From what I understand, Smaug is a beast that must eat other living things to sustain himself. For example, he does not "eat" gold and jewels, he simply admires and covets them.
It's implied that he ate a good deal of dwarves, men, ponies, etc., during his attack on Erebor. But once he had settled in and driven all the dwarves out, what the heck did he eat for 60 years?
The dwarf carcasses would have soon rotten away. I'm sure the dwarves had stores of food in Erebor somewhere, but it probably was in closed-off storerooms that were too small for Smaug to enter. Plus, after a few years, that food would have surely rotted away too.
Did Smaug occasionally leave the Mountain on hunting trips, searching for cattle, sheep, ponies to prey upon? Certainly there were no men living for miles around the Mountain after the attack - they died or moved far away. We know there is a large area around the Mountain that is "desolated". It seems to me, being charred and burnt, that nothing would grow here and no animals would graze. Where then, would Smaug hunt? Surely he did not wander that far away from the Mountain.
Perhaps, he entered a state of hibernation for many years and did not require food or water. But that could not last forever; he was not hibernating when Bilbo came down to him. I doubt he would hibernate for that long.
Please help me answer this question if you can.