Yea, I guess I did leave you hanging :-). The small spoiler is this:
There is an internal counter of how many levels have been created since the
last piece of food. Whenever food is created, the counter is cleared.
Whenever an object is created (initializing a level with goodies or giving
monsters objects (Nymphs...)), food may be created. If the internal counter
shows that it is the fourth level since food was last created, food is
guaranteed to be created. I'll let you think of the implications this has.
One obvious one is that if you are on level four, have killed all the monsters,
and checked all the rooms on all previous levels, then you can be sure a
piece of food is waiting for you.
Now, you will start to keep a food counter in your mind too. Dropping to the
next level may increase your odds when you are getting weak and don't have
any food, but God help you if it was created on that level.
Mike Laman, System Roguer