Typically, Functional Magic is assumed to rely on some universal, pervasive force, like a Background Magic Field as its source of mana. Alternately, it might be drawn from the collective Life Energy of the local environment. In a pinch, it could come from some quasi-benevolent gods. But if these aren't available, it might just be possible for a supernatural being to utilize the stored caloric energy of one's own body as a fantastic power source. This isn't nearly as bountiful as the other options, so it's often used as a desperation tactic.
From a physics standpoint, this actually makes a surprising amount of sense. There's enough chemical energy in a living person to make a reasonably powerful bomb, if it were possible to combust it all at once. If we assume magic has some means of efficiently converting calories into the proper form, the results could be very noticeable- and the subject would find their last few meals negated, if not suddenly suffering malnutrition. Of course, the effectiveness of this process tends to be heavily exaggerated; similarly, the amount one can eat at one time in order to recompense this usage tends also to be exaggerated; many magical entities are Big Eaters who can easily wolf down massive amounts of food, which is apparently changed instantly to a metaphysical form that doesn't take up physical space.
It can be hard to separate this trope from its close relative, Cast From HP, and indeed they can easily overlap, as the difference between physical bodily energy and the metaphysical concept of Life Energy can be hard to clarify. To distinguish them, consider the following question: when feeling the side-effects of supernatural overexertion, which would be a better remedy, a night's sleep or a solid meal?
This trope can also easily overlap with Hyperactive Metabolism, which entails a boost to natural abilities, usually in the form of healing, from eating. Despite the name, has nothing to do with Wizard Needs Food Badly, a video game trope involving the need for food as an arbitrary restriction.
- Literally the case in Slayers. Word of God is that the bodies of those with high magical capacity fuel that capacity directly from the calories in the food they eat, which is why even waifish and petite girls like Amelia and Lina (both powerful spell-casters) eat thousands of calories at each meal and remain thin.
- In Bleach the souls of the deceased don't ordinarily have to eat, but those who have a significant reservoir of spirit energy do. Their intake doesn't necessarily correlate to their spiritual output, however.
- Dragon Ball is probably the Trope Codifier for manga and has definitely influenced other works, especially in Japan. All fighters need calories for Ki Manipulation, and are Big Eaters as a result.
- This is one of the first indications that Goku was different. From a young age, Goku showed off an ability to consume unbelievable amounts of food. In early stories, he would often be weakened by hunger to the point of uselessness. As he grew, his stamina increased and this became less of a problem. A Running Gag was whenever Goku goes for dinner, he will cause whatever unfortunate restaurant he's eating at to run out of food. Pile after piles of bowls will stack up around him, while he eats courses meant for groups of people.
- It's later revealed this is a trait shared by Saiyans in general. Vegeta, Gohan, Goten, Trunks and even Nappa are prone to eating the same enormous amounts. A famous scene during the Buu Arc, has Goku, Vegeta, and Gohan at the World Tournament diner pouring food down and piling bowls up. Krillin even makes a remark on wondering what Saiyan stomachs were made of.
- In the Lyrical Nanoha, the magical energy mages use to cast their spells is generated by their bodies, so they have to replenish their calories at every opportunity. This is particularly true of frontline combat mages, like Subaru Nakajima, who is the series' resident Big Eater (due to also having to continuously expend her mana to power up all the cyber-enhancements built into her cyborg body). It's also implied that different styles of magic are more energy intensive than others with Belkan and Neo-Belkan mages being the most calorie intensive. Hence Subaru and Erio being much bigger eaters than Fate, the most close-combat focused user of Mid-Childan style magic in the series.
- In Naruto, Choji has a Super Mode that uses calories and is triggered by Food Pills. It's considered a Dangerous Forbidden Technique because it consumes so many nutrients that the user dies; however, Choji is so enormously fat that he has barely enough energy to survive, and becomes comically skinny in an instant. Parodied, when Naruto and Sasuke seem to be doing this, gouging down food and asking for more... only to hurl the food they had just eaten back up.
