Vitamin D deficiency is common among those living in areas where there are long, cold winters. Most people need 5,000 to 10,000 units per day to achieve optimal levels. However, if chronic illness is present, the body needs 10,000 to 15,000 units per day. Vitamin D is necessary for bone, cardiovascular, immune, and overall general health. Vitamin D deficiency leads to poor heart health, certain autoimmune disorders, muscle aches, inflammatory disorders, and poor bone and immune health.