Food Chains

If you should ever happen to find yourself in a Metaphysical Place or another locale not entirely bound to this Earth, especially the realm of The Fair Folk, under no circumstances should you eat or drink any of the native comestibles while you're there; the specific results vary, but you'll inevitably wish you hadn't.

This goes double for food that clearly belongs to someone else.

Most commonly, the results fall into four categories:
 * Eating the food traps the unfortunate hungry person in the place from which it originated, either temporarily or permanently.
 * After you eat faerie food, all human food tastes like dust, and nothing else will ever be appetizing. In some cases, it also has the nutritional value of dust, making regular meals not an option; with both versions, unless you have a steady supply of extranormal food, you're likely to waste away from starvation.
 * Eating the food brings you under the control of the entity in charge of the realm, who is typically a tyrant; if he is not an outright villain, they'll be one of the sociopathically amoral Fair Folk.
 * Eating the food brings on Involuntary Shapeshifting onto the one who consume the food, oftentimes it doubles as a skewed Karmic Transformation.

Contrast Sacred Hospitality.

Anime and Manga

 * Spirited Away has both an example and an inversion: The trope is played straight when Chihiro's parents are turned into pigs by the food at the abandoned restaurant, but Chihiro herself has to eat some of the spirit realm's food or else she'll fade out of existence. The difference is that her parents ate food that did not belong to them and transformed into pigs. Chihiro, on the other hand, ate food that was offered to her.

Comic Books

 * Used throughout the Books of Magic, particularly when the main character's girlfriend accidentally eats a berry while trapped in Faerie and can't ever eat human food or touch human soil again or she'll crumble into dust. She goes on a Roaring Rampage of Revenge that sets much of Faerie on fire until the Faerie Queen confronts her and apologizes, giving her a bag of endless Faerie food and a levitation enchantment to compensate.
 * Used as a central plot element in Walt Simonson's famed run on Marvel Comics' Thor, where the Dark Elves of Asgard were revealed to be one and the same as the Fair Folk and enslaved many humans by tricking them into eating faerie food.

Film

 * In Pan's Labyrinth, Ophelia is told not to eat any food while taking the second challenge. She eats a few bits of fruit off the table, and wakes the Pale Man.

Gamebook

 * Completely averted in Book 11 of the Lone Wolf series. All of the food Lone Wolf finds during his stay in the Daziarn is delicious and nutritious despite being unlike anything he's ever eaten in his plane of reality, and there are no consequences whatsoever after eating it. The people in the Daziarn who offer him food and drink are pretty friendly too. Ironically, Lone Wolf is more at risk accepting food and drink from others in his own dimension since it tends to be drugged/poisoned.

Literature

 * The Forbidden Fruit from The Bible, which trapped Adam and Eve (and all their descendants) in sin.
 * In The Spiderwick Chronicles, Lucinda Spiderwick had made the mistake of eating Faerie Food, and is no longer able to eat human food. But that wasn't in the Faerie Realm.
 * A variant occurs in the Mode series by Piers Anthony—if you eat something in a universe which isn't your own, that makes it harder to leave that universe for a few hours, mostly because you can't transport anything across modes that doesn't originate from an anchor mode meaning that you will "drop" the half digested food at the mode boundary. You can wait for it to digest completely but you should avoid doing that too often because your body will slowly become made up of enough of that mode's substance to cause you to lose your connection to the virtual mode and get trapped there.
 * The same trope appears in the Xanth series by Piers Anthony as well: eating the food in the dream world traps a person there (at least, that's what they always said...) No one was ever trapped in the dream realm in the books, but after Nada Naga drank a few drops of wine there, she wound up having to work for the demons. When she threatened to quit, they told her that she was obligated to the dream realm for drinking the wine, and that they'd "acquired the option" on her service. It was implied that if if the demons hadn't needed her, she'd have been stuck in the dream realm for a few years, and that if she'd had more than a few drops it could have been permanent.
 * Goblin Market by Christina Rossetti has the 'tastes like dust' variant.
 * In The Wee Free Men, if you eat when a drome has trapped you in a dreamworld, you'll never get out..
 * Eating in Fairyland itself seems to be safe, though;.
 * Subverted in The Voyage of the Dawn Treader. The heroes find a feast in the middle of some ruins on an island near the end of the world, along with three sleeping men who have obviously been sleeping there for a long time. The heroes, reasonably enough, assume that the men's enchanted sleep was caused by eating the food; because the men are the missing lords Caspian was searching for, they also resolve to sit at the feast all night in case that could break the enchantment. Turns out, the men . The owner of the island and his daughter tell them how to break the enchanted sleep and invites them to winter on the island, which most of them end up doing, with no ill consequences.
 * The Turkish Delight in The Lion, the Witch and the Wardrobe, plays this trope straight.
 * And the Forbidden Fruit in The Magician's Nephew, makes you immortal, but when eaten without Aslan's permission, makes your existence miserable.
 * Variation in Welkin Weasels: the weasels are trapped in Hunters' Hall, which they are told is the land of the dead, and that once they have eaten the food they can't leave.
 * Invoked in The Dresden Files, when Harry tells Billy to not eat or drink anything while they meet with some Faeries. Apparently, however, you can use the ice cold water to bring your libido to its senses.
 * In another book, Harry's inner monologue explains how it works: If you eat Fae food, it counts as the Fae doing you a favor, and you then owe that Fae one in return. Apparently, though, if a Fae offers you a perfectly mundane sandwich and can of Coca Cola, it doesn't count.
 * Mentioned in Percy Jackson and The Olympians, when Percy is warned by Zoe Nightshade not to eat anything from the garden of Hesperides. The characters also briefly see some of the pomegranates that Persephone ate.
 * In Midnight Riot by Ben Aaronovitch, the protagonist, Peter Grant, goes to interview Mother Thames and is told not to eat or drink anything while he's there or he'll fall under her power. She keeps trying to feed him, but seems to view it more as a game than anything, and is in no way insulted when he refuses.
 * While it is played almost for laughs with Peter, Mama Thames has a servant who used to be a Bailiff that came around in the 1970s and accepted a cookie. The book is set in 2010.
 * Mama Thames daughter Tyburn also tries to Mind Control Peter into drinking water from her fountain and binding him to her will.
 * Alice in Wonderland: Any food or drink Alice takes in Wonderland makes her grow or shrink.
 * Dragonback has the concept of the squatter poison; once it's fed to you you need to be given the antidote daily or die. Dragon and Slave does not say whether it has an expiration date.
 * This may be inspired by Dune, which had the Harkonnens poison Thufir Hawat with a poison that will kill him unless he takes an antidote (whose supply they control) every day for the rest of his life.
 * Bone Chillers, a series of horror fiction novels very similar to the more well-known Goosebumps, had one issue "Back to School" involving a school cook who serves extremely delicious and addictive food. It's eventually revealed that the cook is a giant insect who puts larvae in her food for the school children to eat, incubating in the hosts' bodies until they're ready to burst out once they've served their purpose. The cook is only stopped by the efforts of the protagonist (who was infected with the larvae) and a fellow schoolmate who was allergic to anything besides specially prepared lunches.
 * Fairy food in The Girl Who Circumnavigated Fairy World In A Ship Of Her Own Making constitutes a binding magical contract that means the person who eats it has to return once a year.
 * In "Mirror, Mirror, Off the Wall," we're the Fair Folk in the equation: when the reverse-timoline Robert Trebor visits our world, he can eat our food but not metabolize it. He has to keep a small stash of his own world's food on hand to keep from starving. He plans to market reverse-timoline food as a diet aid.

