How interesting, and yet not surprising that we have a vitamin D deficiency in a culture where people spend their entire day indoors or in a car, train, mall, classroom, or bus. Vitamin D comes from: vitamins, vitamin-fortified milk (milk as a "health food" is another topic entirely) or other vitamin-fortified foods and, the only natural source of vitamin D: it is made by the body when the sunlight hits the skin.
A study found that 40% of children are vitamin-D deficient, which can cause thinning bones, rickets, seizures, a weakened immune system. http://www.reuters.com/article/latestCrisis/idUSN02317202 . They also found that drinking a lot of soda and being overweight were also factors that caused demineralization, or leaching of vitamin D and other important nutrients.
I think we as a culture are paranoid about the sun to the point of hurting ourselves by not getting enough of it (with the exception of those who lie in the sun for hours to the point of skin damage). According to this guy http://www.jonbarron.org/blog_published/2008/06/vitamin_d_deficiency_a_silent.html , the incidence of vitamin D deficiency increases in the winter, when there is less sun exposure. And people with darker skin need 3-6 times more the amount of sun exposure than those with lighter skin.
It's a great irony that lupus might be triggered by vitamin D deficiency, because lupus patients are specifically told to *always* avoid the sun. Some of them carry parasols or adjust their schedule so they only run errands after sunset. Those folks definitely need to get some vitamin D.
I am told that calcium doesn't absorb without vitamin D. It seems that folks with lupus really need to take a combination Calcium-vitamin D supplement (doctors usually recommend OsCal, which you can get over the counter), not just to make up for the lack of sun exposure, but also to protect against the bone-thinning caused by drug treatments such a steroids like Prednisone.