Small intestinal bacterial overgrowth: from malabsorption to misinterpretation

Small intestinal bacterial overgrowth (SIBO) was originally described as a cause of maldigestion and malabsorption in situations where disruptions of intestinal anatomy or physiology favored the proliferation of bacteria normally confined to the colon. In this context, the pathogenesis of symptoms resulting from SIBO was well described. More recently, the concept of SIBO was extended to explain symptoms such as bloating, altered bowel habit and discomfort among individuals with irritable bowel syndrome and since then a whole host of gastrointestinal and extragastrointestinal disorders have been attributed to SIBO. In these more recent studies, the diagnosis of SIBO has been largely based on breath hydrogen testing; an approach that is subjected to misinterpretation. Here we critically assess the "modern" (as against the "classical") concept of SIBO and plead for caution in the application of breath tests, and those that employ lactulose as the substrate, in particular, to the diagnosis of this disorder. We look forward to the application of modern molecular microbiological techniques to the assessment of the small intestinal microbiome and metabolome and the delineation of what is truly normal.