People often think that because there's a nutrition tag, nutrition questions are on topic. So:
Alternative proposal 2
This has bugged me for a long time. I recently had yet another problem with a brand new user genuinely misunderstanding the tag (described under Alternative proposal 1)and a title of a new question asked today showed a possible solution.
I think we can rename it to nutrients-composition. The change is subtle, but I think it is important to the perception of inexperienced users faced with the tag for the first time. The words "nutrients" and "nutrition" are closely related, but I think that the second suggests a much more holistic concept of "being healthy", while the first one also has a strong association with the stuff which makes up food. The "composition" part is there basically for the same reason that Jefromi's "content" is, but to my feel of the language, "composition" is much more suggestive of a value-free analysis of the food's components. I don't know exactly why it sounds this way to me, possibly because we are bombarded with marketing messages which say "Contains no fat" or "Contains three important vitamins", but never hear "It is composed of 0% fat" in the context of the "eat healthy" propaganda.
Alternative proposal 1 (and background)
I recently had this happen (screenshot from deleted answers):
This user probably thinks we are a bunch of jerks, or at least that I am a jerk instituting arbitrary rules. And I totally understand why - indeed, after this interaction, it would be very surprising if he thought anything else. And you know what? I hated to see him go, and he will probably never return.
He came, and added what, from his point of view, was a valuable contribution. He put some effort into it, writing a well-formulated text, and added a link to a source. He acted like a good member of the community, as he could not know our unusual stance towards nutrition questions. Indeed, if it hadn't been for that rule, I'd say his post was better than 90% of the first posts we get. And besides, it is one of the cases where the post does not seem to have the potential to cause harm by itself; I thought of leaving it there, but from bitter experience, it would only have created a precedent for other users arguing that, if that nutritional post is allowed, we cannot close their own what-is-good-for-you posts just because they are about nutrition. And they'd have a point.
I deleted with a very curt message, because I was at work and had no time to explain more. The user came back, noticed what had happened, and re-posted the answer, this time adding that the presence of the tag nutrition tells him that we do, in fact, deal with nutrition questions. It impressed me that he did enough research to find that tag, and also that he politely reposted the answer together with an argument for it, without showing any aggression. But the main point was: I had to explain to the poor guy that, we do indeed have a tag called "nutrition", but it is not used to label what most people expect under "nutrition". This is quite surreal to first-time visitors, and it is understandable if it leaves them confused and angry, and they decide to never come back to this (from their perspective) strange and hostile site.
Tags are indeed an important part of our site, and having content tagged correctly enhances finding the right information, which is one of the reasons why we are useful to people. So I normally support having a tag for a distinct category of questions. But I feel that this case is a real exception. Because of the unfortunate circumstances of our tag covering only "the part of nutrition which is not forbidden by our rules", and this fact being completely hidden from users who trust the tags, it does more harm than good.
Renaming it would be a good solution, but as Aaronut points out, anything which is close enough to "nutrition" will suffer from the same problem as the current tag. So, maybe we have to admit that completely removing the tag is the least bad solution available.
The downsides of the tag removal would be: - we couldn't easily find the questions which are related to nutrition. But maybe this also has a silver lining: if people search for "nutrition" first, and don't find good results, they might be more inclined to believe that we don't deal with nutrition. - we may end up with questions completely without tags. I hope that these will be isolated cases, and could be dealt with by the creation of tags which are not all that related to the main point of the question, for example labelling a "does vitamin C remain in kale after boiling" question with just "kale". It reduces the information richness of our content, but I think in this case, the loss is justifiable.
I'm OK with the rename, although going by past experience (e.g. with the renamed ingredient-selection tag), I'm not sure if it will really have the effect that you intend.
Now, maybe "ingredient selection" was just not a good choice, but regardless, we renamed that tag from "ingredients" in the hope that people would stop using it as a generic tag for almost any food question, and particularly recipe requests. What ended up happening, and still happens, is:
Several users who have enough reputation to know better, and unfortunately also have enough reputation to create new tags, end up recreating the old ingredients tag over and over again and we have to keep removing it. Again, just not caring or not paying attention.
So I'm just warning you that you may not see any improvement in the quality of questions. I think a lot of people will type "nutrition", see something that kinda sorta matches, and that'll be good enough for them until they get their question closed.
But as long as you don't mind the extra work - that is, keeping a watchful eye out for abuse of the new tag or recurrences of the old tag - then I think it's fine.