Deficient in D?

Vitamin D is the only vitamin that is also a hormone. After Vitamin D is made by the skin or eaten, the kidney and liver help to convert it into an active hormone form. As a hormone, it controls calcium absorption to help the body build strong bones and teeth, and it helps maintain muscle strength. When you are deficient in calcium and vitamin D, your bones break down to supply calcium to the rest of your body. But being deficient in vitamin D can take a toll on more than just your skeleton.

"There have been concerns about vitamin D status in the U.S. because of increasing reports of deficiencies, with an estimated 10 million Americans over age 50 diagnosed with osteoporosis," says Atlanta-based rheumatologist Eduardo Baetti, MD.

"Vitamin D deficiency is associated with low bone mass and osteoporosis because vitamin D is needed to absorb calcium from the diet. Low levels of vitamin D have also been linked with poor muscle strength and other chronic conditions, such as autoimmune disease and some forms of cancer."

The Sunshine Vitamin

D is also the only vitamin that does not need to be consumed in food or supplements because our bodies are efficient at making it when our skin is exposed to direct sunlight (not through a window). But not all sun exposure is the same, and many factors help determine how much we absorb. In general, the further away you are from the equator, the more efficient the vitamin D production, but cloud cover and air pollution can hinder the sun's ultraviolet (UV) rays.

Many people living in the Southern United States can get enough vitamin D by getting about 10-15 minutes of sun exposure on their arms and face a few times a week -- as long as they don't use sunscreen, which blocks some of the UV rays necessary to make the vitamin.