I always assumed that this was because my brain is part of my body, and doing stuff that is good for my body would be the same stuff that is good for my brain.

I always assumed that this was because my brain is part of my body, and doing stuff that is good for my body would be the same stuff that is good for my brain. For some reason we draw this weird metaphysical distinction between the brain and the other organs in the body that make results like this seem strange, when they clearly are not. The idea that healthy people think better should not shock us. (Think better in terms of lower incidence psychological problems, although i always feel smarter after I exercise as well...)