Here are the top 10 tags:
Tag, No. of Questions, Unanswered %
- Nutrition 198, 30%
- Diet 110, 35%
- Dermatology, 83, 39%
- Medications, 82, 30%
- Sleep, 71, 32%
- Dentistry, 59, 37%
- Pain, 49, 22%
- Side-effects, 49, %18
- Eye, 46, 34%
- Cancer, 42, 31%
As you can see, the Nutrition tag has the highest question count and Diet is a close second. They're largely synonyms so combining them yields 308, which is higher than the total of the next four tags combined. Surprisingly, the unanswered percentage doesn't change much among the top 10 (except Pain and Side-effects, which I suspect is because they are more often added to concrete questions with other tags).
Unfortunately, that's the end of objective facts I can muster from the stats available to me. What I see subjectively is that a very large percentage of nutrition and diet questions are what I would label frivolous or even downright silly. I vote to close questions almost daily along the lines of "Which is better: A or B?" where A and B are largely irrelevant things, often based on superstition or cultural practices that amount to old wive's tales. In fact, the huge number of such questions has surprised me. I had no idea how many people think that drinking liquids before, during or after a meal will have profound effects on your health, or that such questions have even been studied extensively. Quite often no one has bothered studying them at all so scientific answers aren't possible. (If I could answer with what I consider to be common sense, the answered percentage would quickly approach 100%.)
Unfortunately, I consider the OP's example to be a question where common sense is the answer. Vitamin C, for example, is water soluble so cooking potatoes in water is bound to remove vitamin C, and that effect will increase with cooking time (which the supporting article states). The problem is, can I find scientific support for that answer? No, probably not, or at least not directly. I can probably dig into chemistry or biology research to show the general effect, but boiling potatoes specifically? Not likely. I doubt that anyone has done a definitive study that would yield a cooking time/vitamin retention curve for boiled potatoes. They haven't done that study because the answer is unlikely to have an impact on culinary practices or contribute to future research.
Those questions also attract alternative medicine trolls like flies to dead stuff. Asking nutrition questions is a sure-fire way to get long-winded diatribes from brand new users telling us how eating only this or that type of food will change our lives like winning the lotto, and failing to do so will put us in the grave by next week.
And therein lies the problem with nutrition questions. The signal-to-noise ratio with such questions is very low. It's so low that I would consider dropping the tag entirely. In general, I agree with the requirements specified by JohnP in his answer. For example, the potato-vitamin question would be on-topic if it was asking if boiling potatoes directly affected a specific health concern rather than just the generality of vitamin retention of certain cooking methods. That's a cooking question, but it's not even welcome on cooking.SE for pretty much the same reasons.