According to a very well-written (and I suppose accurate, considering
the publication) article in the NUTRITION ACTION newsletter (magazine?)
about two issue ago dealt in depth with calcium absorption and Vitamin
D's role in it.  Living in Vermont, I took special note of the statement
in the story that anyone living above a certain latititude (which
included me) for about three months of the winter even direct sun
exposure yields very little Vitamin D benefit because the sun is so low
in the sky all day that the beneficial rays (part of the UV spectrum)
never reaches the earth because it is absorbed by the greater volume of
atmosphere through which it has to pass during those months.  AND it
said that chemical ingestion of D (tablets) cannot come anywhere NEAR
the value of skin-generated D.

All this is much more quantified in the story--hope you can get a copy
if you're interested.  The periodical is published by the Center For
Science in the Public Interest (in Washington, DC, I think).

Greg Epler Wood
Bennington, VT