The on-topic list is very short by necessity. You will find more pertinent information on the [tag-info of nutrient composition], specifically listing under not permitted questions:
Whether a given food is healthy or not, or healthy in comparison to another food
The best way to understand it though is not by looking up the rules, but understanding the logic behind the line we draw. And it is:
Never write answers which require you to interpret the word "healthy" or "nutritious" in some way, especially by implying that eating a specific food (or a specific nutrient) is better than not eating it.
Let's look at the question which sparked it all: nixtamalization. Your answer relies on the statements (I will pick a single compound to make it easier):
- Niacin in corn [is] not digestible by humans (directly stated in your answer)
- Niacin is desirable in a healthy/nutritious diet
- The bioavailability of niacin matters, and is higher for nixtamalized corn than for untreated corn
- Nixtamalized corn is more nutritious than non-nixtamalized corn.
Here, you directly state 1), and to me is pretty clear that you are concluding 4). This means that you are also implying that 2) and 3) are true - if you thought that they are false, it would be illogical to conclude 4). So, your answer also implies 2) and 3).
And here is where the problem lies, especially in 2). It is maybe a bit difficult to wrap your head around it, because it is quite uncontroversial when it is said about niacin. But imagine a scenario in which somebody creates a new diet fad about barricidine, finds out that corn is full of barricidine and nixtamalization destroys it, and writes an answer saying that non-nixtamalized corn is much more nutritious because it is a great barricidine source, which is much rarer than a good niacin source. What happens in this example? First, we have given the arena to a quack, and second, we are on a good way to a flame war.
And what is the difference between "X is nutritious because it contains niacin" and "X is nutritious because it contains barricidine", if our quack also publishes a couple of studies? Only that the science behind one of them is older and more widespread. Now it would be great if every scientific statement in the field of nutrition was easy to label either as "confirmed by facts" or as "utter nonsense". But the problem is that for almost every statement in nutrition, that's impossible.
The sad state of affairs is that every statement such as "eating X is healthy" (X here can be a food, or a compound) is opinion-based. Even when you are convinced that there is solid science behind it, when you dig a bit, you find that it is not as clear-cut as you'd wish. So nutrition-related statements, be they directly stated or implied, are not allowed, for pretty much the same reasons which cover all other opinion-based questions.
You may wish that we could draw the line somehow such that correct statements are still allowed while incorrect ones are disallowed - but that would require moderation based on the presumed truth content of answers, and this is against the principles of Stack Exchange (and these are very good principles, I must add! - imagine if I was required to delete each of your answers which I find wrong).
This also explains the one exception we make with the tag nutrient-composition whose existence is meant as a concession to the provable part of the discipline, but actually tends to confuse people. Imagine question A, "is nixtamalized corn more nutritious as nonnixtamalized", and question B, "is there more niacin in nixtamalized corn than in nonnixtamalized". Question A is off topic because it requires the answerer to pick a nutritional theory ("it is desirable to eat niacin") and then answer. Question B is on topic because the OP has already done the subjective work of picking their own nutritional theory and asks for the result of a relatively unambiguous measurement.
By the way, we also don't allow questions and answers about bioavailability (such as statement 3 from the list above). In principle, it can be measured for certain compounds under certain circumstances - but I have never seen answerers on the site who understand the complexity of the subject (myself included). Most people seem to actually think that bioavailability is the same thing as bioefficacy. It is not, and while users probably want to know about bioefficacy, this is typically not answerable, and letting them ask and answer about bioavailability instead seems like a good way to spread disinformation. So, we simply don't add bioavailability to the tiny area of allowed-by-exception questions on nutrients.
I hope this cleared up better how to know where the line is. If it still seems like too much, you can try going by the rule of thumb:
Questions on nutrition are forbidden. The exception that is allowed (under the nutrient-composition tag) requires that 1) the OP names a compound in the question, and 2) the question is about the measurable quantity of that compound in the food, before it has been eaten.