i went shopping for some D recently.
$3 - bottle of 400 mg tablets at CVS/ Long's. "Cholecalciferol" - chemical name.
$XX - bottle of D "from lanolin" at the health food store. much more expensive.
i forget the price - bottle of D in Cod Liver Oil, though with slightly alarming label "most mercury removed", something like that.
i went for the cheap D. turns out, Vitamin D IS cholecalciferol.
there was an ad on the radio about "you need 2000 IU a day" and they were talking about the effect on cognition, SAD (seasonal affective disorder), etc.
anyway, i started taking 400 mg a few times a day. also, when the sun comes out, to be outside for 15 minutes or a half hour, though there hasn't been much sun lately.
so i guess part of the good feeling you get from hanging at the beach is from the natural Vitamin D ?