Re: Tom Hardy as Bane XXX
The gas only serves as a painkiller. It doesn't even keep him alive per se, it just prevents the pain from his injuries from overwhelming his senses. I suspect that if he took the mask off and probably hook himself on morphine or something, he could sit to eat with no problem. It sounds like it would be a bother but it's the only explanation I got.
It's either that or he eats through a straw or maybe on some sort of parenteral nutrition. Though this seems unlikely since a large man like him would need to eat a lot of solid and nutritive food to stay in top shape.