Association of body mass index with risk of cognitive impairment and dementia: A systematic review and meta-analysis of prospective studies

•Midlife underweight, midlife obesity and late-life underweight increase dementia risk.•Late-life overweight and obesity conferred 21% and 25% reduced dementia risk.•ACD risk in midlife was significantly elevated when BMI surpassed 29 kg/m2.•AD risk in midlife was significantly elevated when BMI surpassed 30 kg/m2.•VaD risk in midlife was significantly elevated when BMI surpassed 32 kg/m2.•AD risk in late-life was decreased in the case of BMI cutoff under 27 kg/m2. Controversies persist about the associations of body mass index (BMI) with risk of cognitive impairment and dementia. This study aimed to evaluate these associations from various aspects, in which Embase, PubMed and Cochrane databases were searched to identify prospective studies up to May 2019. Random-effects meta-analyses and dose-response meta-analysis were conducted, involving twenty-nine of 20,083 identified literatures. Meta-analysis showed that midlife underweight, obesity and late-life underweight conferred 1.39-, 1.31- and 1.64-fold excess risk for cognitive impairment and dementia, while late-life overweight and obesity conferred 21% and 25% reduced risk. In dose-response meta-analysis, all cause dementia (ACD), Alzheimer’s disease (AD) and vascular dementia (VaD) risk in midlife was significantly elevated when BMI surpassed 29, 30 and 32 kg/m2. AD risk in late-life was decreased when BMI was under 27 kg/m2, while this protection for VaD was absent when BMI surpassed 39 kg/m2. Higher BMI produced opposite exerted opposite effects on dementia in mid- and late-age population. Firstly reported, a dose-response relationship further supports the guideline from the standpoint of dementia prevention.