[mason55]

Melanin blocks sunlight which reduces vitamin D synthesis

http://essays.backintyme.com/item/4

> The lightness adaptation enhances calciferol (vitamin D) synthesis. Too much epidermal melanin for the latitude blocks UV penetration essential to the dermal synthesis of calciferol or vitamin D.

If you have less melanin then more sunlight reaches the lower layers of skin which increases vit D synthesis.