I have been following a regime of healthy food and exercise for the past month and I find the same thing - I'm a bit more able to focus on things I find difficult to concentrate on, and bonus, it provides an excelent outlet for my mild hyperactvie tendancies! I bounce and fidget less if I've jogged and done my yoga and strength training for the day.
It's almost like the more I do, the more "normal" I appear, outwardly, I think.
On a sidenote, however, I'm following said regime for purely health-related reasons. I was just plain sick and tired of being unhealthy and having bad habits. I even quit smoking! Yay me.