Please see the nutritient-composition tag wiki for full information about current policy.
The very short version is that specific questions about specific nutritional content in a culinary context (e.g. "How does ripeness of a fruit affect the amount of sugar it has?")
are okay, but anything which explicitly or implicitly asks for any sort of information about health or effects on the human body (e.g. "Is X healthy?" or "Will X make me lose weight?") is off-topic.
The calcium content question actually wasn't closed, it was deleted by the author. I don't think it was because it was off-topic, it was just a little off the beaten path and there several comments suggesting that maybe it wasn't such a great idea.
The key word in the FAQ is general. As in, not specifically culinary. The issue of nutrition here is a bit like the issue of "Too Localized" being applied to broad regional questions, or even the issue of recipe-related questions here - that is to say, there's a sweet spot, not every question with any reference to nutrition is off-topic, and we should be careful not to step over the line too often.
What our guideline has always been (or at least, how I've always interpreted it) is that nutrition questions as they relate to cooking are fine, but nutrition questions as they relate to dietary needs are not. Somebody (probably myself) should edit the nutrition tag wiki to clarify this but in a nutshell, if the question is about:
- Nutritional content of a specific food: OK
- Effect of a cooking method on nutritional content: OK
- Ingredients or techniques to meet a nutritional need: OK, but must be very specific (e.g. gluten-free substitute OK, but "will help me lose weight" off-topic)
- General healthiness of a food: Off-Topic
- General healthiness of a lifestyle choice (i.e. diet): Off-Topic
We're not trying to create a landscape devoid of anything to do with nutrition, we're just trying to avoid creating little soapboxes for people to rave and pontificate about whatever is currently trendy to believe about nutrition and dieting. We're not dietitians, we're cooks; we're not here to discuss what people's diets should consist of, but assuming somebody has consulted their dietitian and figured out what their needs are, the rest is more or less within our territory.
A question about how cooking changes caloric content is still about cooking. A question about how many calories you need per day isn't about cooking at all. There's a significant difference.
FWIW: I was never a fan of "let's make a list of foods that are [high in calories / low in acid / blah in blah]" questions, but the community almost unanimously declared itself in favour of these some time ago - I think it all started with the cheap sources of protein question - so we'd definitely be fighting an uphill battle trying to close those now. My problem with them isn't so much that they're off-topic, just that they're very poll-oriented. But at least they aren't actively doing any harm by being on the site.
The question about nutrient decay is definitely on the border. My rationale for leaving that one alone is that it's in the scope of commercial food processing, and we generally accept those because they're interesting and challenging and objective, and even though we don't currently have a lot of experts in that field, it would be great to get some experts in that field. The food-science area has one leg in home cooking and another leg in commercial processing (I guess mol-gas is the torso).
Hopefully that's clear. To make a long story short, we're trying to avoid nutrition questions of the "what should I eat" variety; questions that are about nutrition and cooking at the same time are OK.