Despite the back and forth over its importance, every cell in the body has a receptor for the sunshine vitamin, according to Michael Holick, a professor and vitamin D expert at Boston University's School of Medicine. Holick estimates that up to one-sixth of the human genome is directly or indirectly regulated by the sunlight vitamin.
Yet, according to the Harvard School of Public Health, as many as a billion people worldwide may be vitamin D deficient, especially those that live north of the latitudinal lines connecting San Francisco to Philadelphia or Athens to Beijing. Vitamin D deficiency can cause bones to become brittle or misshapen, and may have other severe health effects too, though many of those are still under investigation.
Some people have trouble producing enough vitamin D naturally, and the American Academy of Dermatology recommends supplements instead of the sun for skin health.
But some studies also show that supplements don't effectively provide enough vitamin D, and overdoing it with supplements can lead to kidney problems. Your body, meanwhile, won't naturally overproduce it.
Still, you need only about 15 minutes of sun a day to get adequate vitamin D if you are fair skinned — and more if you have darker skin, since melanin, which makes skin darker, makes it harder for your body to take advantage of the sun's rays.
If you are covered up or wearing sunscreen for those 15 minutes, you also won't produce enough vitamin D. People who are older or overweight also have trouble naturally producing enough vitamin D.