IBS and SIBO are two acronyms being thrown around the internet without much distinction. But distinctions are important in medicine. They allow doctors to hone in on the correct diagnosis. And, as any doctor can tell you (aside, perhaps, from Dr. Google), selecting an effective treatment hinges upon a correct diagnosis.
IBS (Irritable Bowel Syndrome) is a clinically defined disorder characterized by abdominal discomfort or pain associated with changes in bowel habits such as diarrhea or constipation, or alternations between the two. Bloating or distention of the abdomen is also common, and many patients experience other symptoms as well. Importantly, with IBS there are no visible abnormalities seen by x-ray or endoscopy.
In IBS, sensory perception is usually abnormal. The nerves in the intestines are more sensitive to stretch or movement of bowel contents and this can result in pain or discomfort due to normal intestinal functions. Sometimes motility is also abnormal. The intestines contract too much or too often, and sometimes they don't move bowel contents enough or often enough. These abnormalities might be the result of disordered brain-gut communication, genetic factors, infection and altered bacteria in the large intestine, and/or intestinal inflammation.
SIBO (Small Intestinal Bacterial Overgrowth) is a condition characterized by excessive bacteria in the small intestine. As a clinical entity, SIBO is still in its infancy; meaning that it’s poorly understood and that there is no official criteria to guide doctors to making a diagnosis of “SIBO”. Many doctors consider a bacterial population in the small intestine exceeding 105–106 organisms/mL to be indicative of SIBO. Symptomatically, SIBO can be very similar to diarrhea dominant IBS. Bloating and abdominal distention are the primary symptoms, and chronic diarrhea occurs in about 1/3rd of SIBO patients.
Doctors make the distinction between IBS (typically involving alterations in abundance and type of bacteria in the large intestine) and SIBO (typically involving overabundance of bacteria in the small intestine) based upon a timed test that represents bacterial metabolism. The timeline of this test indicates what part of the gastrointestinal tract hosts the bacteria giving a positive result. For example, if the test indicates overactive bacterial metabolism at 80 minutes, then the bacteria are most likely in the ileum (small intestine). If it shows up at 120 minutes, then the bacteria are most likely in the colon (large intestine).
If you’ve been diagnosed with IBS and you suspect you have SIBO, there are a few signs that you may want to discuss with your doctor.
Your symptoms improve while taking antibiotics (and usually worsen a few weeks afterwards)
Probiotics and prebiotics make you feel worse
Fiber gives you constipation (rather than relieving it).
Blood tests show that you have chronically low iron or ferratin with no known cause
A knowledgeable doctor can distinguish between IBS and SIBO by careful review of your symptoms, a physical examination, and selected diagnostic procedures that are often limited to a few basic tests.