So, one thing I've found myself wondering is what happens when one ingests some kind of matter under a temporary Creo or Muto effect--particularly what happens when the effect wears off. This would seem to depend on a few things, namely the duration of the spell, the Magic Resistance of the ingester, and how food is digested and integrated into the body in the Ars universe.
Let's say you were to create a loaf of bread with duration Moon, and eat it. What happens when the spell wears off? It seems to me that the answer probably has something to do with the duration. A Diameter loaf of bread is harmless; it'll disappear from your stomach before you can digest it. But what about Sun? Moon? There's a question of how long and how much of the matter remains in one's body, and the effects of suddenly losing that matter.
I'd suggest that by the end of a Moon, an ounce of material will be left for every pound consumed (I could try and math this out but really don't want to). What happens if you suddenly lose an ounce of flesh scattered all over your body? Probably not much. Call it a Short Term fatigue level--it stings but doesn't do any lasting damage. Losing a pound of flesh might then be a Light wound, assuming its distributed around the body instead of concentrated in one place. For someone who's been eating only temporarily created magic food of Moon duration for a couple weeks, when that food wears off he should immediately have to take ALL of the Deprivation rolls that he missed.
Now let's think about Muto. Those of you who have read Harry Potter and the Methods of Rationality may remember a vignette about a mage who transmuted the gold he owed another wizard into a glass of wine, which he then tricked that wizard into drinking. When the transmutation wore off, the second wizard died a rather horrible death. This sort of thing that is trivially achievable by Muto magic. Here, a short effect is probably better, as it ensures that most of the material is still present when it snaps back to its original, presumably very unpleasant, nature.
There's another question, though, which is what effect a wizard's Parma would have on any such substance before the effect wears off. It seems plausible that, if the material wasn't cast with enough penetration, it simply would not be able to integrate into the wizard's body, and would be passed through--though this could have its own unpleasant consequences, say, a Long Term Fatigue Level.
I might expect that a material transformed with Muto which had been integrated into the body might confer some of its won properties to the person who had ingested it. For example, if you were to transform iron into food and eat it where it would drop the transformation a month later the person might well have certain 'metalic' properties of an unpredictable nature. (tough, puissant terram for a mage perhaps, monstrous form, etc..)
Just a note that could make the question moot in a number of cases: magically created food (unless made "real" and "permanent" with vis) does not nourish. So, when it vanishes you don't really feel the difference!
In theory, the Parma mechanics have been designed by the ArM5 creators exactly to prevent this type of situation. Parma blocks magically "enspelled" stuff away from the wizard. This includes a goblet of poison Mutoed into a goblet of fine wine. Obviously, nothing prevents a wizard from doing this to a mundane; but then, for a wizard it's trivially easy to kill a mundane, and the big question is not if or how it can be done, but what consequences one will face...
The general question seems to be: What happens if you eat magically created food?
Factors: How long is the duration? How often did the creature eat the food - that is, how much of their diet has been magical fluff?
A mundane who has one loaf of bread with Moon duration shouldn't know the difference - at the end of the Moon, they've effectively skipped a meal. The same should apply to someone who eats Sun bread.
However, someone who has eaten magical food for several meals over the course of a duration is in a worse situation. They have been deprived of food - I refer you to ArM p. 180, Deprivation.
Someone who has eaten three days of magical food over the course of a Moon duration should make a Fatigue check, and the check gets a +1 to the Ease Factor every three days. So, having some magical food shouldn't do them much harm, but after nine days of it the check is (3 for the first day and +2 for the next six) 5, and gets worse from there. If you think of a Moon as an average of about three weeks (21 days) the check should be (3 + 6 for 18 days) 9. Failure drops a Long Term Fatigue.
... Which seems kind of light for going three weeks essentially without food ... are there other rules elsewhere?
Yeah, the deprivation guidelines seem really soft. Three days without water is enough to kill most people. You can go a lot longer without food but even a day's fast should probably cost a fatigue level...
A fast of three weeks is possible, if bad; Ghandi did this, and so have others. Water, as you say; you can die of thirst in three days, or last up to ten if not perspiring or otherwise reducing your tank.
I also notice nothing on cold ... hm.
Wrenching this back to food, though, this is all about mundanes. I agree that magi cannot eat their own magical food, at least not without penetrating their own Parma.
Let me restate it. By RAW, magically created food does not nourish. It does not "disappear" at the end of the duration. It does not nourish from the start. Eating a magically created meal is like skipping a meal right from the start.
Actually, before summarily judging the current rules, I'd suggest trying to "roll through" three weeks without food (i.e. make seven deprivation rolls, of ease 3,4,5,6,7,8,9, applying already the penalty from already accrued Fatigue to each roll). On average, with Stamina 0, you're going to end up somewhere around "long term" Dazed, and thus on the verge of passing out (at which point you'll quickly die because you won't be able to drink).
There is going to be quite a bit of variance in this, but high variance is one of the fundamental mechanical problems of Ars Magica. Even ignoring botches the probability is about:
0.3% of being Ok
3.4% of being Winded
11% of being Weary
23.5% of being Tired
27.9% of being Dazed
33.9% of being Unconscious
If you extend that from 3 weeks to 30 days, you end up with a probability of Unconsciousness close to 95%. But if you look at real life, people can live well over a month without food. It is interesting (if somewhat horrifying) to look at the statistics of the ten Irish political prisoners who died of starvation during the 1981 hunger strikes: the majority died after 59 to 62 days of fasting. One died after 46 days, one after 66, one after 71 and one after 73.
So, the food deprivation numbers of Ars Magica are not that "light" compared to reality. Quite the opposite, they are a bit on the harsh side.