If you live in a climate where you don't get much sunlight in Winter months, taking Vitamin D supplements is essential. National Institutes of Health offer an excellent primer on Vitamin D3. At a minimum, NIH's research is impressive (your tax dollars at work).
I don't want to sound like an infomercial, but, after taking 1000 UI (International Units) daily (for several weeks), the difference is noticeable. Wish I had known about this years ago.
