Irritable bowel syndrome (IBS), which affects 11% to 14% of the population,
is a puzzling condition with multiple models of pathophysiology including
altered motility, visceral hypersensitivity, abnormal brain-gut interaction,
autonomic dysfunction, and immune activation. Although no conceptual framework
accounts for all the symptoms and observations in IBS, a unifying explanation
may exist since 92% of these patients share the symptom of bloating regardless
of their predominant complaint.
Ovid MEDLINE was searched through May 2004 for relevant English-language
articles beginning with those related to bloating, gas, and IBS. Bibliographies
of pertinent articles and books were also scanned for additional suitable
The possibility that small intestinal bacterial overgrowth (SIBO) may
explain bloating in IBS is supported by greater total hydrogen excretion after
lactulose ingestion, a correlation between the pattern of bowel movement and
the type of excreted gas, a prevalence of abnormal lactulose breath test in
84% of IBS patients, and a 75% improvement of IBS symptoms after eradication
of SIBO. Altered gastrointestinal motility and sensation, changed activity
of the central nervous system, and increased sympathetic drive and immune
activation may be understood as consequences of the host response to SIBO.
The gastrointestinal and immune effects of SIBO provide a possible unifying
framework for understanding frequent observations in IBS, including postprandial
bloating and distension, altered motility, visceral hypersensitivity, abnormal
brain-gut interaction, autonomic dysfunction, and immune activation.