http://opinionator.blogs.nytimes.com/2010/04/20/brain-damage/

That, at least, is the gloomy conclusion of several recent studies. For example, one long-term study of more than 6,500 people in northern California found that those who were fat around the middle at age 40 were more likely to succumb to dementia in their 70s. A long-term study in Sweden found that, compared to thinner people, those who were overweight in their 40s experienced a more rapid, and more pronounced, decline in brain function over the next several decades.

“What this study tells us is that in general the public commitment principle produces results, especially if the commitment is long-term. But, in the mix of people who make the commitment, those who genuinely fear social disapproval—not a personality trait usually given very high marks–will likely succeed the most. Those who couldn’t care less what others think are, ironically, more likely to come up short.”