Vitamin D is a nutrient required by the body to maintain healthy bones. It also supports the health of our immune system, muscle & brain. Vitamin D is not natrually found in foods unless fortified like fortified milk, cereal & salmon. Your body can only absorb calcium when vitamin D is present. Your body makes vitamin D when direct sunlight converts a chemical in your skin into active form of the vitamin. Risk factors for vitamin D deficiency are old age, lack of sun exposure, dark skin, malabsorption & obesity. Vitamin D deficiency increases the risk of osteoporosis, rickets, diabetes, heart disease & can lead to depression. This is the most widely used test that helps to diagnose whether you are deficient in vitamin D & require supplements.