Higher body mass index (BMI) is associated with increased dementia risk when weight was measured more than 20 years before dementia diagnosis, but this association is reversed when BMI was assessed less than 10 years before dementia diagnosis, according to a study in Alzheimer’s & Dementia.
Researchers, led by Mika Kivimäk, PhD, from the Department of Epidemiology and Public Health at the University College London in the UK, analyzed 39 prospective cohort studies that included a total of 1,349,857 participants from Europe, the United States, and Asia with no history of dementia; were population based with BMI assessed from all participants before the ascertainment of dementia; recorded hospital-treated dementia or dementia deaths; and had accrued a minimum of 3 years of follow-up.
The authors also considered education/socioeconomic position (categorized as high, intermediate, and low), smoking (current smoker vs nonsmoker), and prevalent cardiometabolic disease (coronary heart disease, stroke, and diabetes; 1 or more vs none).
The mean follow-up across studies was 16.1 years (4.3 to 37.7 years). Over the 21,798,141 person-years at risk in the participants, 6,894 incident dementia cases were recorded. The age-, sex-, and ethnicity-adjusted hazard ratio per 5-kg/m2 increase in BMI was 0.87. When BMI was assessed at 10 years, 10 to 20 years, and >20 years before dementia diagnosis, hazard ratios per 5-kg/m2 increase in BMI for dementia were 0.71, 0.94, and 1.16, respectively.
“Taken together, these findings provide new evidence for the hypothesis that the association between BMI and dementia is attributable to two distinct processes; one of which is a harmful effect of higher BMI and the other reverse causation bias contributing to an inverse association between BMI and dementia,” the authors stated. “By dissecting these processes in stratified analyses, our study provides a plausible explanation for the inconsistencies in some of the prior studies on BMI and dementia.”
Kivimäki M, Luukkonen R, Batty GD, et al. Body mass index and risk of dementia: Analysis of individual-level data from 1.3 million individuals. Alzheimers Dement. 2017 Nov 21. doi: 10.1016/j.jalz.2017.09.016