Sam asks the same question, of whether orcs eat and drink or just live on foul air and poison, and Frodo gives him the following answer:
"No, they eat and drink, Sam. The Shadow that bred them can only mock, it cannot make: not real new things of its own. I don't think it gave life to the orcs, it only ruined them and twisted them; and if they are to live at all, they have to live like other living creatures. Foul waters and foul meats they'll take, if they can get no better, but not poison. They've fed me, and so I'm better off than you. There must be food and water somewhere in this place."
-- The Return of the King, Book VI, Chapter I: The Tower of Cirith Ungol