It seems that the higher the level of vitamin D the better.
So VitaminDWiki was wondering why evolution results in > 100 ng of vitamin D
A possible solution: When getting > 50 ng of vitamin D from the sun there is a danger of damaging the skin,
so the body developed a method of limiting skin damage by darkening the skin.
Darkening of the skin is a good trade-off to make vs the benefits of vitamin D when the sun was the only source of vitamin D.
This was appended to Is 50 ng of vitamin D too high, just right, or not enough Jan 14, 2013