You can look at this from one of three perspectives:
- The spells consume based on their specific effect and have the exact impact on the body in terms of the energy output. (See John Robinson's Fireball answer.)
- The spells consume based on their range of effect and have an impact on your calorie count based on what "tier" of magic the spell is. (A Level 1 Spell may used 1,000 calories, but a Level 10 Spell may use 100,000 calories.)
- The spells consume an arbitrary amount of calories, not tethered to any well-detailed and clearly understood measure (similar to MP costs in video games).
- Since John Robinson already explained this, I won't go into much detail. Basically, you find the energy cost of the desired action, and you burn that many calories from your wizard's body. Teleport a mile? They lose the same calories as if they sprinted at their top speed, just without the benefits of actually running. Push a glass off a table from afar? They lose the same calories it takes your typical housecat to do the same. Burn down a village? Lose 80k+ lbs.
- This would look at the spells as being more like abstractions of energy consumption costs as opposed to being wholly based on the law of thermodynamics. In some cases, you may spend more calories on an appraisal spell (1k calories) than what it would take to just look it over (1 calorie). In other cases, you may spend far less calories casting fireball (100k calories) than if you were to let off a big, self-contained explosion (80k+ lbs).
- This would be pure abstraction. The only solid fact of this system is that the costs are not subject to change based on your output, but there will be costs. Basically, whereas Option 2. would have you rank each spell's mana consumption on a scale of 1 to 10 and apply a cost to the mana consumption levels, Option 3. would have you give each spell a mana consumption cost that would be an abstract representation of what they actually do. In a way this is sort of a middle ground between 1. and 2., but in a way it is also taking Option 2. to an extreme.
Depending on which route you take, use of this would cause different effects on the human body both because of the system of magic and because of the culture that would spring up based around it. Overuse would then be a natural progression from the effects of use.
First, converting your energy into an actionable spell would require some kind of organ or cell structure that can perform the process. Something like the pancreas (organ) where the body takes the sugar and fat and naturally processes it into mana, thus giving you a limited supply to work from at any given point; or, something like the mitochondria (cell structure) where the energy is already converted and ready to use or where it takes the energy from the mitochondria and converts it into mana as needed, it just needs the right stimuli for the structure to release it. This would be the case for all 3 options.
However, if option 1. is taken, whoever has the largest storehouse of mana, (i.e. whoever's organ can produce the most mana or whomever's the fattest, meaning whoever has the most energy in their cells,) would be the strongest spellcaster.
If Option 2.: Same as option 1, but at that point, it'd be tied between everyone who has the ability to cast the highest reachable levels of spells.
If Option 3.: It goes back to Option 1's weaknesses, but instead of requiring so much energy availability (instead of needing to way many tonnes), you just need to weigh as much as it takes to cast the spell.
That covers the energy-cost-related effects. But what about the societal effects?
- (organ) Whoever can display the greatest feats of magic would be seen as the most powerful and therefore revered, much in the same way as how the greatest warriors were historically looked up to in our real world.
- (cell structure) Well, since fat people would have more energy in their bodies, then they'd be seen as naturally more powerful by the people. This would encourage more people to overeat when possible, and to become heavyset because fat is power.
What about the effects on the body from CASTING spells? (Not just being prepared to cast spells.) Well, it would depend on how your body prepares the spells.
- (organ) First, if your body stops being able to produce mana from the sugars and fats, then you might wind up with a variation of diabetes which is where the human body stops being able to produce the drug known as insulin. If this happens, then not only will magic be cut off to that person, but depending on how integral mana-production is to survival, you can expect the person to suffer from mana-betic shock where their body slowly starts shutting down due to lacking what is a necessary resource to survive. If it is still able to produce mana, then overtime, your organ may give out, much in the same way as your liver can give out if you overdrink alcoholic beverages frequently. Overuse of a bodily structure will have lasting, damaging consequences. Since your spells would be limited by your body's natural limit to produce mana, then your consequences would simply be limited by your body's own response to the organ and if the organ fails you.
- (cell structure) If spell casting is dependent on energy stored within the powerhouses of the cells, then you would suffer the same adverse effects of burning too much energy too quickly as in real life. For one, if you use Fireball and the cost is 36 tonnes of weight, then when you cast that spell, you need to hope you have enough energy to cast it or that will kill you as your body gets dissolved to make up the remaining energy cost needed if the spell doesn't just "fizzle". But even if you weigh 37 tonnes and the 36-and-change tonnes spent doesn't kill you because it needs to consume your entire being for its equivalent exchange, you'll suffer the obvious health drawbacks of weighing that much, the massive loads of flab as you cast your spell bringing you down to just under 1 tonne, and the fact your body would immediately go into shock because 97.5% of your body's mass just evaporated suddenly. Effectively, if you cast Fireball, unless you weigh something like 1000 tonnes, you will die from the casting, but odds are you would die before you were ever able to cast the spell in the first place: the mana cost being prohibitive.
You ask what the cost of overusing this magic would be? Quite simply, it's the same costs as burning too much energy too quickly in the real world: overexhaustion and fatigue. You'd become tired, pained, and weak. You may get fever or chills or even both. Your body would have just cannibalized itself for the sake of your magic, so you'd feel severe hunger pangs as you crave any form of sustenance, possibly needing a feast to restore your energy, but the severe hypoglycemia would make it hard for you to move. If you do move, you'd be shaking furiously. More likely? You'd just pass out. Death wouldn't be unreasonable either.
The best way to prevent this isn't to eat unhealthy amounts of sugar. No, that'd be the solution to IMMEDIATELY after casting the spell as a temporary way to tide you over til you can get some real food, but even then that would be dangerous depending on how low your blood sugar would have dropped thanks to the spell. The solution would be found in complex carbohydrates like breads and energy bars. Honestly, you'd need the kind of energy bar Barry Allen eats in tv's The Flash in order to balance it out. Even then, it's not a guarantee that your metabolism wouldn't be increased with the use of these greater levels of magic, meaning you may just suffer from hypoglycemia as a constant while your body is already used to burning the energy you have to convert into mana which may just go unused. Your mages would basically need to eat more and more often the more powerful they get, much like any professional athlete. When you consider a teleportation spell basically makes your mage like Barry Allen in that respect, essentially, you'd actually need those energy bars strapped on-hand constantly and make a detour every 5 minutes to get pizza and ice cream. Sounds great until you realize that's all you will ever be able to get away with eating if you want to gain enough calories to survive.
And what about those fat people I mentioned earlier? Regardless of if they actually USE their magic or not, they will have the highest capacity to cast more powerful spells until their bodies too betray them for doing so. Even if they don't use their magic, they will suffer the negative effects of BEYOND morbid obesity in order to have the magic potential necessary to release a Fireball.
Quite simply, in this world, the only way to win with magic is not to use it, but if you do use it, you will be looked upon well meaning some will be foolish enough to use it for wealth, power, and fame. In the end, everyone loses.