The Gut and Brain Are Connected—Here's How to Keep Them in Sync

A two-way connection between the gut and brain—or gut-brain axis—links the central nervous system (CNS)—which includes the brain—with the gut’s own network of nerves, known as the enteric nervous system (ENS) (1).

Our “Second Brain”

Independent of the CNS, the ENS regulates most aspects of gut function, including food digestion; water and nutrient absorption; and regular bowel movements for healthy waste elimination. Of course, like breathing and heartbeat, these gut functions are automatically controlled without consciously using our brain. Hence, the ENS of the gut is often referred to as our “second brain” (2).

So how do the brain and ‘gut brain’ communicate back and forth with each other?

The main link between your gut and brain is the vagus nerve—it's like a long information superhighway that runs from your brain right down to your digestive system. In fact, 80% of the nerve cells in the vagus nerve transmit signals from the gut to the brain, attesting to the gut’s considerable influence on the brain (3).

Besides the vagus nerve, various neurotransmitters, hormones, and other signaling molecules play a role in how our gut and brain communicate. Take serotonin, for instance—this neurotransmitter is well-known for influencing our mood, sleep, thinking, and even appetite. Interestingly, about 90% of serotonin is produced in the gut, where it helps manage different digestive functions, like how food moves through our system. Interestingly, shifts in serotonin levels in both the brain and gut can be linked to our gut microbiota (4).

The Gut Microbiota

In recent years, the gut microbiota—the trillions of microorganisms (predominantly bacteria) inhabiting the gut—has emerged as a vital component of the gut-brain axis. Its role in relaying information up to the brain is critical for gut-brain communication, so much so that the concept of a ‘microbiota-gut-brain axis’ has been introduced (1).

Gut bacteria crosstalk with the brain primarily via the vagus nerve. Microbiota also produce neurotransmitters (e.g., GABA, serotonin, noradrenaline) which contribute to the regulation of brain function.

Another way microbiota plays a role in the gut-brain connection is by helping keep the intestinal barrier strong and supporting our immune system. If this barrier breaks down, toxins and germs can sneak into your body, making you more prone to disease and illness (5).

A number of factors can alter the composition of the gut microbiota, including:

• Genetics
• Environment
• Disease
• Mode of birth
• Exercise
• Diet

Diet and nutritional status are the most effective modifiable factors that can shape the gut microbiota (6, 7).

“All disease begins in the gut”

This famous quote, presumed to have been proclaimed by the Greek physician Hippocrates over 2,000 years ago, is most certainly relevant to the gut-brain connection. Disruption in the gut-brain axis—particularly changes in the gut microbiota—can adversely affect brain function and has been linked to anxiety and depression (8).

Systemic inflammation is associated with symptoms of depression and anxiety (9). A recent review of 26 studies found an association of higher levels of proinflammatory gut bacteria with both anxiety and depression (10).

The review also reported that low levels of gut bacteria that produce short-chain fatty acids (SCFAs) are also linked to anxiety/depression. The SCFAs (e.g., butyrate) serve as an energy source for intestinal cells and are produced from the fermentation of dietary fiber by certain gut bacteria. Among their many health benefits, SCFAs suppress systemic inflammation (8).

Human studies have shown that changes in the quantity and diversity of the gut microbiota, along with various compounds they produce, are associated with neurodegenerative conditions such as Alzheimer’s and Parkinson’s disease as well as autism spectrum disorders (1).

Once again, inflammation is a key player. Studies consistently show that patients with Alzheimer’s disease have high concentrations of proinflammatory gut bacteria and low levels of anti-inflammatory bacteria (11).

Generally, having a greater diversity of microbes in the gut correlates with better cognitive functioning. Some studies suggest that changes in the composition of the gut microbiota in healthy older people could potentially predict the future development of dementia, even before signs of cognitive decline are apparent (11).

As mentioned above, diet and other lifestyle changes can improve gut-brain crosstalk primarily by increasing the abundance of beneficial gut bacteria and reducing levels of harmful bacteria.

Let’s now discuss how to reset your gut naturally.

1. Avoid Ultra-Processed Foods

You probably won’t find many foods worse for your health than those industrially processed options loaded with added sugars and trans fats while being low in protein, fiber, and essential nutrients. This is what we call ultra-processed foods (UPF) — those ready-to-eat or heat meals that often have all sorts of chemical additives. Think soft drinks, sugary breakfast cereals, cookies, packaged snacks, processed meats, and flavored yogurts (12).

It should come as no surprise that multiple studies have shown that consumption of UPF has adverse health effects, including increased risk of diabetes, high blood pressure, elevated blood fats, and obesity (12).

Remarkably, nearly 60% of calories consumed in the U.S. come from ultra-processed foods (13)!

Regarding the gut-brain connection, consumption of UPF—lacking in fermentable fiber—results in lower diversity and changes in the composition of the gut microbiota. Fewer bacteria that produce beneficial short-chain fatty acids (SCFAs) can lead to increased intestinal inflammation (13, 14).

In a study involving over 10,000 adults, high consumption of UPF (＞19.9% of total daily calories) was associated with cognitive decline over eight years of follow-up (15).

