Yeah, it's best to only eat corpses that you've just killed. No, not the one you killed a little while ago, the one you just killed.
But to answer your question, Nethack people just have really fast metabolism. Your character will lose 1 nutrition every turn. Ring hunger and amulet hunger is simply burning 1 extra nutrition every 20 turns. The exceptions to these are:
Regeneration
conflict
hunger
With those three, you'll burn an extra point of nutrition every other turn. Much faster.
Another exception is with a ring of slow digestion, which eliminates normal hunger completely, but you still have to tolerate ring hunger.
Also, there's nutrition penalties when casting spells. Without hungerless or reduced hunger spellcasting, the amount of nutrition you'll burn with every spell is 10 times that of the spell level IIRC.
Hungerless spellcasting is something only wizards have, and basically:
14 or lower INT: 0% reduced hunger spellcasting
15 Int: 25% reduced hunger spellcasting
16: 50% reduced hunger spellcasting
17 or higher: 100% reduced hunger spellcasting
Also, the amulet of Yendor is special in that simply carrying it incurs a penalty similar to amulet hunger, but this penalty stacks ontop of amulet hunger.
To open chests: early on, you can use an edged weapon to #force a lock. This'll always use your equipped weapon, so it's best to get a spare weapon you don't mind breaking until you get a lockpick, key, or credit card, which you can (a)pply to a chest to unlock it. Be wary though, 10% of chests are trapped, so it's good to (u)ntrap them a couple times to be certain you're not opening something that will kill you.
You can also #force with a blunt weapon, or kick a chest until the lock breaks, but I strongly advise against this, as it might break valuable things inside the chest.