As Measured by Vitamin D
Despite its name, vitamin D is not a vitamin, but a prohormone, or precursor of a hormone. Vitamins are nutrients that the body cannot create, and so a person must consume them in the diet. However, the body can produce vitamin D. The body makes vitamin D when direct sunlight converts a chemical in your skin into an active form of the vitamin (calciferol).
Vitamin D plays multiple important roles in the body, including supporting immune, brain and nervous system health. It also plays a vital role in developing healthy bones and teeth, we need vitamin D to support the intestines absorption of calcium and reclaim calcium that the kidneys would otherwise excrete.
Calcium, the primary component of bone, can only be absorbed by your body when vitamin D is present.
Vitamin D deficiency is very common in the US. People who don't get enough sun, especially those living in less sunny climates, are especially at risk. However, even people living in sunny climates might be at risk, due to more time spent indoors, less sun exposure and the use of sunscreen.
People can develop a deficiency for many reasons including having fewer "receptors" in their skin that convert sunlight to vitamin D, having trouble absorbing vitamin D in their diet, and may have more trouble converting dietary vitamin D to a usable form due to kidney issues. Studies suggest that US adults aged between 20-39 have the highest risk for vitamin D deficiency, with 23.8% having inadequate levels of Vitamin D. (1)