As nutrients go, vitamin D is a rarity: It’s one of only two vitamins your body makes on its own. The liver, kidneys and cells and several other parts of the body convert two forms of vitamin D to its active form, the steroid hormone 1,25-dihydroxyvitamin D, and any extra is stored in fat tissue for future use.
Vitamin D2 (ergocalciferol) is found in plant sources and can enter the body only through what you eat. The more potent vitamin D3 (cholecalciferol), which can be produced through sun exposure, from animal products or supplementation, tends to be more readily converted to the active form in the body.
In theory, we can make all the vitamin D we need; in reality, we don’t. “About half of all Americans are chronically deprived of vitamin D,” says Michael F. Holick, PhD, MD, and director of the Vitamin D, Skin and Bone Research Laboratory at Boston University.
Living in northern climates is a problem, because sunshine is weak for about half the year, but that doesn’t mean sunnier locales fare better. Slathering on sunscreen with an SPF of 8 or higher blocks the ultraviolet B rays that generate vitamin D production. In addition, relatively few foods provide vitamin D.