However, over the millennia, more and more people moved to high latitudes, where up to half the year, solar-UV exposure isn't enough to fuel vitamin D production in skin.
More recently, justifiable concerns about sunburns and skin cancer have prompted increasing shares of people—even at high latitudes—to don sun-blocking clothing. Unfortunately, what's good for skin protection is bad for natural vitamin D production.
It also can be bad for health, a host of studies has recently indicated. For decades, vitamin D was appreciated largely for its role in boosting the absorption of calcium, important for bone health. However, over the past decade and especially the past 5 years, research has linked a broad range of additional benefits to having ample vitamin D. It's shown that the nutrient fights cancers and diabetes, is the pivotal feedstock for a hormone that protects muscle, and inhibits autoimmune disorders from multiple sclerosis and lupus to inflammatory bowel disease.
Moreover, many of these newly recognized benefits of vitamin D rely on blood concentrations of the nutrient far higher than those needed to protect bone.
Although high by dietary standards, these aren't really megadoses. Rather, they reflect amounts typical of what people can naturally generate within their skin if they spend a lot of time outdoors in a low latitude.
However, with modern habits and work routines, few of us create these higher doses of vitamin D in our bodies throughout the year. So, we rely on our diets for most of this essential nutrient, even though few foods are naturally rich sources of vitamin D and only a few, such as milk, are supplemented with anything more than meager amounts.
Although vitamin pills can provide much or all of the U.S. recommended daily intake (RDI) of D for children and adults—200 to 600 IU, depending on age—bone and mineral researchers have lately been recommending that people get much, much more.
In fact, some scientists have advised the federal government to boost the vitamin D RDI up to at least 1,000 IU and to bump up the certified-safe limit beyond the current 2,000 IU.
Vitamin D Deficiency