I strongly believe that. Before I gained an insane amount of weight, I'd like to think I was pretty smart. Studying was no difficult task, I'd ace tests. I love to write and my fingers couldn't write or type fast enough to keep up with my thoughts and ideas.
Now I have the worst writer's block in the world, studying, just concentrating takes monumental effort. I forget things VERY easily and my brain just feels... foggy. And I'm only in my mid-twenties. I don't know. Something about blood having to make more of an effort and travel more to reach my brain or something?