alt Hacker NewsWhether he believed in it or not, he passed rigorous physical tests for the Navy and NASA. They don't let just any slob be a fighter pilot, much less a test pilot or astronaut. If you don't have good cardiovascular fitness, you can't handle high G-forces or maintain good judgement while sleep deprived (those jets didn't fly themselves while the pilot napped like modern ones do).
Maybe he was just naturally fit. Some people are. But he was undoubtedly fit.
Look up those tests and see what they selected for, its not as much as raw physical fitness but rather how their bodies reacted to stress + a host of other pyschological factors + flight training. Yes, it is without a fact that they are no slobs but calling them the most fittest is also hyperbole and paints this image of hyper fit astronauts which wasn't true back then. They also didn't care much about long term effects of space travel on the body back then because missions were very short back then.