From The Fat, Drunk, And Stupid Is No Way To Go Through Life, Son Department:

Roto-Reuters: Heavier weight tied to poorer mental function
Overweight middle-aged adults tend to score more poorly on tests of memory, attention and learning ability than their thinner peers do, researchers reported Monday.

The findings, they say, suggest that a heavier weight in middle age may mean a higher risk of dementia later in life.