[Gurthang]

So then, what did they eat? They probably hybernated like dragons would have, but they would still need energy to function. In the case of Durin's Bane, it would only have had orcs to eat. Unless it ate rocks.

Also, it would seem to me that they would need a lot of energy to produce their clothing of fire. They weren't burning themselves, so they had to be emitting a flamable sustance from their bodies. Continually producing any substance takes enormous quantities of energy, so it would make sense that they had to consume a lot because they burned off a lot.