- In Negima!?, the Pactio system is reworked to draw on the partner's calorie count instead of the mage's magic reserves like in the manga. Thus, while Negi was testing Nodoka's Cosplay card, her stomach growled after changing back and he bought her a hamburger to make up for it. This plot-point becomes an issue whenever the students are trapped in a separate dimension without any access to food.
- One Piece:
- Luffy's Gear Second in speeds up his metabolism to a ridiculous degree, granting him Super Speed. As a result, it burns calories very quickly and switches over to Cast from Lifespan if he doesn't have the necessary energy.
- Luffy's Gear Fourth form, Tankman, works in the same way, only even more powerful and more voracious in its energy consumption. He was only able to safely use it when up against an enemy whose power is creating unlimited amounts of food, with Luffy running around eating that food for 18 hours straight. When he did use it though, he utterly curb-stomped the guy in a few seconds.
- Toriko is a story literally about food and eating. It makes a frequent point that those endowed with Gourmet Cells gain superhhuman powers but pay for this with extreme appetites. They need to keep eating vast amounts of food everyday to stay alive.
- Dreams of Alice, the Reality Warping children in Alice & Zoroku, burn calories to use their powers. In the first episode, the man in charge of the research facility Sana's escaping from notes they kept her calorie intake low precisely so they could limit her ability use her powers to get away. Sana ultimately makes her getaway when a third party enters the fray and gives her juice to drink and giving her just enough energy to teleport out of there. Soon after meeting Zoroku, he treats her to food and the other customers in the restaurant wonder if she's in some kind of eating competition.
- The Flash: Wally West had to take in high levels of nutrition to recharge himself. He overcame this weakness later but since he was a Big Eater at heart, he didn't stop eating inhuman levels of food.
- In Totally Awesome Hulk, Amadeus Cho needs to consume lots of calories to maintain the mass and energy needed to Hulk Out.
- Deadpool once explained that he has to eat a lot to allow his Healing Factor to work.
- Wolverine, whose healing factor is what Wade's is derived from, is occasionally depicted as needing to do the same with high protein meals. He gives advice to X-23 to follow suit.
- When using her shapeshifting to heal the new Ms Marvel, Kamala Khan, becomes extraordinarily hungry and needs to eat to recharge her powers.
- Dungeon: The Early Years: Horus and Alcibiade once escaped a trap by casting an anticipative necromancy spell on themselves, starving to death, returning as zombies, and devouring an entire herd of cows to return to living state.
- Iron Man foe Ezekiel Stane bioengineered his own body to be able to fire repulsor blasts like Iron Man, but doing so burns a lot of energy. He compensates by consuming a very high calorie goo.
- In The Amazing Spider-Man, after his powers started developing, Peter also showed a huge gain in appetite. When he gets home he grabs all of the food from the fridge, including Aunt May's meat loaf, which according to Uncle Ben nobody liked.
- Constantine. After Constantine uses his psychic powers to visit Hell for some info, he tells Rachel that he needs food, thus leading to an exposition scene in a diner.
- In Justice League (2017), Barry Allen mentions that he's become "a black hole of snacks" since getting his Super Speed.
- There's a similar effect in the Mage Winds trilogy. At least once when Darkwind is training Elspeth, he has to remind her to eat because use of mage energy 1) uses up the mage's personal energy reserves (a.k.a. previous meal) and 2) suppresses appetite.
- This is the case for wizards in the Liaden Universe novels. For minor everyday workings it doesn't have any obvious effect, but a working of any significant size will leave a wizard ravenously hungry, and there are at least two examples of a wizard visibly losing weight after a major working. (One is in Conflict of Honors, and is followed by Priscilla looking in the mirror and reflecting that she looks like she's the survivor of a famine.)
- The Runelords applies this to people who receive endowments of metabolism. They gain increased speed and reflexes, at the cost of having to eat more. It's implied to also increase their rate of aging, but this can be counteracted by endowments of stamina, which improve healing acumen.
