As you become able to feed yourself to 100% food bar in game, you will become "full" (over 84% food bar = full status). Over time, being "full" generates fat reserves on your character, which currently can be seen by having a fellow player right click, and examine you. It will state that you have "good stores of [[fat]]" or some other such message (will update entry with various fat levels as time goes on)
As of 2/13/20082009, at 0% food bar, you "fast", using up one of your fat reserve levels. This instantly fills your food bar to 100%, and gives a message in your event log. To reverse this loss of fat, stay above 84% food bar for awhile, by eating another meal. (it is known that you must eat *something* to regain fat, not just let the 100% bar sit, but how this works is unconfirmed.)
It has been confirmed that starvation *will not kill you* as of yet, but this could be changed at a later date.