Gut-Brain Axis: Understanding the Connection Between Serotonin and the Gut

Table of Contents

1. What Is the Gut-Brain Axis?
2. Serotonin: The Super Hormone and Mood Regulator
3. What Does Serotonin in the Gut Do?
4. How Gut Dysbiosis Disrupts Mental Health
5. Natural Solutions to Support the Gut-Brain Connection
6. Nurture Your Second Brain
7. FAQs

Have you ever felt butterflies in your stomach before a big presentation? Or experienced digestive upset during a stressful season of life? Most people have.

What many do not realize is that these experiences are not simply emotional responses. They are signs of a powerful biological relationship between your digestive system and your brain. This relationship is known as the gut-brain axis. It is one of the most fascinating systems in the body and one of the most important concepts in functional medicine.

Conventional medicine often treats the brain and digestive tract as separate systems. In my practice, I have found the opposite to be true. Your gut and brain are constantly communicating. When one suffers, the other often follows.

One of the most important messengers in this communication network is serotonin. While serotonin is commonly associated with mood, up to 95% of the body's serotonin is actually produced in the digestive tract. This makes the connection between serotonin and the gut impossible to ignore.

Understanding the gut-brain axis can help explain symptoms ranging from anxiety and depression to constipation, brain fog, and poor sleep. It can also help you uncover root causes and support long-term wellness.

What Is the Gut-Brain Axis?

The gut-brain axis is a communication network that connects your central nervous system and your digestive system.

The central nervous system, or CNS, includes the brain and spinal cord. It coordinates many of the body's automatic functions, including stress responses, mood regulation, and hormone signaling.

The digestive tract contains its own nervous system called the enteric nervous system, or ENS. This system contains an estimated 200 to 600 million neurons embedded throughout the intestinal wall. Because of its complexity, the ENS is often referred to as the body's "second brain."(1)

These two systems communicate continuously.

The primary communication pathway is the vagus nerve. This nerve acts as a direct connection between your gut and brain, carrying signals in both directions. Through the vagus nerve, the brain can influence digestion, and the gut can influence mood, cognition, and emotional health.

This is why I often emphasize the importance of supporting vagal tone. A healthy vagus nerve promotes healthy communication throughout the gut-brain connection. Learn more about vagus nerve connection on this episode of Take Back Your Health® podcast.

Stress also plays a major role in this relationship. Chronic stress can disrupt gut function, weaken the intestinal barrier, and alter communication along the gut-brain axis.

Serotonin: The Super Hormone and Mood Regulator

Serotonin is one of the body's most important chemical messengers.

It functions as both a hormone and a neurotransmitter. Its job is to help regulate mood, emotional balance, digestion, sleep, and many other processes.

Your body produces serotonin from an essential amino acid called tryptophan. Since the body cannot make tryptophan on its own, it must come from food.

Many conventional treatments for depression focus on increasing serotonin activity in the brain. Selective serotonin reuptake inhibitors, or SSRIs, work by slowing the reabsorption of serotonin between nerve cells. While this approach may support some individuals, functional medicine takes a broader view.

Since most serotonin is produced in the digestive tract, I believe it is equally important to evaluate gut health when addressing mood concerns. If the gut environment is unhealthy, serotonin production may be compromised.

Serotonin is often called the body's primary mood stabilizer. It works alongside other important neurotransmitters and hormones, including:

• Dopamine, which supports motivation and reward
• Endorphins, which promote feelings of comfort and resilience
• Oxytocin, which supports connection and bonding

Together, these messengers help regulate emotional and cognitive well-being.

What Does Serotonin in the Gut Do?

When most people think about serotonin, they think about mood. However, serotonin in the gut performs many critical functions throughout the body.

Supports Mood Regulation

Healthy serotonin levels help promote emotional balance, focus, and a sense of calm. Serotonin also interacts with the body's stress response system. It helps regulate cortisol production and supports a balanced response to daily stressors.

When serotonin production becomes impaired, symptoms such as anxiety, irritability, low mood, and brain fog may become more common.

Supports Digestive Motility

One of the most important roles of serotonin and the gut involves digestion.

Serotonin helps regulate peristalsis, the rhythmic contractions that move food through the digestive tract. Low serotonin activity may contribute to slower bowel movements and constipation.(2) Elevated serotonin activity may contribute to loose stools or urgency.

This relationship is one reason researchers continue to explore serotonin's role in irritable bowel syndrome, or IBS.

Supports Healthy Sleep

Serotonin is also involved in sleep regulation.

The pineal gland converts serotonin into melatonin, the hormone responsible for regulating the sleep-wake cycle.

This helps explain why low serotonin levels are often associated with poor sleep quality, insomnia, and mood challenges.

Supports Other Important Functions

Serotonin also contributes to:

• Healthy blood clotting and wound healing
• Bone metabolism and bone density
• Cellular communication throughout the body

Its influence extends far beyond emotional wellness.

How Gut Dysbiosis Disrupts Mental Health

The gut microbiota brain axis refers to the relationship between the trillions of microbes living in your digestive tract and your nervous system.

