Please compare IBS with Gluten allergy. I always thought I had IBS as child, then one doc suggested it could be Gluten allergy - similar simptoms. Now I'm a little confused since the dietary recommendations for the 2 are diametrically opposite.  IB, Amazon Reviewer Nov 2013

Gluten free diet (Celiac Diet) and symptoms of irritable bowel syndrome

It is a well-known fact that Dietary fiber can affect symptoms in irritable bowel syndrome. Whether fiber helps or hurts people with irritable bowel syndrome is a different question. There are people who will benefit from a high-fiber diet when their suffering from irritable bowel syndrome and there are people who will experience more pain if they take high-fiber diet. The improvement in symptoms vary from person to person and time to time in an individual person fiber may help or hurt during different stages of the disease. 

With all this confusion how do we make sense of whether to take the fiber or not?

To understand this confusing interaction we need to understand colonic physiology. During early phases of constipation when the colon is not dilated, the fiber speeds up the movement of stool through the colon. This prevents excessive absorption of water from the stool and prevent the formation of dehydrated hard stool, thus preventing symptoms. As constipation progresses the colon starts to dilate. Once the colon is dilated fiber distends the colon further causing worsening of symptoms. At this stage fiber will worsen the symptoms of irritable bowel syndrome. Low fiber will be helpful as it decreases the total volume of stool in the colon.

Coming back to gluten-free diet, majority of fiber in our diet contains gluten. Once you eliminate gluten from your diet, you practically eliminate significant amount of fiber from your diet. Once the fiber is eliminated patients with dilated colon will experience improvement in their symptoms. However if the colon is not dilated removal of fiber from the diet will worsen the constipation hence the symptoms will become worse. Some of these concepts were reviewed in my previous book “demystifying digestive tract symptoms” including what to do in these situations.

Regardless gluten-free diet is for patients who have documented allergy to gluten and have a diagnosis of celiac disease. The benefit provided by low fiber diet in patients with dilated colon is often thought to indicate subtle celiac disease. This assumption is incorrect. If you have been tested for celiac disease and are diagnosed with the disease, celiac diet will have a role. Otherwise, the results will be unpredictable and more based on association than causation.