If wizards had to draw energy from their bodies, using only existing biological processes and bound to vanilla human metabolism rates, how would they be inclined to maintain their bodies and diets to adapt for different situations?

Source of magic

Magic is produced by an organ located in the pelvis, which is supplied with the same boring old ATP that powers a regular fellow's muscles, organs, etcetera.

Now, humans can only output like 2000 watts (joules/second) at max for short durations, or around 400 watts for a couple of minutes. That's barely enough to boil a teaspoon of water; wizards have got to be more powerful than that. And the human body stores a lot of more energy than it can release, both in the form of ATP in the bloodstream, and well, fat stores on the body.

So, this magic organ is also a capacitor; the ATP is converted to electrical energy and stored in a biological battery. This charge degrades over time, let's say by 50% a month, so the battery is not typically filled to capacity at any given time. The wizard has some control over this process, and can decide to fill up the battery if they expect they will need it in the near future. It can hold up to 200 megajoules of energy, and can release it in about five seconds. That's enough for your typical high-end wizard.

(I get that this is insane electrical energy density, 200 Mj in an organ not weighing more than a kilo. However, the most efficient actual batteries would only hold around a single megajoule with that mass, and while that's enough to do some magic, it's not enough to make this organ dominate metabolism the way I want it to. If 200 Mj/kg sounds too extreme, you could say that the charging process is horribly inefficient, and it takes that much energy to fill it with only 20 Mj. How effective their magic is does not matter much in the context of this question.)

Uses of magic

Magic in this setting is a rough art. You cannot thread a needle with magic: you crack walls, throw winds, and wreak all kinds of havoc.

Just to get an example which is easy to calculate: throwing stuff. A rock of 10 kilograms is about the smallest thing you can reliably manipulate, without loose tendrils of magic latching on to the surroundings and causing undesired side effects. Imagine you want to breach a wall: you just launch the thing at 500 km/h, which is the low end of cannonball speeds. That takes about 100 kilojoules, so on a full battery a wizard can throw two thousand of these.

Other forms of manipulation include heating up stuff, and ionising matter to produce lightning bolts. It's likely more uses exist and they haven't all been discovered, but the common factor is that you cannot concentrate magic on very small objects that well, and expected energy output is quite high per use. That makes stuff like healing out of the question.

Also consider a distance limit of magic projection to be roughly within arm's reach of the sorcerer - outside that range it gets progressively less powerful and even less accurate.

Implications on metabolism

Now onto the meat of this question (literally). I started thinking about the implications of this system, starting with the optimal physique and eating habits of sorcerer or sorceress in question. I had first imagined them as intentionally keeping themselves pudgy, because 200 megajoules is roughly 5-7 kilograms of body fat, and a wizard would want to have that in reserve at all times, perhaps several times over. Though every-day magic tricks will not consume much energy, a wizard generally wants to be prepared for extraordinary situations, like a building collapsing on them or a battle with another wizard, that would require them to fully drain their reserves. And as said, the charge degrades over time, so they cannot just fill it up and forget about it.

But, in a friendly environment, they can simply eat like lions, and get the ATP from the bowels into their bloodstream, to be extracted by the organ. The magic does not make them more efficient at digesting food, but real-life pro athletes like Michael Phelps consume upwards of 40 megajoules (10000 kcal) a day whilst training for the Olympics. So a wizard with sufficient nutrition at his or her disposal can charge up fully in five days.

A full battery in five days a lot quicker than filling it up from the body's native long-term energy stores, aka their body fat. The maximum speed of metabolization of body fat, aka the fastest way for people to lose weight, is obviously subject to much commercially motivated misinformation, but 1 kilograms a week is commonly cited as a safe maximum, and I suppose that these wizards, who are specifically trained and have loads of experience, can double that up. Two kilograms a week, or 80 megajoules, means a wizard otherwise eating normally can "passively" charge their battery in half a month.

The implications for a wizard in a hostile environment is that they would prefer stocking lots of food over fattening their bodies. A single day of gorging is equivalent to three days of metabolising, after all; the only limitation is how long their food stores keep.

Having covered the domestic wizard and the war wizard, I think the wandering wizard may be the only one inclined to maintain a high body mass, especially if they can ride a horse. Although they can also carry food with them, that won't be in Michael Phelps quantities, and they must also anticipate times when they are without food.


So these are my ruminations on the topic, and I was wondering what situations and types of living environments I might have missed. Assuming the organ as described in the first five paragraphs, and a generic late medieval setting (with no other magic), what is the spectrum of situations and possible corresponding habits these wizards could have adopted to have their magical battery at the appropriate charge? If your answer concludes that the same strategy would work just as well for any situation, I would appreciate a suggested adjustment to the organ so that different environments call for different tactics. The goal here is to have wizards who are about as conscious of their food intake as diabetic patients.

