Diarrhoea

Serotonin

Serotonin (5-hydroxytryptamine or 5-HT) plays a major role in gut function and bowel movements. 95% of serotonin is produced in the gut, and it regulates motility (movement of the intestines), secretion, and sensitivity of the digestive tract.

Since you have IBS-D (irritable bowel syndrome with Diarrhea), food intolerances, and gut issues, understanding the serotonin-diarrhea connection can help you manage symptoms better.

How does Serotonin Affect Digestion and Bowel Movements?

Serotonin influences the enteric nervous system (gut’s nervous system) and controls:

Intestinal Motility (Speed of Digestion)

• High serotonin = Faster bowel movements (diarrhea)

• Low serotonin = Slower bowel movements (constipation)

Intestinal Secretions

• Serotonin increases fluid secretion into the intestines.

• Too much serotonin causes watery diarrhea, while too little leads to dry stools and constipation.

Gut Sensitivity (Pain & Bloating)

• • Serotonin affects gut pain perception, making some people more sensitive to bloating and discomfort.

• • IBS-D is linked to excessive serotonin, leading to cramping, urgency, and frequent diarrhea.

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

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

• Loose, watery stools

• post-meal diarrhea (Dumping Syndrome)

• Abdominal pain, bloating, and gas

• Nausea and discomfort

Why Does This Happen?

• IBS-D (irritable bowel syndrome with Diarrhea)

• People with IBS-D have excess serotonin in the gut, leading to overactive intestinal contractions and rapid stool movement.

• This explains the frequent, unpredictable diarrhea episodes.

Serotonin Syndrome (Excess Serotonin from Medications)

• Some antidepressants (SSRIs like fluoxetine, sertraline) increase serotonin, which can cause or worsen diarrhea.

• 5-HTP (a serotonin-boosting supplement) may also cause diarrhea if taken in high doses.

Dumping Syndrome (Post-Meal Diarrhea)

• Some people experience rapid digestion after meals, triggered by excessive serotonin release.

• This causes sudden diarrhea within 30-60 minutes of eating.

Serotonin-Secreting Tumours (Carcinoid Syndrome – Rare)

• In rare cases, tumours called carcinoid tumours produce excess serotonin, leading to chronic diarrhea, flushing, and heart issues.

• If diarrhea is persistent, tests for serotonin levels may be needed.

Low Serotonin and Constipation (IBS-C)

• Slow digestion and infrequent bowel movements

• Hard, dry stools

• Bloating and discomfort

• Straining during bowel movements

IBS-C (irritable bowel syndrome with Constipation) is linked to low serotonin, leading to sluggish bowels.

The Cycle of IBS-D and Serotonin Fluctuations

• Many people with IBS-D experience alternating diarrhea and constipation due to serotonin imbalances.

• High serotonin → Diarrhea → Body compensates by lowering serotonin

• Low serotonin → Constipation → Body compensates by raising serotonin

• Cycle repeats, leading to unpredictable bowel habits

Noradrenaline

There is a clear correlation between elevated noradrenaline levels and the onset or worsening of diarrhoea, particularly in stress-related conditions like irritable bowel syndrome (Diarrhoea predominant).

Noradrenaline is a major stress neurotransmitter released by the sympathetic nervous system during “fight or flight” responses. While its role is to prepare the body for action (increasing alertness, heart rate, etc.), it also has significant effects on the gastrointestinal (GI) tract. Under stress, noradrenaline can:

• Increase gut motility (causing food to move too quickly through the intestines)

• Alter fluid secretion and absorption, leading to looser stools

• Disrupting the gut microbiome, impacting digestion and immunity

• Sensitize nerve endings in the gut, increasing pain and urgency

In short, when noradrenaline levels are high—especially during emotional stress, anxiety, or panic—it can cause the gut to become hyperactive, leading to frequent, loose, or urgent bowel movements. This is why people often experience “nervous stomach” symptoms like diarrhea before exams or public speaking or during periods of chronic stress.

This gut-brain response is especially exaggerated in people with IBS-D, where both the gut and nervous system are hypersensitive to stress and noradrenaline surges.

Diarrhea isn’t always caused by infection or food—it’s often the result of hormonal imbalances triggered by stress or nervous system disruption.

Adrenaline

Adrenaline, the body’s fast-acting stress hormone, activates the “fight-or-flight” response. This often triggers rapid bowel movements as the body clears the gut in preparation for perceived danger. Chronic stress or anxiety that causes repeated adrenaline spikes can lead to frequent, urgent, and loose stools—a common pattern in IBS-D.

GABA (Gamma-aminobutyric acid)

GABA is your body’s natural calming agent. When GABA levels are low, the nervous system stays in a heightened, overactive state, which overstimulates the gut. This disrupts normal digestive rhythms and can result in spasms and loose stools, especially during emotional distress.

DHEA (Dehydroepiandrosterone)

DHEA helps reduce inflammation and regulate immune responses. When DHEA is deficient, the gut becomes more sensitive and reactive. Low DHEA levels are associated with gut inflammation, poor mucosal protection, and increased vulnerability to stress-induced diarrhea or IBS symptoms.

Cortisol

Cortisol, the body’s long-term stress hormone, plays a dual role. In early stress stages, it can speed up digestion, leading to diarrhea. Over time, chronically high cortisol can damage the gut lining and disrupt electrolyte balance, causing unstable or unpredictable bowel patterns.

Glutamate

Glutamate is not just a brain neurotransmitter—it’s also a key player in the enteric nervous system (the “second brain” in your gut).

It acts on glutamate receptors (NMDA, AMPA, mGluRs) that regulate:

• Intestinal motility

• Secretion of fluids

• Sensory signaling (pain, urgency) in the digestive tract

What Happens in Diarrhea?

Excessive glutamate stimulation in the gut can lead to:

• Hyperactive bowel contractions (fast motility = reduced absorption time = loose stools)

• Increased chloride and water secretion into the intestines, which contributes to stool liquidity

• Visceral hypersensitivity, making the urgency and discomfort worse

Gut-Brain Stress Link:

Chronic stress → elevates central and peripheral glutamate levels

Glutamate overactivity in the vagus nerve and enteric circuits can cause:

• Diarrhea during or after anxiety or panic

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

• Inflammatory responses that further increase gut permeability and bowel sensitivity

Clinical & Research Insights:

• IBS-D patients often show elevated glutamate and NMDA receptor activity in gut mucosa

• Animal studies reveal that NMDA receptor blockers reduce diarrhea caused by stress or infection

• Gut microbiome imbalances (dysbiosis) may increase glutamate production by certain bacteria, worsening symptoms