[burrahobbit]

After a quick aim conversation with obloquy this is what I have.

There are different degrees of incarnation, and we don't know how far down the path any particular balrog has/had gone. The Moria balrog spent quite a while locked up in a cave without food, so they clearly don't need to eat all the time. That being said, he could have simply been hibernating. Bears do it all the time, and it does say that the Dwarves woke him. Aside from that period, though, there are typically always orcs around. Take that however you want. Long story short. Either they don't need to eat or they can go a long time without food for whatever reason or eat/be fed by orcs.