For over 10 yrs I have taken Vit D as Rx by my doctors as I have been deficient as evident by blood tests. At that time, 10 yrs, I lived in sun in heat in central Nebraska and I was very active outdoors when I was having a remission. (my CFS journey in early days fluctuated like R & R MS, which they thought I had for several years.) I knew of runners who tested low in the sun vitamin. I now live in western WA state - the Pacific NW - and my doctors and neurologists say every man, woman, and child living here should be taking it. It is cheap over the counter prevention. I take less in the few months of summer we have here, as I like to sit on the patio to soak up the sun. My daily tonic.