Vitamin D helps you absorb calcium and may reduce the risk of cancer, diabetes, and falls. Many people get too little vitamin D from their food or from sunshine (especially in the winter). The Institute of Medicine (IOM) recenly upped its recommendations to 600 IU a day for adults up to age 70 and 800 IU for people over 70.
Those amounts include what you get from the sun, from salmon and other fatty fish, and from fortified foods like milk, breakfast cereals, and some brands of yogurt, margarine, and orance juice. They also include the vitamin D that is in many calcium supplements. Most multivitamins contain 400 IU of vitamin D (the DV, Daily Value).