Constipation

Serotonin (5-hydroxytryptamine or 5-HT) is a neurotransmitter that plays a vital role in digestion, mood regulation, and bowel movements. Since 95% of serotonin is produced in the gut, an imbalance in serotonin levels can lead to either constipation (IBS-C) or diarrhoea (IBS-D).

The Role of Serotonin in Digestion

Serotonin primarily functions within the enteric nervous system (ENS), also known as the "second brain" of the gut. It influences:

Peristalsis (Intestinal Motility)

• Serotonin signals the smooth muscles of the intestines to contract and push food forward.

• Proper serotonin function ensures a smooth and regular bowel movement.

Fluid Secretion & Absorption

• Serotonin stimulates the secretion of fluids and mucus, making stool softer and easier to pass.

• Low serotonin levels lead to hard, dry stools, contributing to constipation.

Gut-Brain Communication

• The gut and brain are connected via the vagus nerve and gut microbiota, both of which influence serotonin production.

• Low serotonin = stress, anxiety, and slowed digestion, leading to constipation.

Pain Sensitivity in the Gut

• Serotonin regulates pain perception in the intestines.

• Low serotonin increases gut hypersensitivity, causing abdominal discomfort and bloating.

Low Serotonin and Constipation: Mechanisms & Causes

• Slow Bowel Transit Time

• Slow Bowel Transit Time

• This delayed transit time allows excessive water absorption from stool, making it hard and difficult to pass.

Stress, Anxiety, and Depression

• Chronic stress & depression lower serotonin levels, affecting digestion.

• People with high stress often experience a “freeze” response in the gut, leading to constipation.

Gut Dysbiosis (Imbalance of Gut Bacteria)

• Healthy gut bacteria help regulate serotonin production.

• Overgrowth of bad bacteria (dysbiosis) can lead to serotonin depletion, resulting in constipation.

Diet & Nutritional Deficiencies

• Low intake of tryptophan-rich foods (precursor to serotonin) can limit serotonin production.

• Deficiencies in vitamins B6, B12, magnesium, and folate can also impair serotonin synthesis.

Medications Affecting Serotonin and Bowel Movements

• SSRIs (Selective Serotonin Reuptake Inhibitors) & Constipation

• SSRIs (like fluoxetine, sertraline) increase serotonin levels but may cause constipation by altering gut motility.

• Some people experience diarrhea instead, depending on how their gut reacts.

Opioids & Painkillers

• Opioids block serotonin release, slowing down digestion and worsening constipation.

• NSAIDs (like ibuprofen) may disrupt serotonin metabolism in the gut.

Medications for IBS-C & IBS-D

• Tegaserod (Zelnorm): A serotonin receptor agonist used to treat IBS-C by increasing serotonin signaling.

• Alosetron (Lotronex): A serotonin receptor antagonist used to treat IBS-D by reducing excessive serotonin activity.

Constipation

Constipation and difficulty concentrating are closely linked through the gut-brain axis, the communication network between your digestive system and your brain.

When you’re constipated, your body is under digestive stress. This can trigger low-grade inflammation, hormonal imbalances, and even disruptions in neurotransmitter production. The gut produces a large portion of key brain chemicals like serotonin (about 90%) and dopamine, both of which affect mood, focus, and cognitive clarity. If your gut isn’t functioning properly, your brain often struggles too, resulting in brain fog, poor concentration, and mental fatigue.

Additionally, constipation can lead to:

• Discomfort and bloating, which distract the brain and reduce mental sharpness.

• Toxin buildup from slowed elimination, which affects energy and clarity

• Increased stress and anxiety, which raise cortisol and noradrenaline, disrupting attention.

• Poor sleep, often linked with digestive discomfort, further worsens focus.

• A general sense of “why bother?”

People with conditions like IBS, dysbiosis, or sluggish bowel motility often report both digestive and cognitive symptoms.

Constipation is more than a digestive issue—it’s often a hormonal response to stress and imbalance. The gut and brain are directly connected, and your hormones play a powerful role in regulating bowel movements.

Adrenaline

Adrenaline, your body’s immediate stress responder, signals the nervous system to prioritize survival over digestion. During chronic stress, high adrenaline levels slow down gut motility, causing food to sit longer in the intestines. This results in harder stools, slower bowel movements, and long-term constipation.

GABA (Gamma-aminobutyric acid)

GABA is the brain’s calming neurotransmitter. It relaxes the nervous system and supports the parasympathetic “rest and digest” response. Low GABA levels keep your body in a state of tension and overdrive, leading to poor gut-muscle coordination and sluggish bowel activity—common in stress-related constipation.

DHEA (Dehydroepiandrosterone)

DHEA helps buffer the effects of stress and inflammation. When DHEA levels are low, inflammation in the gut may increase, and stress tolerance drops. This can impair gut lining integrity and slow peristalsis (the wave-like muscle contractions that move stool), contributing to chronic constipation.

Cortisol

Cortisol, the body’s main stress hormone, directly influences digestion. When cortisol is consistently elevated, it diverts blood flow away from the gut, suppressing digestive enzymes and motility. Over time, this leads to irregular or painful bowel movements. Low cortisol, often seen in adrenal fatigue, can also slow gut function, worsening constipation.