Serotonin is mainly made in the gut, so can it affect the brain? The role of serotonin in IBS symptoms, mood, and the stress response.

The Crucial Role of Serotonin in Gut and Brain Health

Serotonin is often called the "feel-good" hormone due to its well-known influence on mood and emotional well-being, but its functions extend far beyond the brain. Interestingly, approximately 90% of the body's serotonin is produced in the enterochromaffin cells in the intestinal villi of the gut, where it plays a vital role in digestive health and function. People with IBS may have disturbances with how much serotonin is synthesised, resulting in bowel changes and abdominal visceral pain.

Serotonin (5-hydroxytryptamine, or 5-HTP) is also one of the most significant neurotransmitters involved in gut–brain communication.

A recently published paper (Hwang & Oh, 2025) suggests that  gut-derived serotonin activates the vagus nerve, transmitting signals to the brainstem and then higher brain. This interaction influences emotional regulation, stress responses, and immune modulation. Emerging evidence also highlights the role of gut microbiome metabolites, particularly short-chain fatty acids (SCFAs), in enhancing serotonin synthesis and vagal activity, thereby shaping gut–brain communication.

Understanding how serotonin influences both the gut and brain is essential for appreciating its importance in overall health, as well as recognising the potential consequences when its levels are imbalanced.

The Multifaceted Functions of Serotonin

Serotonin serves as both a neurotransmitter in the brain and a hormone in the body, orchestrating a wide array of physiological processes:

• Regulation of Mood: Serotonin in the brain influences feelings of happiness, anxiety, and overall emotional state. This is why selective serotonin reuptake inhibitors (SSRIs), which increase serotonin activity in the brain, are commonly prescribed for depression and anxiety.

• Pain and Sensory Perception: It modulates how we perceive pain and sensory input, contributing to the body's response to various stimuli.

• Digestive Function: In the gut, serotonin regulates bowel movements and general gastrointestinal function. Disruptions in serotonin levels are involved in conditions like irritable bowel syndrome (IBS), where symptoms may worsen or improve depending on serotonin balance.

• Appetite Control and Bowel Movements: The gut-produced serotonin influences hunger and digestion, affecting bowel motility and regularity.

• Nausea and Vomiting: Serotonin in the gastrointestinal tract signals nausea and triggers the vomiting reflex in response to illness, food toxins, or other irritants.

• Sleep-Wake Cycles: Serotonin impacts sleep regulation, influencing waking and sleep patterns.

  
  
  
  

  

Supporting healthy Serotonin levels: The Key to Health

Having the right amount of serotonin is crucial. Chronic gut issues, poor diet, or damaged gut lining can impair serotonin production, affecting mood and cognitive function as well as digestive function and pain.

Gut bacteria produce the metabolites called short-chain fatty acids (SCFAs): acetate, proprionate and butyrate, which have been shown to enhance the expression of tryptophan hydroxylase 1 (TPH1), the enzyme involved in serotonin synthesis. Here is four ways to help our microbiome make these metabolites and regulate the production of serotonin.

1. Aim for a diet rich in various fibre-rich foods. Short-chain fatty acids such as butyrate are produced when the gut microbiome ferments dietary fibre

2. Incorporate resistant starch: Cooked and cooled potatoes, rice, and green bananas are good sources of resistant starch, which is fermented by butyrate-producing bacteria. 

3.  Fermented foods like sauerkraut and kimchi, and yoghurt as well , can also promote butyrate production.

4. If dietary changes are not enough, you can explore prebiotic supplements like partially hydrolyzed guar gum or inulin, but consult with a healthcare professional before adding supplements as they encourage the growth of aa wide variety of bacteria.

Conclusion

Serotonin is a vital chemical messenger with a dual role as both a neurotransmitter and a hormone, impacting mood, digestive health, sleep, and more. Maintaining optimal serotonin levels—through diet, and gut health—can significantly influence overall well-being. Recognising the interconnectedness of gut and brain health highlights the importance of a holistic approach to wellness, ensuring that serotonin functions effectively across the body for both mental and physical health.

Hwang YK, Oh JS. Interaction of the Vagus Nerve and Serotonin in the Gut-Brain Axis. Int J Mol Sci. 2025 Jan 29;26(3):1160. doi: 10.3390/ijms26031160. PMID: 39940928; PMCID: PMC11818468.