Because it’s a crock?
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Mainstream quackery is the crock!
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Vitamin D supplementation provides some risk reduction, especially in those who are D deficient according to this:
http://www.bmj.com/content/356/bmj.i6583
Doc, seriously. Nutrition is no crock. See my previous for a great success in the elderly with adequate b12, which no one can have without supplementing.