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Serotonin

Serotonin, also known as 5-hydroxytryptamine or 5-HT, is crucial for gut function and bowel movements. A staggering 95% of serotonin is generated in the gut, where it plays a key role in regulating motility (the movement of the intestines), secretion, and the sensitivity of the digestive system.

Given your condition of IBS-D (irritable bowel syndrome with Diarrhea), along with food intolerances and other gut issues, grasping the connection between serotonin and diarrhea can significantly aid in managing your symptoms more effectively.

How Does Serotonin Impact Digestion and Bowel Movements?

Serotonin has a profound effect on the enteric nervous system (the nervous system of the gut) and governs:

Intestinal Motility (Speed of Digestion)

• Elevated serotonin levels = Quicker bowel movements (diarrhea)

• Reduced serotonin levels = Slower bowel movements (constipation)

Intestinal Secretions

• Serotonin enhances the secretion of fluids into the intestines.

• An excess of serotonin can lead to watery diarrhea, while insufficient amounts can result in dry stools and constipation.

Gut Sensitivity (Pain & Bloating)

• Serotonin influences the perception of gut pain, making some individuals more susceptible to bloating and discomfort.

• IBS-D is associated with elevated serotonin levels, which can result in cramping, urgency, and frequent diarrhea.

High Serotonin and Diarrhea (IBS-D, Dumping Syndrome, etc.)

• Frequent, urgent diarrhea (typical in IBS-D)

• Loose, watery stools

• Post-meal diarrhea (Dumping Syndrome)

• Abdominal pain, bloating, and gas

• Nausea and discomfort

What Causes This?

• IBS-D (irritable bowel syndrome with Diarrhea)

• Individuals with IBS-D tend to have an excess of serotonin in the gut, which leads to heightened intestinal contractions and rapid stool passage.

• This phenomenon accounts for the frequent and unpredictable episodes of diarrhea.

Serotonin Syndrome (Excess Serotonin from Medications)

• Certain antidepressants (SSRIs such as fluoxetine and sertraline) can elevate serotonin levels, potentially causing or exacerbating diarrhea.

• Additionally, 5-HTP (a supplement that boosts serotonin) may also induce diarrhea when consumed in high doses.

Dumping Syndrome (Post-Meal Diarrhea)

• Some individuals undergo rapid digestion following meals, which is triggered by an overproduction of serotonin.

• This results in sudden diarrhea occurring within 30-60 minutes after eating.

Serotonin-Secreting Tumours (Carcinoid Syndrome – Rare)

• In uncommon instances, tumors known as carcinoid tumors generate excess serotonin, resulting in chronic diarrhea, flushing, and heart complications.

• If diarrhea persists, it may be necessary to conduct tests for serotonin levels.

Low Serotonin and Constipation (IBS-C)

• Characterized by slow digestion and infrequent bowel movements,

• Hard, dry stools,

• Bloating and discomfort,

• Straining during bowel movements.

IBS-C (Irritable Bowel Syndrome with Constipation) is associated with low serotonin levels, which leads to sluggish bowel function.

The Cycle of IBS-D and Serotonin Fluctuations

• Numerous individuals with IBS-D face alternating episodes of diarrhea and constipation due to imbalances in serotonin levels.

• High serotonin → Diarrhea → The body compensates by reducing serotonin

• Low serotonin → Constipation → The body compensates by increasing serotonin

• This cycle continues, resulting in unpredictable bowel patterns.

Noradrenaline

There is a distinct link between increased levels of noradrenaline and the development or exacerbation of diarrhoea, especially in stress-related situations such as irritable bowel syndrome (diarrhoea predominant).

Noradrenaline serves as a key stress neurotransmitter released by the sympathetic nervous system during 'fight or flight' scenarios. While its primary function is to prepare the body for action (boosting alertness, heart rate, etc.), it also significantly influences the gastrointestinal (GI) system. In stressful situations, noradrenaline can:

• Enhance gut motility (causing food to transit too swiftly through the intestines)

• Modify fluid secretion and absorption, resulting in looser stools

• Disturb the gut microbiome, affecting digestion and immune response

• Heighten sensitivity of nerve endings in the gut, leading to increased pain and urgency.

In summary, elevated noradrenaline levels—particularly during emotional stress, anxiety, or panic—can result in a hyperactive gut, causing frequent, loose, or urgent bowel movements. This explains why individuals often suffer from 'nervous stomach' symptoms like diarrhoea before exams or public speaking engagements, or during prolonged periods of stress.

This gut-brain interaction is particularly pronounced in individuals with IBS-D, where both the gut and nervous system exhibit heightened sensitivity to stress and surges in noradrenaline.

Diarrhoea is not solely the result of infections or food; it frequently stems from hormonal imbalances induced by stress or disruptions in the nervous system.

Adrenaline

Adrenaline, the body's rapid-response stress hormone, triggers the 'fight-or-flight' mechanism. This often results in swift bowel movements as the body clears the gut in anticipation of perceived threats. Ongoing stress or anxiety that leads to repeated adrenaline surges can result in frequent, urgent, and loose stools—a typical pattern observed in IBS-D.

GABA (Gamma-aminobutyric acid)

GABA serves as the body’s natural relaxant. When GABA levels drop, the nervous system remains in an overstimulated state, which can lead to gut overstimulation. This disruption affects normal digestive processes and may cause spasms and diarrhea, particularly during times of emotional stress.

DHEA (Dehydroepiandrosterone)

DHEA plays a role in reducing inflammation and managing immune responses. A deficiency in DHEA can make the gut more sensitive and reactive. Low levels of DHEA are linked to gut inflammation, inadequate mucosal protection, and a higher risk of stress-related diarrhea or IBS symptoms.

Cortisol

Cortisol, known as the body’s long-term stress hormone, has a complex role. Initially, during stress, it can accelerate digestion, resulting in diarrhea. However, prolonged high levels of cortisol can harm the gut lining and upset electrolyte balance, leading to erratic bowel movements.

Glutamate

Glutamate is not only a neurotransmitter in the brain but also a crucial component of the enteric nervous system, often referred to as the “second brain” in the gut.

It interacts with glutamate receptors (NMDA, AMPA, mGluRs) that control:

• Intestinal motility
• Fluid secretion
• Sensory signaling (pain, urgency) within the digestive system

What Happens in Diarrhea?

Excessive stimulation of glutamate in the gut can result in:

• Hyperactive bowel contractions (rapid motility = decreased absorption time = loose stools)
• Increased secretion of chloride and water into the intestines, contributing to watery stools
• Visceral hypersensitivity, intensifying urgency and discomfort

Gut-Brain Stress Link:

Chronic stress → raises both central and peripheral glutamate levels

Excessive glutamate activity in the vagus nerve and enteric circuits can lead to:

• Diarrhea during or following anxiety or panic

• Worsening of IBS-D (Irritable Bowel Syndrome – Diarrhea Type)

• Inflammatory reactions that further heighten gut permeability and bowel sensitivity

Clinical & Research Insights:

• Patients with IBS-D frequently exhibit increased glutamate and NMDA receptor activity in the gut mucosa

• Research on animals indicates that NMDA receptor antagonists can alleviate diarrhea triggered by stress or infection

• Imbalances in the gut microbiome (dysbiosis) might enhance glutamate production by specific bacteria, exacerbating symptoms