People produce vitamin D from sun exposure; however, 40 to 75 percent of people are vitamin D-deficient as of 2015, states WebMD. Time of year and day, air pollution, age and skin color affect the skin's ability to manufacture vitamin D. Doctors recommend obtaining vitamin D from supplements and food.
Even when there is sufficient sunshine, the sun is not a reliable source of vitamin D, according to WebMD. A deficiency in vitamin D can result in osteoporosis. The body needs vitamin D to efficiently absorb calcium from the intestines, which contributes to building strong teeth and bones. Since few foods naturally contain vitamin D, manufacturers have added it to products including milk, cereal and orange juice.