Your microbiome plays a major role in neurotransmitter production, immune regulation, and inflammation control. When the microbiome becomes imbalanced, a condition known as dysbiosis can develop. Dysbiosis may influence both gut health and mental health.

SIBO and Chronic Inflammation

Small intestinal bacterial overgrowth, or SIBO, occurs when excessive bacteria accumulate in the small intestine.

These bacteria can produce compounds that increase histamine levels and trigger chronic inflammation.

Inflammation affects communication throughout the gut-brain axis and may contribute to symptoms such as anxiety, fatigue, mood changes, and difficulty concentrating.(3)

Candida Overgrowth and Brain Fog

Candida overgrowth can also disrupt the gut-brain connection. Excess Candida may irritate the intestinal lining and interfere with normal digestive function. It can also release metabolic byproducts that affect cognitive performance.

Many individuals with Candida overgrowth report symptoms such as:

• Brain fog
• Fatigue
• Poor concentration
• Mood fluctuations

When gut health improves, many people notice improvements in mental clarity and emotional resilience as well. This is one reason I always consider digestive health when evaluating concerns related to mood, focus, and overall wellness.

Natural Solutions to Support the Gut-Brain Connection

Supporting the gut-brain axis requires a comprehensive approach.

Here are some of my favorite strategies.

Eat Tryptophan-Rich Foods

Since serotonin is produced from tryptophan, consuming adequate protein is important.

Focus on nutrient-dense, AIP-friendly options such as:

• Turkey
• Wild-caught fish
• Grass-fed and pasture-raised meats
• Organ meats
• Bone broth

These foods provide amino acids and nutrients that support healthy neurotransmitter production.

Prioritize Stress Management

Stress directly affects communication along the gut-brain axis.

Daily practices may include:

• Deep breathing exercises
• Prayer or meditation
• Gentle movement
• Time outdoors
• Adequate sleep

Consistent stress management supports both digestive and emotional wellness.

Get Daily Sunlight and Exercise

Regular movement and natural light exposure help support healthy serotonin production. Aim for daily outdoor activity whenever possible. Even a short walk can provide meaningful benefits.

Support Your Microbiome With Targeted Probiotics

A healthy microbiome is essential for a healthy gut-brain axis.

For daily maintenance support, I often recommend Probiotic 30 Billion. For individuals seeking more comprehensive microbiome support, Probiotic 100 Billion offers a higher potency option.

For those addressing SIBO or bacterial overgrowth concerns, I often recommend Primal Earth Probiotic, a soil-based probiotic formula.

Nurture Your Second Brain

Your gut and brain are in constant communication. When you support your digestive health, you also support your mood, focus, sleep, and resilience.

Addressing the gut-brain axis is a foundational part of The Myers Way®. By nourishing your microbiome, reducing inflammation, and supporting optimal serotonin production, you can create a stronger connection between your gut and your mind.

FAQs

• What is the gut-brain axis, and how does it work?
  The gut-brain axis is the complex, bidirectional physical and biochemical communication network that connects your central nervous system (brain and spinal cord) with your enteric nervous system (the network of neurons lining your gut). These two systems communicate continuously through hormones, neurotransmitters, and electrical signals. The primary physical connection is the vagus nerve, which serves as a communication superhighway allowing gut bacteria and digestive processes to influence brain function and vice versa.
• How does serotonin in the gut affect my mental health?
  While serotonin is well known as a brain chemical that regulates mood and anxiety, up to 95% of your body's serotonin is produced and stored in your gut. Specialized cells in the intestinal lining synthesize serotonin from the amino acid tryptophan. If your gut is inflamed or compromised by infections, your ability to produce serotonin is impaired, which directly reduces the amount of this feel-good neurotransmitter available to travel via the vagus nerve to your brain, impacting your overall mood and mental health.
• Can gut infections such as SIBO or Candida overgrowth cause anxiety and depression?
  Yes. Through the gut microbiota brain axis, your microbiome directly influences your brain function. SIBO leads to an overgrowth of bacteria that produce excess histamine, triggering an inflammatory response that can manifest as anxiety and panic. Candida overgrowth coats the intestinal tract, physically suppressing your gut's ability to produce serotonin, and releases toxic by-products that cross the blood-brain barrier, contributing directly to feelings of depression, fatigue, and brain fog.
• How do probiotics support the gut-brain connection?
  Probiotics introduce beneficial bacterial strains that help restore balance to your microbiome, crowd out opportunistic yeast and harmful bacteria, and support the health of your intestinal lining. By reducing gut inflammation, probiotics improve your body's natural capacity to produce and utilize serotonin and other key neurotransmitters. Choosing a high-quality daily formula such as Probiotic 100 Billion or a soil-based formula like Primal Earth Probiotic™ helps maintain this delicate communication.
• What is the relationship between stress, cortisol, and serotonin?
  When you experience chronic stress, your adrenal glands produce high levels of cortisol to help your body manage the perceived threat. While brief cortisol spikes are natural, prolonged elevation can exhaust your system and impair your serotonin receptors. Serotonin helps modulate your body's stress response and regulate cortisol levels, but chronic stress can overwhelm this mechanism, leading to adrenal fatigue, gut permeability, and depleted serotonin production, which often culminates in mood imbalances.