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.