You're not the first to wonder about Superman's "super-metabolism". In fact, the issue was poked fun at during the first episode to Lois & Clark.
Lois arrives at Clark's apartment to pick him up for work, and catches him dressed in nothing but a towel. While waiting for him to get dressed, she notices that his fridge is full of nothing but junk food and makes the following observation:
LOIS: Explain something to me.
CLARK: What do you mean?
LOIS: Um... well, you eat like an eight-year-old, yet you look like
Mr. Hardbody. What's your secret, and can I have it?
CLARK: (looks perplexed, shrugs)
LOIS: (sighs, heads towards door) We'd better go.
CLARK: (smirks at the underlying compliment)