Since there seems to be a lack of information about food effects out there, I thought I'd dig some up. I think SOE made a mistake by not giving us some sort of indicator to judge food's effectivness. As it is, anyone who's not a provisioner wouldn't know.
First of all, you should *not* be using the summoned food from a Priest unless you have no other choice. It has a horrible regen rate. Second, the 6c ration and water you buy from the NPC is better but not optimal. Of course, you can get by with the cheap store bought food but it's going to get harder and harder as you increase your level.
There are several "Tiers" of food/drink. Crafting lvl 1-10 is tier 1. Crafting lvl 10-20 is tier 2, etc., etc. Here's the breakdown I found and some more information:
http://eqiiforums.station.sony.com/eq2/board/message?board.id=a3&message.id=409