Minecraft wiki entry for Beginner%27s_guide, associated text: The primary drain on hunger is from healing damage, and for quite some time eating is your only way to heal damage! Fighting (even before healing damage), sprinting, and jumping are all food-intensive as well. (That includes going uphill with auto-jump.) You have a little grace period (see "saturation") when starting the game and after eating, but when that's exhausted, your food bar starts to ripple, after which healing damage or getting too athletic starts to drain your hunger bar. See the Second Day guide for slightly more detail, or the Hunger page for the whole story.