Did DC Comics ever say if Superman works out? I mean, above and beyond fighting crime and saving the world, what does Superman do to stay physically fit? If he didn't excercise and there was a peaceful lull in crime, would he get a pot belly?
Overall, no. DC has had the general consensus that Superman's superb physiology keeps him fit and trim living on fresh air and solar radiation. We will see if they add the need for him to eat and engage in other bodily activities in the DCnU as he develops.
In previous continuities, as long as he was empowered by the sun, he did not NEED to eat or excrete and barely needed to breathe or sleep. His body is maintained in his perfect condition by whatever genetic technology gave him his powers in the first place. We know that he did eat on occasion and would sit down to dinner with the Kents or friends during the holidays. In appearance and function, his body seems to work just like ours, with the added feature of being able to subsist completely on energy if he needed to.
Under a red sun, where he has no superpowers, he would likely revert to his human-appearing un-powered state and that may necessitate him having to consume food/calories as any other human would. (Yes, this is terribly inconsistent, but that has been the nature of the character for 60+ years. No one is willing to commit to a version of the character that could make scientific sense and still be, well...super.)
There was a time during the John Byrne run that Lois remarks that Clark Kent's weights seemed a bit light for a fellow who is as in shape as he was to be using. This was during a time when she was still trying to compare Clark and Superman's physical builds. It never occurred to Clark that his weights were not sufficient for maintaining a physiology as well cut as his, since he never used the weights in the first place and had no idea what was necessary to maintain his in-shape appearance.