The Gut Microbiome’s Role in Mental Health

Ever wonder why your mood sometimes sinks for no reason? The answer might lie deep in your gut, literally. While most people know serotonin as the brain’s “feel-good” chemical, fewer realize that nearly 90% of it is made in the gut. Even more surprising is that your gut bacteria play a major role in how much serotonin your body can produce, thanks in part to a microbial metabolic route called the shikimate pathway. Let’s explore how your microbes may be influencing your mood.

What is serotonin?
Serotonin is a neurotransmitter, it instructs your nervous system to make a change. In the realm of mental health, it has a potent calming and mood lifting effect. There are other functions of this neurotransmitter however, it also helps with bowel motility, how we sense intestinal pain, immune regulation, cardiovascular tone, sleep quality, hormone balance and even reproduction.

Serotonin is made in the brain and the gut

Serotonin is made in the Central Nervous System (Brain) in the Raphe nuclei in the brainstem neurons. Serotonin that is made in the brainstem is the true “feel-good” messenger. It plays a key role in lifting your mood, supporting deep sleep, calming pain, balancing appetite, regulating temperature, and sharpening focus. This is the serotonin that directly influences how you feel mentally and emotionally. But here’s the fascinating part, it has to be made right there in the brain, because serotonin can’t cross the blood-brain barrier. That means your body depends on the right building blocks (such as the amino acid tryptophan from meat, eggs, legumes) and signals to keep this internal mood engine running smoothly.

Serotonin is made in the Gastrointestinal Tract (Gut) mainly in the Enterochromaffin cells of the small intestine using tryptophan, an amino acid. This valuable chemical we get from food and from the good bacteria that live in the large intestine via the shikimate pathway. 90–95% of total body serotonin is made in the gut.

Serotonin in the gut does so much more than most people realize. While it doesn’t directly control your mood, it plays a powerful role in helping to keep digestion smooth, digestive enzymes flowing, and your immune system balanced. It also sends messages up to your brain through the vagus nerve, creating a strong gut-brain connection. Gut serotonin helps set the stage for how your body feels and how your brain responds. I’ll explain more about how this works below.

What is the Gut-Brain Axis?

Here are a few concrete examples of how gut-derived serotonin (which does not cross into the brain) can still influence mood and brain function indirectly via the gut-brain axis, vagus nerve, and immune signaling:


Vagus Nerve Activation is Brain Feedback
When gut serotonin levels rise (e.g., after a meal), it stimulates local enteric neurons which have a calming vagal feedback that can modulate anxiety and depression, even without serotonin entering the brain. It does so by signalling the brain neurons to make more receptors for serotonin to have its effect.

Serotonin Balances Our Immune Response
When gut serotonin interacts with immune cells in the intestinal wall (like dendritic cells and macrophages) it can trigger pro or anti inflammatory cytokine release. When the gut is chronically inflamed, serotonin triggers inflammatory cytokines which can cause anxiety and depression. Some things that cause our gut to be inflamed are eating processed food, high sugary foods, drinking excessive amounts of alcohol, certain medications such as aspirin and ibuprofen, and chronic stress.

Microbiome and GABA Which Makes Us Calm
Some gut bacteria respond to serotonin levels by changing their gene expression and enhance particular types of good bacteria to grow. For example, serotonin can enhance the growth of spore-forming bacteria that produce metabolites like GABA that affect brain chemistry in a calming way, even without serotonin crossing the blood brain barrier.

Intestinal Serotonin Affects Pain and Sensory Processing
Intestinal serotonin plays a key role in how we sense and respond to digestive activity. When levels are balanced, digestion flows smoothly, gut sensations feel normal, and mood remains steady. But when serotonin is too high, it can lead to cramping, diarrhea, and heightened gut sensitivity. When we have too low serotonin, it can cause constipation, sluggish digestion, and a dull or disconnected feeling in both body and mind.

The Essential Support for Mental Health

As we have discussed, serotonin production is controlled partly by the health of the gut microbiome and the health of the gut in general. When the gut is healthy, you have a proper amount of good bacteria (Lacto, Bifido, etc), your lining is strong, you have a thick protective mucus layer and very little inflammation.

Some things that create an unhealthy gut:
Infection
Chronic stress, trauma
Poor diet including processed foods or non-organic foods, lacking in whole fresh organic foods
Toxin exposure
Excessive alcohol use

When the gut lining is damaged it becomes porous and weak. This allows particles of dead bacteria called lipopolysaccharides (LPS) into the blood which causes a major inflammatory reaction and higher levels of cytokines IL-6, IFNg, and TNFa. These circulating inflammatory cytokines cause a tryptophan shift from making serotonin to making IDO instead. IDO feeds the much needed immune functions that have been called into action by the cytokines. However, IDO then causes a higher amount of neurotoxins that directly cause depression and or anxiety.

Even mild overgrowth of yeast (like Candida) or opportunistic bacteria (like Klebsiella or Proteus) in the gut can produce toxic metabolites, such as acetaldehyde or LPS. These byproducts can irritate the gut lining, damaging it and making it more permeable (“leaky”), which allows inflammatory signals to reach the brain via the bloodstream or vagus nerve. The result is a low-grade, chronic inflammation that reduces serotonin availability and activates the brain’s stress circuitry, contributing to symptoms like anxiety, low mood, brain fog, and sleep issues—even in the absence of overt gut symptoms.

Summary

In closing, understanding the microbiome’s influence on serotonin shifts how we think about depression and anxiety. It’s not just a chemical imbalance in the brain, it may also be a microbial imbalance in the gut. Supporting beneficial bacteria, avoiding microbiome disruptors like eating non-organic foods containing toxic pesticides and herbicides, and fueling the system with nutrient-dense, organic, fiber-rich foods can be powerful tools for your mental health. The next time you're feeling low, don’t just ask what’s going on in your head, ask what’s happening in your gut!


Wendy Wells, NMD