Nutrition is essential for keeping your character alive. Too little and you starve; too much and you choke.
Sources of nutrition
Comestibles are the main source of nutrition. eating increases your nutrition (equivalently, decreases your hunger) by a set amount. For example, a food ration grants 800 units of nutrition. If polymorphed into a metallivore, metal objects can also be eaten for nutrition.
Wraith corpses can be eaten to gain levels, but provide zero nutrition. It is nevertheless possible to choke on them (see below).
A player starts with 900 nutrition points.
On the status line, your hunger is displayed if you are anything other than "Not hungry". The following table lists the hunger states and the corresponding amount of nutrition remaining.
|2000 or more||Oversatiated|
|1000 or more||Satiated|
|150 to 999||Not hungry|
|50 to 149||Hungry|
|0 to 49||Weak|
|Below minimum||Starved to death|
If your nutrition is 1500 or more, you may get a warning that you're having a hard time getting the food down. If you continue to eat, you risk choking to death. Sometimes NetHack will give you an option to "Stop eating? (yn) <y>". If you continue to eat, you risk choking. If you select y and eat something else, you will immediately choke. You cannot choke unless you are already satiated, but if you are, you won't always get a warning. You cannot choke to death if wearing an amulet of magical breathing - instead you will "stuff yourself, then vomit voluminously", losing 1000 points of nutrition.
If you consume any food (even a zero-nutrition wraith corpse) while oversatiated, you immediately choke.
While weak, your strength is decreased. While fainted, you are vulnerable to attack as if paralysed. Being satiated abuses dexterity for all classes, and wisdom for Monks only. Being hungry exercises wisdom for Monks.
Occasionally, while hungry you will receive the message "(player class) needs food badly!" (This message always occurs when you become hungry if playing as a Valkyrie or Wizard, or as an Elf.) This is most likely a reference to the series of Gauntlet dungeon crawler arcade games, where food is usually a far more pressing issue since it is a healing item. In the Gauntlet games, the narrator will boom the same message when your player is dying.
Every turn, you lose one point of nutrition, unless you are wearing a ring of slow digestion or are polymorphed into an inediate monster. If you are asleep, there is only a 10% chance this point of nutrition will be lost. Every twenty turns, you lose one point of nutrition for each ring you wear (the turn that this is calculated is different for your left and right hands), unless it is a chargable ring and is at +0. If you have regeneration that does not come from an artifact, you lose 1 point of nutrition every odd turn. If you are generating conflict and this is not caused by an artifact, you lose a point of nutrition every even turn. If you have intrinsic hunger, you will lose 1 point of nutrition every even turn. Every twenty turns, you lose one point of nutrition if you're wearing an amulet. Every twenty turns, you lose one point of nutrition if you're carrying the Amulet of Yendor. Being Stressed or worse incurs a point of nutrition loss every odd-numbered turn. All of these sources stack, so it is possible to burn nutrition at up to 320% of the normal rate. Note that 'turn' refers to game turn, not movement turn. The nutrition is lost as the turn counter advances, not as you move.
Attacking a monster uses the same amount of nutrition you'd consume on a whole turn, in addition to the normal hungering.  For this purpose, attempting to move into the same square as a monster counts as an attack, even if, e.g., the monster is a pet which you displace. The m command never incurs this extra hunger.
Spellcasting without hungerless casting or reduced-hunger casting (both of which are granted only to high-intelligence wizards) incurs a hunger penalty, unless the spell is detect food, which costs 0. The base penalty is ten times the level of the spell you're casting. Spellcasting with the Amulet of Yendor incurs an additional d(spell level * 4) hunger penalty (which is also affected by wizard hunger reduction).
Nutrition and pets
All pets, except for inediate pets, require nutrition in order to stay alive, though 2-8 times less than the player, depending on its size. Carnivorous and omnivorous pets will generally kill enough monsters on their own to be able to live off the corpses of the monsters they kill. Herbivorous pets have a harder time, since the majority of monsters do not leave vegetarian corpses. However, fruit and vegetable items such as apples, carrots, melons, and so on provide a herbivorous pet with many more turns of nutrition than they provide to you. It is thus optimal to save these items for a herbivorous pet, if you have any, rather than consuming them yourself.
When a pet is running low on nutrition, it may become confused. Pets who run out of nutrition points completely will die if they are on the same dungeon level as you, and otherwise they will go feral. Chatting with a pet often gives a clue about its nutrition level. If a pet is about to starve, and you have no way to feed it, one way to save its life is to abandon the dungeon level and come back later once you have a way of re-taming the pet. This works especially well with domestic animals, which are easy to tame.
Starvation is a common cause of death during the early stages of the game. Here are some tips for maintaining a healthy caloric intake. This discussion concerns only quantitative aspects of nutrition; for a more sophisticated overview of the relative qualitative advantages of different types of food, see the article on food.
The most basic advice is, of course, to eat. Non-perishable comestibles should be picked up and saved in your inventory. Monster corpses are perishable, so when a monster dies, eat its corpse immediately if the corpse is safe to eat. Novice players often forget to eat corpses when the opportunity presents itself, and end up dying of starvation as a result. However, don't eat while satiated, as this abuses dexterity and there is the risk of choking to death.
If you have a pet, your pet will often eat monster corpses before you have a chance to get to it. To lessen this problem, stand next to the monster while your pet is fighting, and pounce on the corpse afterwards. This works best if you are fast or if you can whistle away your pets. inediate pets will not cause this problem.
Once you reach either Minetown or Sokoban, you will most likely encounter a lot of food, since these levels tend to be generated with large amounts of food. At this point it becomes possible to subsist on non-corpse food items. Some players prefer to descend quickly to one or the other of these destinations, in order to get the food. Generally speaking, Minetown is easier to reach if you are playing a dwarf or a gnome, and Sokoban is easier otherwise.
In an emergency, if you are not trying to be an atheist, you can pray while weak to restore your nutrition status, provided that it is safe to pray. Since all games start with an initial prayer timeout of 300 turns, this means that, unless you do something out of the ordinary (such as receiving a wish), you can always pray safely after 300 turns. However, prayer is a very useful tool, which you may want to save for other things.
The foodless article describes a number of the more obscure ways of gaining nutrition without consuming food.