The Gut-Brain Connection: How it Works and The Role of Nutrition

March 01, 2021 6 min read

Nervousness or feelings of anxiety are associated with "butterflies in the stomach."

Hearing terrible news or anticipating it often leads to a "gut feeling."

Many athletes have described an extra urge for number two before significant events.

Just thinking about a good meal can get your stomach grumbling.

These experiences are not just coincidences or all in your head; the brain and stomach have a direct connection.

The gut-brain connection goes both ways, your thoughts and emotions can influence how your stomach feels and works, and the gut sends signals to the brain as well.

Today, we'll explore the crucial gut-brain axis and how you can improve it for better mental and physical wellbeing.

What's the Gut-Brain Connection?

Your gut and brain communicate via a network called the gut-brain axis¹. The gastrointestinal tract has the enteric nervous system (ENS), often referred to as the "second brain." The ENS controls several gut functions and communicates closely with the spinal cord and brain's central nervous system. There are several physical and biochemical connections between the gut and brain organs.

Vagus Nerve

The 100 billion neurons in the brain and central nervous system control the body's behavior. We often talk about neurons connected with the brain, but the brain is not the only place to find neurons. The guy contains 500 million neurons that are connected to your brain through the nervous system².

The gut and brain are connected by the vagus nerve, which signals in both directions. Animal studies have found that stress disrupts the signals sent through the vagus nerve and causes gut problems³. One study of mice examined how a probiotic impacted the stress hormone in their blood⁴. When the mice were given a probiotic, it reduces the blood's stress hormone content, but the probiotic has no effect when the vagus nerve was cut.

However, there's also been a study in humans⁵. This study found that those with gastrointestinal problems (like IBS or Crohn's disease) had reduced vagal tone. The reduced vagal tone indicated a lower function of the vagus nerve.

Gut-Brain Neurotransmitters

Neurotransmitter chemicals also connect the gut and brain. Often, we think of neurotransmitters for their role in controlling feelings and emotions.

For example, the neurotransmitter GABA is connected with a calming effect, and abnormalities in GABA are associated with feelings of anxiety and depression⁶.

The gut also produces many neurotransmitters, including most of your serotonin⁷. Serotonin is associated with your internal clock and feelings of happiness.

GABA is also produced in the gut and can help regulate feelings of anxiety and fear⁸. One study in mice found that some probiotics can boost GABA production, resulting in less behavior associated with anxiety and depression⁹.

Gut Microbes and the Brain

Your gut also contains trillions of microbes that produce chemicals that affect the brain. For example, when you digest fiber, your gut microbes produce short-chain fatty acids (SCFA) like propionate, acetate, and butyrate¹⁰. One fundamental way SCFAs affect the brain is by reducing appetite, which makes sense. After you eat, your stomach must communicate with your brain that you're full and no longer hungry. The phenomenon has been demonstrated in one study that found when people consume propionate, it reduces food intake and the activity in the brain associated with reward from high-energy food¹¹.

The SCFA butyrate is associated with forming the blood-brain barrier¹².

Another role of gut microbes is to metabolize amino acids and bile acids to produce other chemicals. Those chemicals affect the brain as well. For example, bile acids made by the liver can affect the brain. Studies in mice found that stress and social disorders lower the production of gut bacteria bile acids¹³. More so, stress and social disorders altered the genes associated with bile acid production.

Immune System

The immune system also connects the gut and brain. Gut microbes control what is passed into the body vs. what is excreted, which means that the gut is essential for your immune system and inflammation levels¹⁴.

When the immune system is active for too long, it causes inflammation associated with feelings of depression and other mental health issues¹⁵. If the gut barrier becomes leaky, it can release too much lipopolysaccharide into the blood. High LPS levels and inflammation are connected with severe mental health issues¹⁶.

Changing Gut Bacteria Can Impact Brain Health

Yes, you can alter your gut bacteria to improve your mental health. Probiotics are live bacteria that have several health benefits when consumed, but there are several different probiotics.

Psychobiotics are probiotics known for affecting the brain¹⁷. Probiotics can improve stress and feelings of depression and anxiety. For example, one study examined people with IBS and mild-to-moderate feelings of anxiety or depression. The study found that taking the probiotics Bifidobacterium longum NCC3001 for six weeks drastically reduced symptoms¹⁸.

