There is plenty of evidence to show that being overweight for any great length of time in life causes harm, either fairly directly by boosting levels of chronic inflammation, or because that fat tissue is associated with a lack of exercise and consequent development of vascular dementia, or for a range of other possible reasons. Here is another study on this topic: "High midlife body mass index (BMI) has been linked to a greater risk of dementia in late life, but few have studied the effect of BMI across midlife on cognitive abilities and cognitive change in a dementia-free sample. ... We investigated the association between BMI, measured twice across midlife (mean age 40 and 61 years, respectively), and cognitive change in four domains across two decades in the Swedish Adoption/Twin Study of Aging. ... Latent growth curve models fitted to data from 657 non-demented participants showed that persons who were overweight/obese in early midlife had significantly lower cognitive performance across domains in late life and significantly steeper decline in perceptual speed, adjusting for cardio-metabolic factors. Both underweight and overweight/obesity in late midlife were associated with lower cognitive abilities in late life. However, the association between underweight and low cognitive abilities did not remain significant when weight decline between early and late midlife was controlled for. ... There is a negative effect on cognitive abilities later in life related to being overweight/obese across midlife. Moreover, weight decline across midlife rather than low weight in late midlife per se was associated with low cognitive abilities." The weight decline association shows up in a range of studies on weight and health; one common conclusion is that it reflects the impact that more serious medical conditions - related to weight or otherwise - can have on people.