Finally, note that the wizards have been doing their thing for many generations. Assume every process, habit or recipe to be optimised by centuries of experience.

  • 1
    $\begingroup$ I have a similar magic system, and people keep telling me that 2500W is too low for "short burst" power. I think you can also maintain around 300-400W for more like an hour or so, but you're going to notice that you're doing it. $\endgroup$
    – Matthew
    Feb 14, 2020 at 16:04
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    $\begingroup$ Have you considered that the presence of this very hungry organ would influence human evolution to the point that they would probably have better ATP production than us "vanilla" humans with no such organ? $\endgroup$
    – Muuski
    Feb 14, 2020 at 16:17
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    $\begingroup$ @KeizerHarm, it's a great question. May I also recommend putting some combat-related strategies in your question context as well? These are very important for consideration, as they affect how quickly a mage may need to output their magic, or how long a mage can output magic before they're empty. $\endgroup$
    – Enthu5ed
    Feb 14, 2020 at 21:19
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    $\begingroup$ @Enthus3d I was going to, but I'm honestly conflicted. Going with my usual approach of calculating joules for everything, the setting is starting to feel more restricted: the most efficient use of energy is shoving things at great velocities. The same energy that would heat a rock by 500 degrees is enough to throw it at Mach 1. There's ways to do creative things with the energy-based approach, but I feel like they belong in a question focused on that; this one was specifically about metabolism. $\endgroup$
    – KeizerHarm
    Feb 14, 2020 at 22:05
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    $\begingroup$ @KeizerHarm great, the question feels a lot clearer now, thank you for putting the time to clarify it :) $\endgroup$
    – Enthu5ed
    Feb 14, 2020 at 22:46

2 Answers 2


To answer your question, I will split my answer into 3 distinct types of mages, the Domestic, Combat, and Wanderer mages.

Domestic Mages

Domestic mages will have access to food supplies when needed, and thus only need to consume as much food as the magic that they use. Also, domestic mages will be mainly decided based on the capacity of their magic organs, rather than output speed. The organ itself is more important than their body weight reserves, although mages with extremely long, drawn out jobs would probably prefer a larger body fat reserve to help tide them through.

Combat Mages

For combat, combat mages would be similar to the professional athletes or soldiers of today. They will have to take up strict diets like sumo wrestlers to train up their organ's capacity and magic output speed starting from childhood, and their innate talent for storage and output speed of their organ will decide their future. These are the elites of the elites, with only those with the best capacity and/or output intensity allowed to occupy their ranks.

Combat mages can be potentially split into two categories.

Crowd Killers and Siegers (High Capacity)

One type of combat mage would probably be attrition-based combat mages specialized for dealing with large crowds of targets, or siege mages, who specialize in laying siege, both of which may need to output magic over long periods of time. Their body fat would be reserves to last them through long, drawn out fights.

A large group of Crowd Killers acting in tandem could release waves upon waves of flames to drown out any dissidents or crowd tactics, and also depend on their capacity to release large amounts of liquids/substances to drastically lower enemy mobility, or put up solid cover for other mages.

A large group of Siegers could potentially release waves upon waves of arrows, rocks, and other projectiles at city walls, making use of their capacity to continuously release magic until the enemy is drowned.

Mage Killers (high-output)

Mage Killers are the cream of the crop amongst combat mages, specializing in killing other Mages. These are the special troops and leading generals, possessing the greatest combat potentials.

For Mage Killers, there is one very important deciding factor: more mass on the body means lesser maneuverability, as well as higher energy consumption when engaging in combat. The amount of energy needed to accelerate a larger mass is definitely more than that of a smaller one. Also, Mage vs. Mage combats end extremely quickly, considering the lethality involved. It does not take much energy to launch a rock at the speed of a bullet, and any human hit by such a projectile would be fatally wounded.

If a combat mage were to have more weight, and was especially immobile, they would be a sitting duck for any other mage spells. In mage vs. mage combat, a Mage Killer relies on their lesser body weight and high magic output to explosively increase their speed, evasion and maneuverability, making it more difficult for their opponent to land a hit. Their ability to more quickly output magic would also come in handy, as more output means faster and more powerful spells, allowing them to attack faster and further than their opponents.


As speculated, Wanderers are probably the only class of mage that needs a high amount of body fat reserves. The higher body fat reserves mean being able to more liberally use magic when the situation arises, especially if situations where food or water for recharge are scarce. This comes in handy when they run into tricky situations in their travels, wandering across the lands.

