Association of bacterial overgrowth in the small intestine with cortical thickness and functional connectivity in Parkinson's disease involving mild cognitive impairment

This study explored potential associations of bacterial overgrowth in the small intestine, as detected based on levels of hydrogen and methane in breath after lactulose consumption, with cortical thickness and resting-state functional connectivity in different brain regions. Prospective comparison of 35 patients with Parkinson's disease (PD) involving mild cognitive impairment, 35 patients with PD with normal cognitive function and 17 healthy controls showed the largest level of hydrogen alone and the largest combined level of hydrogen and methane in patients with mild cognitive impairment. The comparison also revealed a significant negative correlation between those levels and thickness of the right insular cortex. Mild cognitive patients showed different functional connectivity between the right insula and cognition-related brain networks from normal cognitive patients. Our results suggest that bacterial overgrowth in the small intestine may contribute to cortical thinning and alterations in resting-state functional connectivity in PD involving mild cognitive impairment. These insights support and deepen previous observations implicating the gut-brain axis in the neurological disorder.