Assessment of small intestinal bacterial overgrowth in Alzheimer's disease

There is great interest in crosstalk between the gastrointestinal and immune systems. Small intestinal bacterial overgrowth (SIBO) is a bowel disorder prevalent among patients with Parkinson's disease; SIBO treatment has been shown to modulate neurological inflammation, motor and cognitive outcomes there. However, to date, no link between Alzheimer's dementia and SIBO has been established. This pilot study sought to estimate the prevalence of SIBO in Alzheimer's dementia in the outpatient setting in Singapore General Hospital. It entailed performing a hydrogen breath test and objectively scoring gastrointestinal symptoms and their severity in 48 patients, comparing symptom scores and mean breath test values in those with mild to moderate Alzheimer's against age- and sex-matched controls that did not fulfill DSM-V criteria for probable Alzheimer's. Here, the prevalence of positive breath tests and symptoms of SIBO were no greater among Alzheimer's patients than in controls. This suggests that the gut microbiome changes and increased bowel inflammation seen in previous studies on Alzheimer's patients are likely effected through pathways other than SIBO, and are likely more complex than a mere increase in small bowel bacterial volume. Rather, future research could be directed along the lines of qualitative changes in small bowel microbiota, or pathologies in other parts of the gastrointestinal tract such as the colon or stomach, aspects which are not adequately captured by the hydrogen breath test. Keywords: Alzheimer's disease; dementia; gut-brain axis; small intestinal bacterial overgrowth; microbiome.