On the other hand, prebiotics is fibers that your gut ferments. Prebiotics may also impact the brain. According to one study, taking the prebiotics galactooligosaccharides for three weeks reduced cortisol (stress hormone) levels in the body¹⁹.

The Best Foods for the Gut-Brain Axis

The gut-brain connection means that what you put into your stomach can affect your brain, and your brain can affect how your stomach works. Different foods impact the gut-brain axis differently. Here are some of the best foods for your gut-brain axis.

High-fiber foods. Foods with prebiotic fibers reduce cortisol in humans¹⁹. Some high-fiber foods with prebiotic fibers are nuts, whole grains, seeds, fruits, and vegetables.
Omega-3 fats. Studies have shown that omega-3 fatty acids promote good bacteria in the gut and lower the risk of brain disorders²⁰. Oily fish are a good source of omega-3.
Fermented foods. Fermented foods contain healthy microbes and alter brain activity²¹. Try cheese, sauerkraut, yogurt, and kefir.
Tryptophan-rich foods. The brain converts tryptophan into serotonin²². Turkey, cheese, and eggs are good sources of tryptophan.
Polyphenol-rich foods. Polyphenols promote good gut bacteria, improving cognition²³. Some foods with polyphenol include green tea, olive oil, cocoa, and coffee.

Tips to Improve the Gut-Brain Axis Health

Eat a well-balanced diet filled with nutrient-dense foods. Limit consumption of added sugar and processed foods.
Monitor how different foods make you feel.
If you experience digestion or gastrointestinal issues, consider the role that stress or feelings of depression may be playing.
Exercise regularly. Exercise has been shown to promote gut bacteria diversity²⁴.
Evaluate your medications. Antibiotics can limit the diversity of the microbiome, so only take them when necessary.
Reduce stress. Stress can disrupt your gut and wreak havoc on the gut-brain axis. Some ideas for managing stress include journaling, meditation, progressive muscle relaxation, and deep breathing.

Research for the gut-brain axis has already shown the power of the connection, but the research is ongoing. Ultimately, mental and physical health are two parts of comprehensive wellness, and they never exist in isolation. By focusing on the bacteria in your gut, you can improve your mental health. Follow the tips above and incorporate foods omega-3, probiotics, polyphenol, and fermentation to improve the gut-brain axis.

(1) Carabotti, Marilia et al. (2015) The gut-brain axis: interactions between enteric microbiota, central and enteric nervous systems

(2) Mayer, EA (2011). Gut feelings: the emerging biology of gut-brain communication

(3) Sahar, T; Shalev, AY; Porges, SW. (2001). Vagal modulation of responses to mental challenge in posttraumatic stress disorder

(4) Bravo, Javier A et al. (2011). Ingestion of Lactobacillus strain regulates emotional behavior and central GABA receptor expression in a mouse via the vagus nerve

(5) Pellissier, Sonia et al. (September 2014) Relationship between vagal tone, cortisol, TNF-alpha, epinephrine and negative affects in Crohn's disease and irritable bowel syndrome

(6) Petty, F (1995). GABA and mood disorders: a brief review and hypothesis

(7) Anguelova, M; Benkelfat, C; Turecki, G. (2003). A systematic review of association studies investigating genes coding for serotonin receptors and the serotonin transporter: I. Affective disorders

(8) Mazzoli, R; Pessione, E. (2016). The Neuro-endocrinological Role of Microbial Glutamate and GABA Signaling

(9) Janik, Rafal et al. (2016) Magnetic resonance spectroscopy reveals oral Lactobacillus promotion of increases in brain GABA, N-acetyl aspartate and glutamate

(10) Ríos-Covián, David et al. (February 2016) Intestinal Short Chain Fatty Acids and their Link with Diet and Human Health

(11) Byrne, Claire S et al. (2016) Increased colonic propionate reduces anticipatory reward responses in the human striatum to high-energy foods

(12) Bourassa, Megan W et al. (2016). Butyrate, neuroepigenetics and the gut microbiome: Can a high fiber diet improve brain health?