More on the link between obesity and increased risk of dementia from the New Scientist: "Brain regions key to cognition are smaller in older people who are obese compared with their leaner peers, making their brains look up to 16 years older than their true age. As brain shrinkage is linked to dementia, this adds weight to the suspicion that piling on the pounds may up a person's risk of the brain condition. Previous studies suggested that obesity in middle age increases the risk of dementia decades later, which is accompanied by increased brain shrinkage compared with leaner people. Now brain scans of older people have revealed the areas that are hardest hit, as well as the full extent of brain size differences between obese people and those of average weight. ... High insulin levels and type 2 diabetes tend to accompany being overweight and are risk factors for brain tissue loss and dementia. However, the relationship between brain size and body mass index still stood when the researchers accounted for these conditions, indicating that body fat levels may be linked directly to brain shrinkage. Thompson suggests that as increased body fat ups the chances of having clogged arteries, which can reduce blood and oxygen flow to brain cells."

Link: http://www.newscientist.com/article/mg20327222.400-expanding-waistlines-may-cause-shrinking-brains.html