London, Jan 19 (IANS): Researchers have identified the biological mechanism that explains why some people experience abdominal pain when they eat certain foods.

According to the researchers, up to 20 percent of the world's population suffers from irritable bowel syndrome (IBS), which causes stomach pain or severe discomfort after eating.

The researchers revealed a mechanism that connects certain foods with activation of the cells that release histamine (called mast cells), and subsequent pain and discomfort.

"With these new insights, we provide further evidence that we are dealing with a real disease," said researcher Guy Boeckxstaens, Professor at Katholieke Universiteit Leuven in Belgium.

In a healthy intestine, the immune system does not react to foods, so the first step was to find out what might cause this tolerance to breaking down, according to the study published in the journal Nature.

Since people with IBS often report that their symptoms began after a gastrointestinal infection, such as food poisoning, the researchers started with the idea that an infection, while a particular food is present in the gut, might sensitise the immune system to that food.

They infected mice with a stomach bug, and at the same time fed them ovalbumin, a protein found in egg white that is commonly used in experiments as a model food antigen.

An antigen is any molecule that provokes an immune response. Once the infection cleared, the mice were given ovalbumin again, to see if their immune systems had become sensitised to it.

The results were affirmative -- the ovalbumin on its own provoked mast cell activation, histamine release, and digestive intolerance with increased abdominal pain. This was not the case in mice that had not been infected with the bug and received ovalbumin.

The researchers then went on to see if people with IBS reacted in the same way.

When food antigens associated with IBS (gluten, wheat, soy and cow milk) were injected into the intestine wall of 12 IBS patients, they produced localised immune reactions similar to that seen in the mice. No reaction was seen in healthy volunteers.