Hate to break it to you, but there ARE hitpoints in the game, they just aren't exposed. It's a digital game.
Yes, I realize that. I was referring to the concept of a character's entire bodily health represented by a single HP meter.
I just had an interesting idea for magic: how about adding a tag to body parts to specify that they are used specifically to implement magic (or specific kinds of magic)? This way it may be easier to tie in calorie use to magic, and you could get some pretty quirky ways some beings might cast spells: twitching one's nose like in Bewitched, waving arms, blinking, or maybe just pure thought if it's tied to a brain. Or even an ethereal magic body part, which wouldn't be vulnerable to physical damage but could be harmed by another magic user with specific spells?
Say, for example, you want humans to rarely be spellcasters. You design the tag something like this: [MAGIC:1:100], where the body then has some body part which has a 1 in 100 chance of having a working [MAGIC:x:x] tag - otherwise the part is severed upon birth. Attach the tag to an [ETHEREAL] "arm," which the person wouldn't normally miss anyway, as without magic it's virtually useless. Viola, an easy way to tie in rare magical ability to a creature's body. Then, simply using the "arm" magically uses calories and makes one tired just like all other intense actions like physical combat and pumping. There's no odd exception coding to add (except the spells themselves). Yes, this still makes a kind of "mana pool," but just like with physical skills, it's divided up a bit to make things more interesting: in order to use magic, the creature must both have calories to use (not be exhausted), and have that ethereal "arm" in working condition. Just as physical combat requires a body part with which to strike or grapple or whatever the opponent, or [STANCE] parts to move around easily, or [FLYER] parts to fly.
And other creatures could have magic tags tied to other odd parts of one's body: you could put a specific magic tag [PETRIFY_ON_SIGHT] on a creature's head, for example, to create a Medusa with the ability to petrify opponents. Pretty much exactly like the game works now, just with more tags added. I guess the [MAGIC:x:x] tag would be somewhat unique, as we'd want that one to imply that the creature must learn magic rather than having an innate usable ability right away.