Zuckerberg did 5000 pushups throughout the course of a week[1], which proves that he's at least moderately fit.
Richard Branson's main piece of productivity advice is "work out"[2].
A strong counterexample would be Warren Buffett, who was once the world's richest man, never worked out, and ate terribly.
Ultimately though it's best to look at the studies other responders to your post cited, which do show some correlation between exercise and mental performance.
Source? I didn't see anything like that in his authorized biography The Snowball. And it specifically mentioned several times that Buffett always hated manual labor, always ate badly etc.
You're right-the quote showed up in the Google summary of the page, but not the page itself. Weird.
Here is a video where Branson says he achieves "twice as much" on a day when he works out compared to a day when he doesn't exercise: http://www.youtube.com/watch?v=QFjgMKwpz_k
Richard Branson's main piece of productivity advice is "work out"[2].
A strong counterexample would be Warren Buffett, who was once the world's richest man, never worked out, and ate terribly.
Ultimately though it's best to look at the studies other responders to your post cited, which do show some correlation between exercise and mental performance.
Edit: forgot my links
[1]: http://venturebeat.com/2009/10/02/mark-zuckerberg-the-evolut...
[2]: http://www.entrepreneur.com/article/222739