- In The Stormlight Archive's second book, Words of Radiance, Lift is a Surgebinder — a person with magical abilities which are powered by Stormlight. However, unlike most Surgebinders who must get Stormlight from an external source, Lift can generate her own from the food she eats. While this ability is pretty convenient, it also means that she can not use her magic unless she has the calories to do so. Consequently, she is always hungry.
- Although normal magic in The Wheel of Time draws from the inexhaustible True Source, Healing uses the energy within the patient's body to heal them. This usually feels like a cold shock, with the severity and duration proportionate to the amount of healing. As such, Healing someone who is on the brink of death may cause them to die from the shock as their body repairs itself. Part of learning Healing is learning how to heal a severely wounded person enough that they don't die from their injuries but not so much that it uses up too much of their energy reserves. In the third book, Mat is Healed of a powerful curse and the lingering effects of the work cause him to become a Big Eater, having to eat many huge meals each day without gaining any weight, in order to eventually recover. An improved technique is eventually developed that doesn't have this drawback.
- In the third book of John Maddox Roberts's Stormlands series, Fyana plows through a fairly respectable spread as if she hasn't eaten in days after performing a major healing ritual.
- In the Tower and the Hive series, it is explained that this is true of people with Psychic Powers. The energy to move things around with one's mind has to come from somewhere, and Talents are mentioned as requiring a much greater calorie intake than ordinary people. Even then, the majority of telekinetics bolster themselves by siphoning power from electric generators to do heavy lifting.
- Discussed in Everworld: when the group wonders why Loki isn't coming after them, April suggests that he might be tired from his battle with Merlin that morning. Jalil, the resident skeptic who is constantly annoyed to find himself in a fantasy world, sarcastically wonders if magic burn calories.
- In Princess Holy Aura, doing magic or supernatural fighting burns a lot of energy, some of which must come from the magic user. This puts a severe hamper on Silvertail's magic usage.
The worst trial school currently presented was that they couldn't eat as much as they wanted to without making spectacles of themselves. Seika had discovered that the day after her first transformation. She didn't quite keep up with Holly in the eating department, but she was now eating more than anyone else in her house, easily.
- In Mark A. Garland's The Demon Blade series, wizards cast from their body's energy reserves, so before trying any serious feat of magic they have to fatten themselves up.
- In The Flash (2014), after first gaining his Super Speed Barry would often succumb to passing out during the day. It's explained that for Barry to run at such high speeds he consumes a staggering amount of energy, and hence he needs to eat that same amount each day. For convenience, Cisco makes Barry some high calorie energy bars. Barry also would have a habit of going to Big Belly Burger and eating about 20 or so burgers in one sitting. During the crossover with Supergirl, Barry himself says he eats about 10000 calories a day.
- The Defenders: Luke Cage is incredulous at how Danny Rand is able to eat so much and still be thin.
Luke Cage: How are you still hungry?
Danny Rand: It takes a lot of energy to summon my chi.
- Warhammer Fantasy: The ogre's spellcasters use Gut Magic, which, well, works on what the ogre has eaten.
- In Vampire: The Masquerade, Vampires can spend Blood Points on vampiric abilities; they must then recharge their Blood Pool by snacking on some mortal.
- Tachyon of Sentinels of the Multiverse, like the Flash, is a speedster and is almost constantly eating — there's a few pieces of official art that have her scarfing down burgers, and in the single promotional comic, she takes a slight detour through the villain's evil lair to hit its cafeteria.
- In Red Markets every skill check involving strength or speed requires the character to expend a charge of rations. Edible food doesn't keep well in the Loss.
- In NetHack using spells makes you hungry. If you're a wizard, having a high Intelligence stat reduces how hungry it makes you, and having a high enough intelligence can even negate the effect.
- Dungeon Crawl also makes you hungry when you cast spell. The amount of hunger goes up with the spell's level and goes down with a certain trainable skill.
- This happens to Melissabelle in Granblue Fantasy. Using her powers is really taxing on her stomach. Luckily, she's a competent farmer so food is usually not much of an issue, but she first meets the crew after getting careless and ending up lost in the forest with not even the energy to move.
