Ask the Doctors: SIBO, similar to IBS, affects the small intestine

Dear Doctors: I’ve seen stories about people who thought they had irritable bowel syndrome, but it turned out to be something called “SIBO.” Can you explain what it is?

Dear Reader: Small intestinal bacterial overgrowth, often shortened to SIBO, is similar to irritable bowel syndrome, or IBS.

IBS affects the large intestine. SIBO involves the small intestine.

They share many symptoms, including loss of appetite, abdominal pain, cramping, abdominal bloating, nausea, gas and bouts of constipation and diarrhea.

People with SIBO also can have deficiencies of vitamins including B12 despite eating a balanced diet.

Both SIBO and IBS can range from mild, with minimal symptoms, to severe enough to interfere with daily life.

SIBO occurs when the optimal balance of bacteria in the small intestine is disturbed. That’s the “overgrowth” in the name of the condition. This includes an abnormal increase in the bacteria in the small intestine and the presence of bacteria that don’t normally live in that portion of the bowel.

As this altered population of bacteria ferments the food that enters the small intestine, it causes unpleasant, often painful symptoms.

It’s not clear what triggers SIBO. Structural issues might allow bacteria from the large bowel to mistakenly migrate across the muscular valve that separates it from the small bowel.

Something known as low motility also is believed to be a factor. That’s a slower-than-normal rate at which food moves through the gastrointestinal tract. Certain medical conditions — including Crohn’s disease, Parkinson’s disease and diabetes — can contribute to low motility.

Low levels of stomach acid can help allow bacteria to proliferate.

And some cases of SIBO have been linked to gastric surgery for weight loss.

When SIBO is suspected, a test to measure the concentrations of methane and hydrogen in the breath is the next step. A more definitive — also more invasive — test involves retrieving a tissue sample for examination from the small intestine during an endoscopy.

SIBO has a wide range of causes, so treatment can be a challenge. It typically begins with antibiotics to control the overgrowth of bacteria. Patients also might be asked to interrupt treatment that includes antacids and to use medications to improve motility.

A promising area of research into SIBO and IBS is the use of fecal transplants. But this remains experimental and is not approved by the federal Food and Drug Administration for treating SIBO.

Dr. Eve Glazier and Dr. Elizabeth Ko are internists at UCLA Health.