2. Increase Fiber Intake

Dietary fiber is widely known for promoting bowel regularity and helping to keep blood sugar and cholesterol levels in check. But it also has more obscure health benefits related to gut microbiota.

Missing in ultra-processed foods but plentiful in whole plant foods, fiber is basically ‘food’ for beneficial gut bacteria. Fiber cannot be broken down by our digestive enzymes in the upper GI; rather, it is fermented in the colon by certain species of gut bacteria, generating various metabolites including short-chain fatty acids (SCFAs) (16).

The SCFAs (e.g., butyrate, acetate, and propionate) are the primary mediators of the metabolic health benefits of dietary fiber. Specifically, SCFAs can reduce elevated blood levels of glucose, cholesterol, triglycerides, and insulin, thus supporting metabolic health (16).

The composition and diversity of the gut microbiota change in response to greater exposure to fiber. Diets high in fiber are associated with a greater abundance of bacteria that produce SCFAs. Conversely, low-fiber diets foster a different community of gut bacteria that can lead to chronic diseases such as diabetes and cancer (17), as well as anxiety and high psychological distress (18).

The bulk of your dietary fiber intake should come from non-starchy vegetables, as these foods do not impact blood sugar levels. You may consider taking a fiber supplement as well. Preferably, choose one with both soluble and insoluble fiber—and gluten-free.

Native Note: If you’re struggling to eat enough fiber, here’s a helpful list of 26 high-fiber foods to add to your cart next time you go shopping.

3. Consume Prebiotic Foods

Prebiotics are compounds that function as a food source for beneficial gut bacteria (e.g., Bifidobacteria and Lactobacilli) while also inhibiting the growth of harmful bacteria, thereby fostering a balanced gut microbiota. As noted above, fiber is a major food for gut bacteria and is classified as a prebiotic (19).

Many of the most familiar prebiotics are soluble fibers, including inulin, fructooligosaccharides (FOS), and galactooligosaccharides (GOS). They are found in various fruits and vegetables, notably asparagus, leeks, and bananas (8).

Polyphenols—natural plant compounds found in colorful fruits and vegetables and known for their anticancer properties—also act as prebiotics. Notably, byproducts formed from the metabolism of tea polyphenols by beneficial gut bacteria have been found to have significant neuroprotective effects (20).

In a study of elderly participants (≥ 65 years), dietary intake of fructan—a common prebiotic found in many fruits and vegetables—was associated with a reduced risk of developing Alzheimer’s disease. The effects of fructan were likely related to its ability to boost the activity of friendly gut microbiota (21).

4. Add a Probiotic Supplement

Probiotic supplements contain live bacteria and yeasts that exert beneficial health effects in the body. Lactobacilli and Bifidobacteria are common probiotic bacteria, whereas Saccharomyces cerevisiae is a popular yeast.

Clinical trials have shown that probiotics—primarily through altering the composition and/or activity of the gut microbiota—can help mitigate a wide variety of disorders, including inflammatory bowel disease (IBD), irritable bowel syndrome (IBS), metabolic syndrome, and type 2 diabetes (19).

Probiotics can also influence nervous system functioning and behavior. Growing evidence indicates that specific probiotic strains can effectively improve behavioral symptoms of various disorders such as anxiety and depression. As with most conditions, the benefits of probiotics in neurological and psychiatric disorders involve their interactions with beneficial gut bacteria (8).

A brand new study from Chinese researchers provides further evidence that specific gut bacteria can impact the brain. The study reported that mice with symptoms of Alzheimer’s disease were less likely to develop colorectal cancer. The researchers discovered that mice with low levels of Prevotella (a type of gut bacteria) were less prone to cognitive decline but more susceptible to developing colorectal cancer (22).

When choosing a probiotic supplement, look for products with multiple strains of Lactobacilli and Bifidobacteria, and a potency of at least a billion CFUs (colony forming units) per serving (23). Ideally, it should also include a prebiotic.

5. Reduce Stress

Stress—both acute and chronic—can significantly impact the gut-brain axis by causing profound changes in the composition of the gut microbiota. This can lead to dysbiosis (imbalances in gut bacteria) and low diversity, which, in turn, can compromise immune function and health (24).

In fact, an experimental study demonstrated that stress hormones can rapidly increase levels of certain harmful bacteria by 10,000-fold (24)!

A fascinating new study found that people who were more resilient to stress had gut bacteria that were associated with anti-inflammatory effects and improved gut barrier integrity (25). Thus, the composition of your gut microbiota could potentially impact your ability to cope with stress.

Some effective ways to de-stress include (26):

• Get enough sound sleep
• Exercise regularly
• Stay socially connected
• Eat regular, well-balanced meals
• Practice mindfulness (paying attention to the present moment)
• Journaling
• Try controlled breathing
• Consider meditation

Practicing mindfulness weekly for three months has been shown to significantly improve cognitive functions in elderly patients with mild cognitive impairment (MCI). These improvements coincided with favorable changes in the abundance and diversity of gut microbiota (27).

Long-term meditation has also been demonstrated to increase the diversity and quantity of gut bacteria (including Prevotella bacteria). These changes in the microbiota were associated with a lower risk of anxiety and depression (28).