Irritable bowel syndrome: Why IBS patients experience abdominal pain after eating certain foods?

According to a new hypothesis, abdominal pain in people with irritable bowel syndrome is caused by a kind of localized allergic reaction to food in the gut.

Irritable bowel syndrome (IBS) is a gastrointestinal disorder that causes cramping, abdominal pain, bloating, gas, and alternating diarrhea and constipation. Symptoms are often worse after consuming certain foods. Now, researchers have identified the biological mechanism that causes abdominal pain or severe discomfort in IBS patients after eating.

Guy Boeckxstaens, a gastroenterologist at Katholieke Universiteit Leuven in Belgium, and his team have presented a new hypothesis that this pain is caused by a kind of localized allergic reaction to food in the gut.

In their study results published in the journal Nature, the researchers also noted that 20 per cent of the world's population suffers from the irritable bowel syndrome (IBS).

Intestinal infections, such as acute food poisoning, are a known trigger of IBS symptoms. Earlier it was believed that low-grade inflammation can persist in the gut after infection, leading to chronic pain. But Boeckxstaens hadn't found any such inflammation when he previously examined intestinal biopsies of IBS patients. But he had another idea: When battling an infection, the intestine's immune system may mistakenly perceive food antigens (protein fragments) as enemy forces. A persisting gut reaction to these food antigens could explain the pain and cramping that often accompany a meal.

About 15 years ago, Giovanni Barbara, a gastroenterologist at the University of Bologna, had first suggested that the immune system of IBS patients was different. Barbara and her team had found that in IBS patients who had no active infections immune cells called mast cells were activated. Normally, mast cells act as an alert system for the body, releasing chemicals such as histamine when threatened with infection. But nobody had believed Barbara at that time as most physicians and researchers doubted that the pain of IBS was rooted in gut biology.

IBS patients show localized reaction to food antigens

To test his hypothesis, Boeckxstaens and colleagues infected mice with harmful gut bacteria, and at the same time fed them ovalbumin, a protein found in egg white that is commonly used as food antigen in experiments.

After the gut infection is cleared, the mice were given ovalbumin again, to see if their immune systems had become sensitised to it. Their hypothesis turned out to be true. The ovalbumin set off a chain reaction similar to what happens in food allergies, causing mast cell activation, histamine release, and increased abdominal pain.

However, mice that didn't get egg white protein while they were infected had no trouble.

To see if people with irritable bowel syndrome reacted in the same way, the researchers infected food antigens found in gluten, wheat, soy and cow milk into the intestine wall of 12 IBS patients. Subsequent tests showed that every volunteer had a localized reaction to at least one of the antigens. No reaction was seen in healthy volunteers.

This indicates that IBS pain is rooted in biology, noted Bana Jabri, a pediatric gastroenterologist and mucosal immunologist at the University of Chicago. Science Magazine reported that Jabri had pursued a similar line of inquiry into celiac disease, in which patients can't tolerate gluten. Her team had reported in 2019 that ingesting gluten causes a strong immune reaction specific to the substance, which triggers abdominal symptoms such as pain and nausea.