- Mass Effect: While not a game mechanic, codex entries state that biotics (telekinetics) need higher calorie requirements than those without as using them is as strenuous as several kilometers' jog note . There's even a scene in the third game where Jack is telling her students to fill up on ration bars and energy drinks during a break in the battle.
- Spellcasting in Golden Krone Hotel causes your hunger to increase a bit. Increasing your intelligence lessens the effect, and the Scholar, the game's main magic user, starts with honey and curry noodle soup potions to combat this.
- In The Fourth, magic is explicitly said to be powered by calories, Skarva claims that "mana potions" are just milkshakes.
- Robin in the Walkyverse is a speedster whose abilities are powered by eating gargantuan amounts of sugar. This is played for laughs in Shortpacked!, where she will occasionally eat something truly outrageous in its caloric content and then wake up from a fugue state weeks or months later to find she's accomplished some lofty achievement in the interim.
- In The Adventures of Dr. McNinja, Frans Rayner comes back from the dead as a Cyborg with a single laser eye. But this laser eye is powered by his organic body — and it uses so much energy that a single shot leaves him too tired to stand, so he only uses it in one-on-one fights when he's sure it will kill his opponent.
- In an addendum to the roleplays of White Dark Life, Author!Luigifan shows off his biology knowledge by explicitly stating that many supernatural abilities in the setting, including those of the Belnades sisters, are powered by ATP. He further states that this is replenished over the short term by resting and over the long term by eating.
- Magic in El Goonish Shive is usually cast through Mana, but it's stated that magicians can burn calories instead if they want-Nanase even explots this to eat whatever she wants and still stay trim.
- The heroine of the MSPA Fan Adventure Alanna is struck by hunger when she unconsciously manifests powerful ice magic, or survives underwater longer than she naturally should. Turns out she's partially possessed by the Destroyer Deity, who gains power from devouring stuff, and used lots of such power saving her. Before facing the Big Bad, the god insists she only do so well-rested and well-fed.
- This is referred to as the "Joel Duscher Bird Theory" in Plumbing the Death Star; Joel Duscher answers "Why Isn't Nightcrawler Fat?" by positing that using mutant powers like Nightcrawler's teleportation would expend energy like flight does for birds. Problem is, other mutants like Cyclops and "the one with the bees" can't turn their powers off, so under this theory those characters should be constantly exhausted and starved.
- In Shadow Unit, merely having a paranormal ability doubles a person's daily caloric intake; the team considers an increase in caloric uptake a sign of Traumatic Superpower Awakening. Actively using the power can starve you to death in hours (or even faster). Chaz considers himself to be middle-aged - burning all those calories is medically proven In-Universe to be wearing out various organs - and nearly starved to death as a child on numerous occasions; his foster families treated it as an eating disorder, so he had to resort to scamming his fellow students for extra lunch money and stealing sticks of butter out of people's fridges just to swallow them whole. When Chaz is held captive by his uncle/father, he's reduced to a third-world refugee skeleton in a matter of days.
- Glowfic: Eclipsed need to eat a lot to do magic and they get hungry when they use a lot of magic and the scale of things one can do with it is influenced by metabolic efficiency.
- In Savage Divinity with enough skill a healer can cure anything short of death, the cost is that healing still needs to convert excess body mass to heal the wound, overtax your healing without eating enough and you will waste away.
- In Justice League, The Flash is frequently seen eating high-calorie food whenever he's not actively performing deeds of superheroics. He is, on at least one occasion, seen devouring an armful of hot dogs, which he comments is for keeping up his caloric intake which, as a speedster, is understandably high.
- Kid Flash in Young Justice keeps a small supply of food in a hidden compartment in his right glove, due to his accelerated metabolism.
- In The Avengers: Earth's Mightiest Heroes!, the Incredible Hulk can often be seen sitting in the kitchen of the Avengers Mansion eating entire chickens as if they are drumsticks.