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http://www.vitamindcouncil.com/newsletter/2006-aug.shtml
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I continue to be shocked and amazed at the prevalence and magnitude of vitamin D deficiency in the people I see every day. It's been a beautiful summer with very little rain. Most days have been in the 70-80 degree range--very comfortable to be outdoors in the sun and getting skin expoxure to activate vitamin D in the skin.
Yet, in the vast majority of people I see, summer blood levels of vitamin D are virtually indistinguishable from winter levels. Both hover around the 30 ng/ml range. Summer levels in Wisconsin people seem to be no more than 10 ng/ml higher than winter levels. This remains true even in people who spend a lot of their day outdoors gardening, walking, etc. wearing shorts and a short-sleeved shirt, i.e. with plenty of skin surface area exposed.
I'm at a loss to explain precisely why. Yes, it is Wisconsin. But a direct sun overhead, 75 degree day should be providing plenty of sun. My suspicious is that a combination of factors are at work: people are not spending as much time outdoors as they claim; they often seek shade; use sunscreen; and they're overweight. (Excess weight decreases vitamin D blood levels dramatically, yet another reason not to get fat!)
Read more about vitamin D by checking out Dr. Cannell's insightful comments on the unfolding vitamin D story. He holds nothing back.