Vitamin D is called the "sunshine vitamin" because of the body's unique ability to produce its own vitamin D through sun exposure, according to U.S. News & World Report. When skin cells are exposed to the sun's UV-B rays, a chemical reaction occurs that produces the vitamin.
Vitamin D helps prevent and manage a host of conditions, including obesity, heart disease, hypertension, bacterial vaginosis and influenza, explains the National Center for Biotechnology Information.
The government recommends 200 IUs of vitamin D for adults up to age 50, 400 IUs to age 70, and 600 IUs for those over age 70, but many experts think these guidelines are far too low, according to U.S. News & World Report. They suggest supplementing with 2,000 IUs per day in the winter and daily sun exposure in the summer to maintain optimal levels.