Hunger

As of the pre-release of 1.8, hunger is a new aspect added to the game.

If you have 90%-100% food you will regain health slowly, but if you have 0% you will lose health instead. On easy the player's health would slowly drop to 5 hearts, 0.5 hearts on normal, and all the way to death on hard.

The hunger bar does not keep your health bar from completely depleting.

While the player is sprinting, the hunger bar will deplete faster than when not sprinting.

This game mechanic would allow food to be stacked, increasing the amount of food one can carry in their inventory at a time. It has been hinted that this will be added when Hardcore mode is added. Despite that fact, some community modders have already created Single Player mods and Multiplayer mods, both of which incorporate the hunger mechanic.

Hunger does not diminish when playing on Peaceful mode, but its effects on your character's health bar remain.

When you eat rotten flesh, your hunger bar "Zombificates" and depletes faster.

Bugs
Sometimes the hunger bar will stop depleting. This may be due to a glitch with the new food items.

The hunger bar also seems to periodically 'jiggle' like the health bar does when the player has less than 2.5 hearts, but it happens at any level.