1. Don't get stressed in the UT if you can help it. It decreases satiation a lot and also hurts your combat abilities a lot (-50% modifier). Even though you may not ever see anything as cool as that mithril plate mail in the rest of your game, it won't really matter if you don't have the strength to carry the rest of the stuff you need, and when you could just put on some hardened leather armor instead there's not really any point to having it besides the gold it will fetch in a shop and the reasonably rare instance of a weak enemy finding something mean and nasty enough to break through said hardened leather armor.
2. Of the enemies you meet in the UT, hedgehogs and jackals are 100% safe to eat, and giant/magic mushrooms, while they give you some magical side effects, do not by themselves give you anything permanent. Kenny also makes good eating; IMO killing him as soon as you find a decent weapon, get good unarmed skill, or find some kind of body armor is a policy to live by (though if he eats a zombie corpse, you probably should avoid his meat since I believe I've caught leprosy from him that way).
3. Eat *everything* you come across that is undeniably food if you're not satiated (the holy banana is an exception [because it is a bit more like a real wand than other bananas], and you'll always want to eat carrots and ommel substances before non-stat enhancing food). The only other exceptions to this policy are terribly spoiled food and school food. If you are satiated, school food is a semi-reasonable indulgence for the reward of extra endurance (which has enemy generation risks but does technically make it easier to survive), but eating more normal food risks becoming bloated which penalizes Agi/Dex training.
4. If you find a wand of polymorph, you can very, very easily get overfed on all the loaves, carrots, bananas, kiwis, and pineapples that pop up. If you find a pickaxe it will be even easier to handle it properly (polymorph wands can polypile 25 items at a time, given that you put down 5 items on 5 squares in one direction or another and zap them all). Given that polymorph wands also tend to have more than one charge, you're bound to make out like a bandit.
5. If you find an altar or book of Seges, you can pray to him for instant satiation. He's probably one of the best gods you can get to know in the game for this alone. Loricatus will also, when angry, turn whatever you're wielding into banana flesh, so if you're really hungry wield a chest, pray a couple times, and boom! instant hunger destroyer.
6. UT3 has constantly spawning carnie plants which sometimes have kiwis that drop when they die. I don't precisely recommend this, but you can hang around and scum the level for food (and *why* I don't recommend this is simple; the plants get bigger and I've never had a lot of luck surviving long against giant carnie plants). The plants in Jenny's room, as well as Jenny herself, also have lots of food (and I definitely recommend eating as much of it as you can before going to Attnam; if you aren't at least satiated by the pineapples and kiwis in that room you have either done something wrong or passed up food elsewhere
EDIT: 7. If you have a pickaxe, you can mine your way to a secret staircase leading to UT4 that is usually available in UT2 (which is hidden within a diamond shaped wall structure in a distinctly unrandomly designed room). UT4 is notable for having lots of edible enemies as well as a high rate of enemy generation, valuable stones, and random stashes of food; however if you go in while weak, while this will set the majority of enemies up to be not quite as nasty as they could be, you are nigh-guaranteed to find at least one or two crystal light golems which will quickly end your game if you try to fight them. Run from them whenever possible instead if you want to live to the gloomy caverns.