AbstractAimsWhite matter lesions (WMLs) are involved in the pathological processes leading to cognitive decline and dementia. We examined the mechanisms underlying the exacerbation of ischemia‐induced cognitive impairment and WMLs by diet‐induced obesity, including lipopolysaccharide (LPS)‐triggered neuroinflammation via toll‐like receptor (TLR) 4.MethodsWild‐type (WT) and TLR4‐knockout (KO) C57BL/6 mice were fed a high‐fat diet (HFD) or low‐fat diet (LFD), and subjected to bilateral carotid artery stenosis (BCAS). Diet groups were compared for changes in gut microbiota, intestinal permeability, systemic inflammation, neuroinflammation, WML severity, and cognitive dysfunction.ResultsIn WT mice, HFD induced obesity and increased cognitive impairment and WML severity compared with LFD‐fed mice following BCAS. HFD caused gut dysbiosis and increased intestinal permeability, and plasma LPS and pro‐inflammatory cytokine concentrations. Furthermore, HFD‐fed mice had higher LPS levels and higher neuroinflammatory status, including increased TLR4 expression, in WMLs. In TLR4‐KO mice, HFD also caused obesity and gut dysbiosis but did not increase cognitive impairment or WML severity after BCAS. No difference was found between HFD‐ and LFD‐fed KO mice for LPS levels or inflammatory status in either plasma or WMLs.ConclusionInflammation triggered by LPS–TLR4 signaling may mediate obesity‐associated exacerbation of cognitive impairment and WMLs from brain ischemia.