IBS and SIBO: Is There a Connection?

Irritable bowel syndrome (IBS) and small intestinal bacterial overgrowth (SIBO) are two gastrointestinal conditions that share many symptoms, including bloating, abdominal pain, and changes in bowel movements.

But IBS affects the large intestine, while SIBO, as the name suggests, is a problem with the small intestine.

While researchers are still working to better understand these conditions, many believe there is some overlap and that one may lead to the other.

IBS and SIBO: Symptoms and Causes of Each

IBS is a common disorder, affecting 10 to 15 percent of U.S. adults, according to the American College of Gastroenterology. It is characterized by recurrent abdominal pain, discomfort, or cramping related to bowel movements, as well as gas, bloating, and changes in stool like diarrhea, constipation, or both.

The causes of IBS are not fully understood, but the condition is thought to be related to problems with how the bowels work, per the American College of Gastroenterology. It’s also believed that what’s referred to as the “gut-brain connection” plays an important role.

“The thought is that episodes of pain, bloating, changes in stool frequency and consistency in IBS are related to the way the brain and GI tract communicate with each other,” says Elizabeth Schaefer, MD, a pediatric gastroenterologist at Ascension St. Vincent and Peyton Manning Children's Hospital in Indianapolis.

She notes that people with IBS can have nerves in the gastrointestinal tract that tend to be overactive and hypersensitive, which can result in abnormal GI tract motility (movement), including diarrhea and constipation.

Other potential causes of IBS include severe infection of the intestines and changes in the gut microbes that live in the intestines, such as bacteria, fungi, and viruses, according to Mayo Clinic.

Similarly, SIBO occurs when there is an abnormal increase in the bacteria in the small intestine — especially types of bacteria that are not commonly found in this part of the digestive tract, per Mayo Clinic.

Signs and symptoms of SIBO often include loss of appetite, abdominal pain, nausea, bloating, an uncomfortable feeling of fullness after eating, diarrhea, unintentional weight loss, and malnutrition, notes the Mayo Clinic.

SIBO, also called blind loop syndrome, can have a number of causes, including complications of abdominal surgery, structural problems in or around the small intestine, and various medical conditions, including Crohn's disease, radiation enteritis, scleroderma, celiac disease, diabetes, or other conditions that slow motility of food and waste products through the small intestine.

When you have intestinal dysmotility, the muscles or nerves in the digestive system do not work as they should.

“This can allow the colonic bacteria to creep up and overflow into the small bowel,” explains Christine Lee, MD, a gastroenterologist at Cleveland Clinic.

Small bowel dysmotility disorders include gastroparesis and hypothyroidism.

Other conditions can also increase the risk of SIBO, Dr. Lee notes, including multiple sclerosis, lupus, Parkinson’s disease, chronic pancreatitis, and kidney disease.

Symptoms of SIBO are similar to IBS. “It can be easy to confuse the two conditions, as both can have nonspecific symptoms that can overlap,” Dr. Schafer says.

The Relationship Between IBS and SIBO: ‘Which Came First, the Chicken or the Egg?’

In addition to shared symptoms, IBS and SIBO are thought to be related in other ways.

According to a research review published in March 2017 in Gut and Liver, as many as 78 percent of people with IBS have SIBO, compared with up to 40 percent of people who do not have IBS.

Dr. Lee notes that people who have constipation as a predominant symptom of their IBS (referred to as IBS-C) have a higher risk of SIBO.

“This is because of the slow motility, meaning if you have difficulty emptying your colon completely, you always have a little bit or a lot of stool in your colon,” she says. “So there’s going to be a higher risk for developing SIBO, because the colonic bacteria will creep up to the small bowel and inhabit that area.”

Some researchers theorize that the gut imbalance that occurs with SIBO may lead to IBS, notes James Tabibian, MD, the director of endoscopy and the house staff research director for the department of medicine at Olive View-UCLA Medical Center in California.

According to a research review published in 2018 in F1000 Research, it remains unknown whether SIBO is a cause or a consequence of IBS — or both. “In other words, it is biologically plausible to suggest that SIBO can cause IBS symptoms in some but that, in others, alterations in motility, gut immune function, or microbiome predispose to the development of SIBO,” the researchers say.

If this is true, one might feed the other, leading to a circular cycle. Or as Dr. Tabibian puts it, “sometimes it’s unclear which came first, the chicken or the egg.”

This is an active area of research, with scientists trying to understand the nature and relationship of the two conditions.

How Are IBS and SIBO Diagnosed and Treated?

There is no specific test for IBS, so doctors typically take a complete medical history and rule out other conditions, such as celiac disease, inflammatory bowel disease (IBD), and lactose intolerance, according to Mayo Clinic.

Once other conditions have been ruled out, the criteria to diagnose IBS include:

• Abdominal pain and discomfort at least one day per week on average in the previous three months
• At least two of the following: paint and discomfort related to defecation, a change in frequency of bowel movements, or change in consistency of stool

SIBO is commonly diagnosed with a noninvasive hydrogen or methane breath test, per Mayo Clinic. Another way to diagnose SIBO is to take a small intestine aspirate and fluid culture. Doctors pass a long, flexible tube down your throat and through your upper digestive tract to your small intestine and take a sample of intestinal fluid to test for bacteria growth.

Treatment for IBS focuses on managing symptoms and stress, and may include medications as well as changes to your diet to avoid trigger foods.

These foods can be different for everyone, and Lee recommends keeping a food journal and taking a careful history of diet and symptoms. “Whether it's oily, greasy food, high carb foods, spicy food, or a particular nut, it’s just a matter of avoiding it to prevent symptoms,” she says.

According to a meta-analysis published in the European Journal of Nutrition in February 2021, certain people are sensitive to foods that are high in FODMAPs (fermentable oligosaccharides, disaccharides, monosaccharides, and polyols), and a low-FODMAP diet can reduce IBS symptoms.

The first line of treatment for SIBO is to treat the underlying problem; for example, surgery to fix anatomic abnormalities. When that isn’t possible, treatment focuses on antibiotic therapy and nutritional support, according to the Mayo Clinic. It can also be beneficial to better control diabetes, which helps motility.

In fact, rifaximin (Xifaxan), an antibiotic used to treat SIBO, can also improve symptoms in people with IBS, notes the F1000 Research review, suggesting a further link between the two conditions. More research is needed to better understand this relationship and which IBS patients would benefit from this treatment.

Lee says that since disorders that slow bowel motility, including IBS-C, can increase a person's risk of SIBO, treatment for the underlying issue should also help.

“Sometimes just treating SIBO is not enough,” she says. “Ideally, one should get to the root of the problem, otherwise incidence of relapse SIBO is high.”