Vitamin D deficiency means that your body is not getting enough vitamin D to stay healthy. Why do I need vitamin D and how do I get it? Vitamin D helps your body absorb calcium. Calcium is one of the main building blocks of bone. Vitamin D also has a role in your nervous, muscle, and immune systems. A vitamin D deficiency is when your levels fall below 12 nanograms of vitamin D per milliliter of blood, or 30 nanomoles of vitamin D per liter of blood, per the NIH. Vitamin D deficiency occurs when the body doesn’t get enough vitamin D from sunlight or diet. Vitamin D deficiency can cause loss of bone density, osteoporosis, and broken bones. Vitamin Causes Symptoms Diagnosis Prevention Treatment Spotlight on Aging: Vitamin D Deficiency Vitamin D deficiency is most commonly caused by a lack of exposure to sunlight. Some disorders can also cause the deficiency. cy5O.