Vitamin D was first identified in the early 20th century with its deficiency leading to rickets. Contrary to popular belief vitamin D is not absorbed from the sun, but rather synthesized in the body from a chemical called 7-dehydrocholesterol found in the skin.

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To activate the chemical and make vitamin D, you must get sufficient quantities of wavelengths in the ultraviolet B range. Unlike ultraviolet A wavelengths, these are not associated with cancer, but are blocked by window panes.

If you live in a Northern climate and are currently enduring winter, chances are you probably aren’t getting enough of the sunshine vitamin. Not only do we have less hours of sunlight, but the angle at which the sun hits the Earth makes it extremely difficult and maybe even impossible for you to get enough of the vitamin.

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Moreover, if you’re tanned or dark-skinned, you are at an even higher risk of vitamin D deficiency as your skin naturally has more protective melanin; thus limiting the sun from penetrating your skin. The older you are, the more clothes and sunscreen you wear all limit your skin’s ability to absorb the sun’s rays.

For more:

NYTimes: Can you suntan through a window?

Dietary Reference Intakes for Calcium and Vitamin D

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