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An unfortunately common misconception is that being out in the sun during winter will allow the body to synthesize vitamin D. In Slovakia, I have seen everyone from well respected doctors (Krčmery), to politicians (Krajniak), to my local post-lady expressing the belief that with adequate sun exposure in winter they are getting their needed D3. For much of the world, this is simply impossible for a significant part of the year:
Human skin ... exposed to sunlight on cloudless days in Boston (42.2 degrees N) from November through February produced no previtamin D3. In Edmonton (52 degrees N) this ineffective winter period extended from October through March. Further south (34 degrees N and 18 degrees N), sunlight effectively photoconverted 7-dehydrocholesterol to previtamin D3 in the middle of winter.