IBS: Where and Why Symptoms Occur

Symptoms of irritable bowel syndrome (IBS)—including abdominal pain, gas, diarrhea, and constipation—are thought to occur from many causes. The two major causes are changes in the motility (movement) of your gastrointestinal (GI) tract and changes in sensory perception.

Most people without IBS don’t sense the movement of food from their esophagus to their stomach and through their intestines. But people with IBS tend to be very aware and are more sensitive to discomfort during the process.

Illustration of a human digestive system

Where IBS Hurts

While intestinal, or bowel, problems are a trademark of IBS, symptoms can occur in any part of your GI tract.


Spasms in your esophagus can make it:

  • Hard to swallow
  • Feel like you have a lump in your throat


Indigestion due to poor motility (inability to properly contract) can cause:

  • Belly pain
  • Bloating
  • Nausea
  • A heartburn sensation
  • Burping
  • Regurgitation

Biliary System

Your biliary system makes bile to help your intestines digest food. If motility problems affect your biliary system—including your gallbladder and bile ducts—it can feel like a gallbladder attack. You may have:

  • Bloating
  • Nausea
  • Vomiting
  • Abdominal pain

Small Intestine

Sudden, abnormal spasms (muscle contractions) in your small intestine also can cause:

  • Gas
  • Bloating

Due to these spasms and those of the large intestine, IBS used to be called "spastic colon."

Large Intestine (Colon)

IBS symptoms in your colon vary depending on how motility is affected.

  • If food runs too quickly through your large intestine, you can have diarrhea. That’s because food residues aren’t staying long enough in your intestine for water to be absorbed in your body.
  • If food moves too slowly through your large intestine, the opposite problem—constipation—can occur because too much water is absorbed by your body.

Changing Diagnostic Methods

IBS is similar to a lot of digestive diseases, which can make diagnosis difficult. IBS has always been considered a “diagnosis of exclusion,” meaning other possible conditions have to be ruled out before it's diagnosed. That may be changing, though.

In guidelines published in 2021, the American College of Gastroenterology (ACG) recommends changing the IBS diagnostic process from one of exclusion to the opposite, which is called a "positive diagnosis." Instead of looking for what it's not, the ACG says, doctors should be able to diagnose IBS based on what they see.

However, the change in guidelines doesn't mean all doctors will suddenly change how they're diagnosing IBS, so it helps to be familiar with both methods so you understand what's going on.

Diagnosis of Exclusion: How it Goes

A gastroenterologist is the doctor most likely to diagnose IBS. When making a diagnosis of exclusion, they'll go over your symptoms and medical history, and then perform a range of blood tests and imaging of four different areas of your digestive system:

  1. Esophagus and stomach: An upper endoscopy uses a long, thin, and flexible scope to see inside the upper GI tract.
  2. Small intestine: A CT enterography, or a small bowel series, is a set of x-rays that can help detect conditions in the small bowel.
  3. Large intestine: A colonoscopy can give the most detailed look at the inside of the large intestine to detect or rule out conditions.
  4. Everything outside the GI tract: A CT scan and blood tests can detect or rule out other diseases that may be affecting your digestion.

You may be tested for a range of conditions, including:

  • Celiac disease: Blood antibody testing, genetic testing
  • Food allergies or sensitivities: Skin-prick tests, blood tests, imaging, an elimination diet
  • Enteric pathogens: Stool testing
  • Pelvic floor disorder: Anorectal physiology testing
  • Inflammatory bowel disease: Fecal calprotectin and/or fecal lactoferrin plus blood tests for inflammatory markers
  • Other inflammatory diseases: Blood tests including C-reactive protein (CRP) and erythrocyte sedimentation rate (ESR or sed rate)

The primary drawback of the exclusionary diagnostic method is that it can take a long time, which means it takes you longer to get started with the treatment process. It's also a rather expensive process.

Positive Diagnosis: How it Goes

The ACG says doctors should be able to diagnose IBS based on a few simple things:

  • Your medical history
  • A physical exam
  • Your symptoms
  • Minimal diagnostic testing

The IBS symptoms doctors look for include abdominal pain, abnormal bowel habits, at least a six-month duration of symptoms, and the absence of "alarm features" (symptoms that could indicate a more serious condition.)

The ACG recommends that only the following tests be performed, and only in certain situations:

  • If diarrhea-predominant IBS (IBS-D) is suspected: Fecal calprotectin or fecal lactoferrin and CRP to rule out IBD; Blood tests to rule out celiac disease.
  • If constipation-predominant IBS (IBS-C) is suspected: Anorectal physiology testing if you have symptoms of a pelvic floor disorder and your constipation doesn't respond to standard therapy.
  • If food allergy/sensitivity symptoms are present and reproducible: Standard testing for food allergies/sensitivities

What Isn't Included?

The positive diagnosis method eliminates a lot of tests that are otherwise routinely performed, including:

  • Automatic testing for food allergies/sensitivities
  • Automatic anorectal physiology testing
  • Sed rate blood tests for inflammation
  • Automatic stool testing for infectious agents
  • Automatic colonoscopy and other imaging tests for those under 45 who don't have alarm symptoms

The ACG says this process will save both time and money and—most importantly—get you on treatments faster than the old method. Even so, both methods are considered valid and should give you a firm diagnosis that paves the way for proper treatment and management.

When New Symptoms Arise

If you have IBS, never assume that new GI symptoms are just part of your chronic condition. Always have new symptoms, like unexpected weight loss or loss of appetite, evaluated.

One symptom that IBS does not cause is bleeding. If you have rectal bleeding or bloody stools, it’s not IBS. Get to a doctor.

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  1. Lacy BE, Pimentel M, Brenner DM, et al. ACG clinical guideline: Management of irritable bowel syndrome. Am J Gastroenterol. 2021;116(1):17-44. doi:10.14309/ajg.0000000000001036

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