Is temporary, magically-created food nourishing?
This has been discussed at length and the rules seem to support more than one interpretation. This question has a long history as the rules for creating things with magic, and how "real" those magically-creating things will be, have gradually changed since First and Second Edition.
One of the main threads on this discussion on the official Ars Magica Forum is: Consensus Building on Magically Created Food
The first thing to say is that food created using a ritual (CrHe, CrAn, CrAq) is real, permanently, and behaves like mundane food in every way. The rules are clear on this point and there is strong consensus about food created through rituals. The question is what happens when a character tries to subsist on food that created by a non-ritual spell.
Next, some relevant excepts from the rules:
The entire second-to-last paragraph under Creo on page 77, which is too long to reproduce here, says in part that "...magically created food only lasts as long as the duration lasts, and someone who has eaten it becomes extremely hungry when the duration expires."
The first paragraph under "Creo Herbam Guidelines" on page 136: "Any food created is nutritious only if the creation is a ritual.
Strictly speaking, these two references contradict one another. The CrHe guidelines do not say non-ritual food is not permanently nourishing, they say it's not nourishing at all. There are, at present, no official errata to resolve the contradiction.
Opinions in the community seems to be divided approximately equally between three choices:
Magically created food is nutritious for as long as it lasts, and if a character fully digests the food before its duration expires, then he suffers no ill effects when the spell ends.
The rules don't explicitly say this is possible, and in several places they seem to imply that it's not. This option should be considered more of a house rule than an interpretation of the rules-as-written.
The principle is that magically-created things can have effects on the mundane world that outlast the spell's duration. A magically conjured horse leaves hoofprints on the ground, a magically created fire can burn down a house, so logically, magically created food should have its normal effects on a body after being fully digested. After all, once digested, the food is gone anyway, so its disappearance may not matter.
There is a precedent in ArM5 for a magically-created substance being able to sustain life without need for a ritual: the spell Chamber of Spring Breezes (page 125) explicitly states that it creates breathable air. It could be considered inconsistent that non-ritual magic can create breathable air, but not nourishing food. However, things may not be that simple.
Someone very knowledgeable about medieval medicine might be able to say whether, in the Medieval Paradigm (with a big "P"), food and air merely nourish the body or whether they are actually converted into bodily humors or something; the FAQ maintainer is not qualified to discourse on that subject so he'll offer no conclusion one way or another. It is quite possible that food and air are used by the body in very different ways, so the ability of magic to create breathable air but not nourishing food may not be inconsistent at all.
This interpretation (as stated above, it's more of a house rule) would have significant effects on play. Magi would not really need to grow or buy food at their covenants if a Moon-Duration, non-ritual spell can create food that is nourishing for all practical purposes. This has implications for both the economics of covenants, and for the fantasy flavor of the setting. On one hand, such a ruling could open the door to abuse if magi decide to make a living selling magically-created wine or foodstuffs. On the other, it makes a lot more sense for magi to take up residence in the midst of a tangled forest or in a lonely tower on top of a mountain if they don't have to worry about where their groceries come from.
Magically created food is not nourishing at all (unless created by a ritual).
This essentially is interpreting the CrHe guidelines on page 136 to be correct ("...food created is only nutritious if the creation is a ritual...") and to overrule the sentence on page 77 that says "...[magical] food only nourishes for as long as its duration lasts, and someone who has eaten it becomes extremely hungry when the duration expires."
One advantage of this approach is that the Troupe doesn't have to worry about difficult questions coming up in play. If characters try to live solely on magical food, it's pretty clear what would happen (malnourishment, weakness, eventual death).
This interpretation is consistent with the legacy of past Ars Magica editions, where magically-created things of all kinds (water as well as food) were ephemeral and in vague ways partly unreal.
The drawback, mainly, is that under this rule magi can't use magically-created food to skip more than a few meals in a row. This could be seen as a hindrance to adventuring magi, especially those who undertake long journeys into desolate areas. Some troupes may not want to bother with magi being tied down to details such as where their next meal is coming from.
Magically-created food is fine as long as the spell lasts and causes a problem when the spell ends.
The section on page 77 is a lot longer and more detailed than the rule on page 136, and this interpretation can be seen as giving more weight to the page 77 rules.
This interpretation immediately leads to another question: what sort of problem does the character suffer when the magical food expires?
The answer may depend on how much normal food, compared to magical food, the character ate in a given period. The rules on page 77 say that the character becomes "extremely hungry;" it's not clear whether this implicitly assumes the character had eaten only magical food for a considerable length of time, or whether it's meant to imply extreme hunger is some kind of side effect of eating any magical food. (An extreme interpretation is that extreme hunger must result from eating one magical pea amid an otherwise mundane diet; though people who read the rules that way generally seem to prefer a house-rule "fix" rather than actually playing them that way.)
Some players treat the magical food as never having existed, once its duration expires. The character is treated exactly as if he had not eaten it. If this had been one meal, several days ago (and the food had a Duration of Moon) then the character may not even notice; if he had subsisted entirely on magical food for weeks, then he would be severely weakened by hunger or might even die of starvation.
Others take a less scientific approach and say that if the character ate (a sufficient amount of) magical food, he becomes supernaturally hungry when the spell ends. Presumably he needs to eat approximately as much real food as he had consumed in magical food, all at once. This has more of a fantastic feel, and some players prefer it for that reason.
It has been pointed out that feeding a character on magical food could be a subtle form of attack (causing malnutrition or starvation when the spell ends). Opinions are divided, though, whether that's an abuse of the rules or a great story seed for some devious NPC plot. In either case, feeding magical food to magi would probably not work, as their Magic Resistance would keep out the magically-created food.