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More than 40% of Americans have a vitamin D deficiency. That's not a problem for everyone, but a severe deficiency can lead to bone density loss and rickets (a softening of the bones in children). The body can't absorb calcium without vitamin D. Vitamin D also helps the immune system fight off bacteria and viruses. Many people take vitamin D supplements that are available over-the-counter, but when it's prescribed, it's usually for hypoparathyroidism. That's a condition that causes calcium deficiency, weakness, fatigue, and muscle cramps and spasms, Dr. Luykanovsky says. Vitamin D is also prescribed for osteomalacia, a condition where calcium is lost from the bone.