What's causing your IBS

While the exact cause of irritable bowel syndrome, or IBS, is unknown, studies have suggested that IBS might be related to a few specific changes in the body. Some symptoms may be caused by spasms, uncontrolled contractions in the muscles of the colon. The nerve endings in the intestines also may become unusually sensitive, magnifying pain. The reasons for these changes are not always known, but factors that have been linked with IBS include bacterial overgrowth, use of antibiotics, and stress, among others.

Psychological factors. The brain and gut are intimately connected. Your thoughts and emotions can trigger symptoms in the gut, and the health of your gut can shape your mental well-being. Stress can cause more contractions in the intestines and increase sensitivity. It's not clear whether stress or other psychological factors may be a cause of IBS or vice versa. However, we do know that people with IBS often have higher levels of stress and anxiety and that this distress also can make IBS symptoms worse. A 2017 study in the Journal of Neurogastroenterology and Motility found that people with IBS have higher levels of depression and anxiety compared with those who don't have the disorder. IBS also is more common among people who experienced psychological trauma as children.

Antibiotics. The human digestive tract contains trillions of bacteria, viruses, and fungi known collectively as the gut microbiota or gut flora. These microorganisms play several critical roles in our health, including digestion and immune system function. When we take antibiotics to combat bacterial infections, the drugs also kill helpful bacteria in the gut. Repeated treatments or long-term use of antibiotics may alter the gut flora in a way that disrupts the colon's normal function. Some animal and human research suggests that this disruption may lead to IBS in some cases. However, studies have not had consistent results, so more research is needed.

Bacterial overgrowth. Some people with IBS also have a surplus of bacteria in the small intestines, a condition called small intestinal bacterial overgrowth (SIBO). It's unclear whether SIBO can be a cause of IBS, but people with IBS are more likely than others to test positive for SIBO. In addition, some research has found that IBS symptoms often decrease after antibiotic treatment that focuses on bacteria in the small intestine.

SIBO occurs when extra bacteria in the colon back up into the small intestine. In this situation, people often have symptoms typical of IBS such as bloating, constipation, abdominal pain, and diarrhea. Estimates vary about how many people diagnosed with IBS also have SIBO, but research in the March 2017 issue of Gut and Liver suggested it may be between 19% and 37%. Most studies diagnose SIBO with a breath test that measures gases released by the body's breakdown of sugars such as glucose and lactulose. There is a debate, however, about whether this test is reliable.

For more information about the causes and treatment of IBS, buy Managing Irritable Bowel Syndrome, a Special Health Report from Harvard Medical School.

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