Minecraft wiki entry for The_second_day, associated text: Fighting: Both attacking mobs and receiving damage costs hunger, even before you start trying to heal damage. (60 blows either way matches healing .) You can slaughter a few animals but pick your fights carefully. Taking damage in other ways counts for this too, but each time you take damage it counts as one "blow" no matter how much damage you took.
Sprinting: You sprint (run faster) by double-tapping the forward movement key (W by default), or pressing the sprint key (Left Ctrl by default). While you move faster, sprinting uses up food. (60 meters matches healing .)
Jumping: Obviously, you'll need to jump some just to get around, but don't bounce around randomly or unnecessarily. (120 jumps matches healing .) Sprinting jumps are especially costly, 4 times as much as a regular jump, although they are the fastest mode of transportation early in the game. Don't neglect the automatic jumps from Autojump, going uphill can add up!
Swimming and mining blocks cost a little hunger, but those are fairly minimal compared to the activities above.