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How does irritable bowel syndrome lead to stomach cramping and diarrhea?

It's not known exactly what causes irritable bowel syndrome. The walls of the intestines are lined with layers of muscle that contract and relax in a coordinated rhythm as they move food from your stomach through your intestinal tract to your rectum. If you have irritable bowel syndrome, the contractions may be stronger and last longer than normal. Food is forced through your intestines more quickly, causing gas, bloating and diarrhea. In some cases, the opposite occurs. Food passage slows, and stools become hard and dry.

Abnormalities in your nervous system or colon also may play a role, causing you to experience greater than normal discomfort when your intestinal wall stretches from gas. There are a number of other factors that may play a role in IBS. For example, people with IBS may have abnormal serotonin levels. Serotonin is a chemical messenger that's normally associated with brain function, but it also plays a role in normal digestive system function. It's also possible that people with IBS don't have the right balance of good bacteria in the intestine.

The Rome III criteria define Irritable Bowel Syndrome (IBS) as “recurrent abdominal pain/discomfort occurring for at least 3 days per month in previous 3 months (onset of symptom should be 6 months prior to diagnosis)  and the pain is often associated with 2 or more of following symptoms:

  1. Improvement (not relieved) with defecation
  2. Onset associated with change in frequency of stool
  3. Onset associated with change in form/appearance of stool”

Four subtypes of IBS have been described including a diarrhea subtype (IBS-D) when loose or watery stools are reported for ≥25% of the bowel movements or hard or lumpy stools occur <25%.

The mechanisms of diarrhea and abdominal pain may include:

  • abnormal intestinal motility
  • increased visceral sensitivity (or hypersensitivity/hyperalgesia)
  • increased perception (hypervigilance - the symptoms are amplified)
  • symptoms may be preceded by an acute infectious diarrhea event (post-infectious)
  • change in the bacterial flora or an increased number of bacteria in the small intestine (small intestinal bacterial overgrowth)
  • immune modulation
  • intolerance to certain foods (which has led to the use of FODMAPS diet or lactose or gluten elimination diets)

Patients have correlated increased symptoms (abdominal discomfort and /or diarrhea) when they are stressed.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.