I was under the impression that Vitamin D was produced by your body naturally and is one of the good things about getting outside and soaking up some sun...Have you been a shut-in lately? (Wink is implied and easily inferred, I hope...)
Just about all Americans over 40 are low on Vitamin D. A friend who spends hours a day in the sun walking her dog has a worse problem than I do. I've been trying to push mine up for about three years, with some success (but not enough). Blood tests soon, so I'll find out if doubling the supplement dose made a difference.