Live Action TV

 * Another variant occurs in an early Star Trek: Deep Space Nine episode, wherein Kai Opaka journeyed to a planet where she was killed, but microbes in the atmosphere brought her back to life and healed her. The catch is that, in order to continue living, she can never leave that planet.
 * In True Blood, any human who eats the light-fruit can never leave Fairyland without turning to dust.

Myth and Legend

 * Izanami from Japanese creation mythology is forced to remain in the underworld (Yomi) after eating there when Izanagi tries to rescue her.
 * The legend of Persephone (Roman name: Proserpine) in Greco-Roman mythology: Persephone ate six pomegranate seeds while in the underworld, and is thus forced to stay there for six months out of every year. Her mother Demeter, goddess of the harvest, gets depressed when she's gone, and now we have seasons. (In the Mediterranean, the nasty season is the summer, when everything is too dry to grow food.) That's the story with any mythological World of the Dead. Or elves. Or with the Lotophagi.
 * Chinese mythology features a literal example. Once there was a Goddess, Shui Mu Niang Niang (roughly meaning "Old Water Mother") who was constantly, and rightly antagonized by the Celestial Bureaucracy due to her hobby of causing devastating floods with her magic buckets. After the latest attempt to stop her by having a disguised god use his donkey to try and fail to drain one of her buckets dry, Shui Mu retaliated by overturning her other bucket and casually obliterating a city, Ssu-Chou, forming what is now the Lake Hung-tse. The Goddess of Mercy, Guan Yin, was then tasked to subdue the Water Mother. Guan Yin did this by disguising herself as a noodle vendor, and convinced Shui Mu to try some of her noodles. When Shui Mu was halfway finished, all of her noodles turned into iron chains, including the ones she had eaten. The chains then magically dragged the Water Mother down a well, fastening her to the very bottom. People visit the well where Shui Mu is still imprisoned, and it's often said that when the well water is very low, you can see the ends of her chains.

Tabletop Games

 * One of the ways in which a mortal might catch the attention of the True Fae (resulting in kidnap and durance) in Changeling: The Lost is by eating food the Fae's claimed as its own. Also, some of the fluff indicates that humans transform into Changelings in Arcadia, in part, by eating the food, drinking the water, and breathing the air; this is because Arcadia operates under contract law, so in order to gain nourishment from any of those elements, you must become a willing signatory to their respective Contracts—and, in doing so, become an entity of Arcadia.
 * In GURPS Time Travel, Time Agents need to be very careful not to eat or drink anything in past-time. Sure, you can leave past-time whenever you want ... but matter from past-time stays in past-time, even if it's been broken down into proteins and incorporated into a Time Agent's body. The consequences are ... messy.

Video Games

 * In Arthur's Knights - Tales of Chivalry - on both paths - upon ending up in Avalon, Branwyn's squire accepts a gift from a fairy. The guy got turned into a giant mushroom for it.

Web Comics

 * A variation/subversion in Sluggy Freelance: the demon food in the Dimension of Champagne does not tie you to the dimension (the demons do that with, y'know, chains) but its calories can never be exercised off afterwards. Women in particular do not take this revelation very well...

Western Animation

 * Adventure Time: In "Dad's Dungeon", Jake almost eats a magical fruit set forth by a trio of floating fruit witches out of despair when Jake makes him think his adoptive father is not proud of him (in fact, Jake was forced to do so under a promise from their deceased father's hologram message to toughen Finn up). The Genre Savvy Jake knocks the fruit out of his hand and force feeds it to one of the witches, where it transforms into a giant apple and is devoured by its kin. Note the apple had BONES jutting out of its core after the witches were done with it (shiver).

Real Life

 * A non-supernatural example, but children can't work to meet their own needs and are reliant on others for food.
 * Any addiction.
 * Animals that learn to beg for human food sometimes become dependent on it, and forget how to find food on their own.