I'm afraid the answer is pretty simply that health is off-topic. We've long since decided this, and if we don't want to change it, then not only should we disallow questions about health, we should disallow answers about health. (Your question sort of suggests that you're wanting to change that - "we all are having food just to keep our health normal" and "why can't this be allowed here?" - but I'm assuming you don't actually mean to be asking for health to be on topic.)
Look at it this way: the question you're answering should never be asking about health, because that'd be off topic, so if you say anything about health, it's not related to the question. I guess there might be a couple exceptions - for example, if someone asks "why do people always add X to Y?" and there's no culinary reason and it's entirely because of supposed health benefits, then pointing that out would answer the question. But that's definitely an exception, not the rule.
So in general, really, adding health information is at best a bit of a digression and at worst an invitation for debate. A lot of health things you may be quite confident are true are not actually terribly well-verified scientifically, and since we really, really don't want health debates on the site, we shouldn't invite them by letting people post answers about health. Don't get me wrong, I think we discourage or ban pretty much any off-topic content in answers, but health is especially bad because of that potential for debate.
As for the specific question/answer/decision, I think I'll let rumtscho answer more fully, but for what it's worth, I think the question was a bit iffy, as Stephie pointed out - it's kind of inviting health-ish answers. So I don't blame you for ending up writing about nutrition, but that doesn't mean it was the right thing to do. As far as I can tell, your answer is basically "use it in soups or sauces" and the entire rest of the answer is health claims; that suggestion is fine but the health claims aren't.