but their taboo nature in our culture leaves many people suffering in silence. On top of that, Western diets consisting of highly processed, carbohydrate-rich foods increase chronic inflammation in the body. This inflammation has been linked to poor gut function and decreased biodiversity in the gut microbiota. (Coping with a global pandemic with extra carbs isn’t doing our guts any favors either.) Runners tend to chalk gut distress as a normal, expected part of running.

But GI problems—nausea, constipation, diarrhea, bloating, urgency, and pain—can be signs of much bigger issues that affect much more than your “bathroom business,” even extending to your mental health and energy levels.

Why Your Gut Matters for Mental Health

Stephanie Small, a Boulder, Colorado–based mental health nutritionist says, “the gut is the gateway to health.” Hall is a licensed clinical social worker and nutritionist with a bachelor’s from Yale and master’s in social work from Smith College. While she sees many clients for therapy (for diagnoses like eating disorders), she also sees people for mental health nutrition, which uses supplements and nutrition to stabilize someone’s biochemistry, thereby reducing or eliminating mental health symptoms. For both approaches, every client first undergoes an assessment of gut function, and then focuses on healing the gut with nutrition.

The focus on the gut for mental health is often misunderstood: “It’s not about eating more salads,” Small says. “There are a lot of connections between gut function and mental health. And Western medicine doesn’t really have a lot to address digestive issues. It’s more like symptom suppressants—take Tums, take antacids—instead of looking at the root of the issue and going, ‘What is causing the acid reflux?’”

With her patients, Small focuses on three main benefits of improving gut function. First, breaking down food properly for proper nutrient absorption. Without proper digestion, “you will be low in certain nutrients, and that alone can cause mental health issues,” she says.

Second, maximizing neurotransmitter production. Several neurotransmitters, like serotonin and dopamine, are manufactured in your gut. “Those are feel-good chemicals. When we’re low in them, we’re more likely to have issues with mood, sleep, energy, and focus,” says Small.

And third, reducing chronic inflammation. “That’s due to how we eat, it’s due to medication that we take, it’s due to alcohol, a lot of things,” she says. “And that actually flips on switches downstream for anxiety and depression.”

Ready to tackle your gut health and, in turn, your mental health? These seven steps will get you started.