Well, there are many causes for depression.pretty much all of Shizune's route sees Misha suffering from [depression].
I don't know if calcium deficiency is among them.
I'm pretty sure unrequited love is, though.
Guess which cause I think is more likely for Misha.
Besides, the day calcium deficiency (and similar stuff) is regognized as a disability 95% of all humans will be disabled.
Regarding the "dizzyness on stairs" stuff, that's been suggested dozens of times before and no argument so far was convincing:
- Misha says that when she wants Hisao to go fetch something for her. It is obviously an excuse.
- In the Act 1 finale Misha is perfectly able to ascend to the roof alone packed with foodstuffs without problems.
- There are other scenes as well with Misha on the roof. In on she's leaning against the fence without getting dizzy.
- Class 3-3's homeroom is on the third floor. The student council room is on the ground floor. Hisao probably walked that route with Shizune and Misha hundreds of times during the schoolyear. He never mentions a single occasion of Misha getting dizzy on the stairs or of her preferring to use an elevator. If something like that had happened, it would have been relevant to the story and would have had to be mentioned in some way.