The players do not feel the hunger of their bodies, nor any pain or essentially anything from their bodies in real life. The game essentially controls when they are hungry, what they feel in the way of pain and everything else. "hunger" in the game and hunger in real life are two different things. Even if they're not hungry in real life they can feel "hungry" in the game due to the NERVgear. So to remove that "hunger" they eat in-game foods. They don't NEED to but, the feeling is bothersome.

I could go into a long version or a short version here. I'll go with short because the long version confuses me. 

Short version: In a few complex video games it's required to feed your characters, example The Sims, they also have to sleep, see again The Sims. Continuing with the The Sims example, the more your character does the faster they run out of energy and need sleep, just like in The Sims

EDIT: The game stimulates pain, so while there would be some sensation of pain when running for long periods, or getting cut or bitten or attacked, it would not be debilitating, unless the pain sensor is up to real world levels