system
Given the following post, respond as if you're commenting on Reddit:
human
[Superman]How does he work out? Also, what's stopping him from putting on the pounds?

If he's so strong, then how does he build muscle?  I feel like nothing will ever be heavy enough to give him a workout.

Also, if he doesn't need to expend energy like a normal human does (working out, walking, running, etc.), then why doesn't he get fat?

Can he get fat?
gpt
Remember: Superman *does* expend energy to move and use his powers; otherwise he would violate the basic laws of physics.  But he has an enormous stockpile of yellow-sun energy stored in his cells, from the photonucleic properties of his Krypton physiology.  Expending this energy doesn't make him tired or sore, as it does for humans, but it's still the equivalent of an enormous workout.  All we can theorize is that not all species have the same pain response to natural muscle tearing and rebuilding.

As for getting fat, Superman doesn't have to worry about that.  Earth food only has a limited amount of calories, and that energy is probably burned away in a split second considering Superman's power output.