Vitamin D3 is necessary for building and maintaining healthy bones. That's because calcium, the primary component of bone, can only be absorbed by your body when vitamin D is present. Your body makes vitamin D when direct sunlight converts a chemical in your skin into an active form of the vitamin (calciferol).
Without vitamin D your bones can become soft, thin and brittle. Insufficient vitamin D is also connected to osteoporosis. If you don't get enough vitamin D through sunlight or dietary sources, you might need vitamin D supplements.