Vitamin D may decrease from certain medications such as certain anti-seizure, corticosteroids, heparin, certain antibiotics, mineral oil, and antacids; or certain disease states such as celiac disease, Crohn’s disease and Whipple’s disease. Insufficiency may cause symptoms such as bone loss, osteomalacia (in adults), and impaired Calcium absorption occurs due to very low levels of the vitamin. You should not go over that amount in supplementation, 2000 IU is the tolerable upper intake level for adults and can be at risk for toxicity (weak muscles, weak bones, excessive bleeding, and kidney stones) with higher amounts. Vitamin D is available through our foods - Fortified milk, breakfast cereals, egg yolks, fatty fish, fish oils, and the sun. Sitting by a window, in the sun will not give you the amounts of vitamin D as if you were in the sun without the window, however, the combination of eating Vitamin D rich foods and sitting by a window (about 1/2 hour few times a week) will give you the Vitamin D your body needs. People who do not get outside or unable to go outdoors, a sunny window is the next best thing for you. It is better than not at all. Hoped this helped you.