The researchers exposed 55 adults between ages 18 and 65 to an artificial light containing ultraviolet radiation (UV) equivalent to what they'd absorb in 10 minutes spent outside at noon on a summer day. Each person was exposed to the light over 88 percent of his or her body (basically, everywhere not covered by underwear) either once a week, once every two weeks, or once every four weeks over the course of 16 weeks, after which, the subjects' blood was sampled and vitamin D levels measured.
As it turns out, it didn't take much to keep vitamin D at healthy levels. Though the people who received the weekly treatment saw their vitamin D levels increase, those who received the treatment once every two weeks achieved what were considered healthy levels, and they maintained those levels throughout the 16-week study.
Granted, surrounding yourself with artificial light while standing in your underwear is one thing. It's totally different to walk around nearly naked outside when temperatures are hovering around 20 degrees. But you can still benefit from a daily walk in the winter. The UV dose used in this study is equal to two hours and 20 minutes outside under the weak winter sun. That equals 10 minutes of sun exposure a day over the course of two weeks.
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To make the most of your wintertime sun exposure let as much of your skin show as you comfortably can, but make sure to stay safe when exposing bare skin in frigid temperatures; aim for "solar noon"—the time the sun is at its highest point in the sky—whenever that is in your area; this solar calculator will tell you when that is. You can also use your skin as a cue. You've had the necessary amount of UV exposure when your skin starts to turn pink.
If sun exposure is just too hard to get where you are, supplements are a good backup. Because your body doesn't absorb vitamin D from supplements as well as from sunlight, you generally need to take higher doses than you might get from sunlight. Aim for a 600-IU supplement of vitamin D3—which is a more beneficial form of the vitamin than vitamin D2, another supplement you might see at the store. And buy supplements certified by US Pharmacopeia or ConsumerLabs.com to ensure you aren't getting contaminated pills.