Wanderers are the most variable of the mages, potentially possessing low talents for output or capacity compared to other kinds of mages. Their greatest strength, however, is their versatility, using both their greater body reserves and wits to come out on top. The high body mass impacting maneuverability in Mage on Mage scenarios is a drawback, but Wanderers can rely on their high sense of intuition, instinct, and combat experience from their travels to avoid direct combat with deadly opponents like Mage Killers, and slowly whittle them down with their own considerable reserves. Some Wanderers, now on the run from the authorities, have even slain well-known Mage Killers. Their magic style is also the most unorthodox of all mage classes, given their tendency to wander and pick up experiences from all over the great unknowns.


In general, domestic mages will have the most variation on body weight and metabolism, with their body weight depending upon their day to day magic usage. Higher standing and social positions would go to those with better ATP.

Combat mages can consist of high intensity/output Mage Killers, who act as special troops, generals, and the elites. They rely on lesser body weight and high output magic organs to perform explosive maneuvers and spells. High metabolism would be a boon for these mages, as the increased metabolism of the body means faster energy output, and more powerful magic.

Another class of combat mage would be high reserve type Crowd Killers/ Siegers, who rely on their body weight reserves to continuously output magic. A lower metabolism in this category is still acceptable, since a lower metabolism means a slower burn rate of energy and steady consumption, both boons to these endurance combat mages. A high metabolism would also work, allowing them to partly replenish their reserves during sieges.

Wanderers are the most versatile class of mages, relying on their high reserves and wits, as well as their highly unorthodox magics to even the tide. Metabolism is not the main concern for these mages, instead, storage, although low metabolism would be favorable due to lesser energy consumption over time, useful for when food or supplies are low.


From these examples, we can conclude that the only class of mages that require high metabolism are Mage Killers, as they have more need for faster energy output and consumption. On the other hand, Wanderers are the ones that benefit the most from a low metabolism, allowing them to lower their general idle consumption of energy, as well as for when food and supplies are running low. The other classes of mages are less reliant on general metabolism, and this can vary from case to case.

Of course, these are just some potential archetypes for the mages; there could be specialized Mage Killers who rely on their senses and spells to snipe other mages, and keep a high reserve of body fat for more shots. There could be Wanderers who engage in mostly disreputable activities like theft and robbery, and so need to keep their maneuverability and combat power at a maximum.

Hopefully these archetypes provide some examples you can use to help define your world. Thanks for reading to the end!

  • $\begingroup$ A belated thank you for this answer! You went a lot deeper into differences between individual magic users than I had considered,and this selection of archetypes feels like it runs the gamut of situations quite well. $\endgroup$
    – KeizerHarm
    Feb 14, 2020 at 22:39
  • $\begingroup$ @KeizerHarm no problem, I get a kick out of writing answers too, haha :) $\endgroup$
    – Enthu5ed
    Feb 14, 2020 at 22:40
  • $\begingroup$ This is such a unique and interesting idea. $\endgroup$
    – cowlinator
    Feb 15, 2020 at 1:25

As you've pointed out, even in the modern age there are a lot of competing opinions and vague studies regarding nutrition and optimal physique, and it seems likely that a similar cacophony of standards would arise around the system you describe.

It could be helpful to address a couple of additional questions about your organ:

  • How sensitive is the performance of your organ to the performance of other organs in the body? Unusual or short-term benefits in relation to other aspects of health -- e.g. temporarily higher capacity when the wizard is highly caffeinated or while there is an excess of glucose in the bloodstream -- could affect accepted wisdom on the subject.
  • How dependent is the performance of your organ on genetics? If its capabilities are difficult to compare directly between individuals, there might be a lot more justification for variations in habits among wizards.
  • Are there ways to artificially stimulate or improve the capacity of the organ in an obvious way? I.e. is there an effective means of ATP supplementation that boils down to taking steroids or B-12?

In a situation where these interactions are complex and wisdom about such an organ is based on oral tradition or, at best, semi-empirical research, I see eating habits breaking down along organizational lines that are finer grained than what you've described above.

Combat wizards conscripted into a military, for example, might rely on the scheme you've described, while bandits or hedge wizards might rely on stimulants in the absence of proper nutrition.

A clan of wizards from the same ancestral line might have found that the more naturally pudgy they are, the better they perform.

Or, in high society there may be a popular fad among wizards to stay as lean as possible.

The existence of special, proprietary foods or nutrition regimens (effective or not), provided by enterprising individuals both sincere and not, would also be a near certainty in this scenario.

This type of nuance would also provide the basis for lots of plot points.

  • $\begingroup$ Good point. I hadn't considered that variation in organ effectiveness, combined with a non-scientific era and a tendency towards secrecy, would naturally spawn different habits associated with different magician communities, based on their experiences. $\endgroup$
    – KeizerHarm
    Feb 17, 2020 at 8:44

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