"Vitamin D deficiency is an unrecognized epidemic among both children and adults in the United States" http://www.ncbi.nlm.nih.gov/pubmed/15585788
For those of us who are healthy and drink plenty of milk, wearing sunscreen when we're out in the sun for a long time is a great way to stay healthy.
From the article:
""" Sensible sun exposure (usually 5-10 min of exposure of the arms and legs or the hands, arms, and face, 2 or 3 times per week) and increased dietary and supplemental vitamin D intakes are reasonable approaches to guarantee vitamin D sufficiency. """
I believe that milk contains Vitamin D2, which is has a low bioavailability. Vitamin D from your skin, created by UVB exposure from sunlight, is Vitamin D3, which has a high bioavailability.
So, it's not true to say that milk is a replacement for sunshine.
Of course, there are Vitamin D3 supplements that you could take.