I don't think it's any great secret that failing to exercise regularly and accumulating visceral fat around the organs are both bad for your long term health - but I suspect that most people don't realise just how bad it is. Either that or it's the way in which we are hardwired to discount the future and cause harm to the person we will one day be - time preference at work. In any case, here is a reminder from the scientific community:

Adiposity and Alzheimer's disease

Alzheimer's disease is the most common form of dementia. There are no known preventive or curative measures. There is increasing evidence for the role of total adiposity, usually measured clinically as BMI, and central adiposity [or visceral abdominal fat], in Alzheimer's disease.

...

Salient publications in 2007 and 2008 showed that (a) central adiposity in middle age predicts dementia in old age; (b) the relation between high adiposity and dementia is attenuated with older age; (c) waist circumference in old age, a measure of central adiposity, may be a better predictor of dementia than BMI; (d) lower BMI predicts dementia in elderly people; and (e) weight loss may precede dementia diagnosis by decades, which may explain seemingly paradoxical findings.

All a long way of saying if you get fat and stay fat, your biochemistry is more likely to destroy the structure and function of your mind. And those thinner old people who are suffering as well? Their developing dementia - partially caused by earlier excess fat - led them to lose weight during their decline. There are easier ways through life than this, and unlike many of the slings and arrows we suffer, for the vast majority of us our level of body fat is a choice.