Comment on The essential guide to vitamin D and the sun by Myrtonos.
How much UVB you need to get enough vitamin D might vary according to skin type.
Myrtonos Also Commented
The essential guide to vitamin D and the sun
I should note that not all races evolved in the tropics. Although the first humans may have come from the tropics, Caucassian people have been living in mid-latitudes for thousands of years. And certain races, such as the Inuit, Aluit, Sami and the Nordic peoples have all been living north of the 50 possibly for hundreds of generations.