The makeup of the skin is also a factor. "Ol Red" the pink guy, compared to the folks who live in equatorial regions naked all mixes inbetween.
Also, the health of the system, the skin. I have noticed how sauna and scrapping has changed the character of my skin. The efficiency is improved as modern toxins and clogging are removed, thus the manufacture of "D" can take place to a higher degree. Intake of good cholesterol seems to be a factor.
There are many different studies with diverse aspects converging for our home team. Basically "D" come from skin and diet, "D" good. Get lot of "D".
The cancers mostly come where parts commonly get red and burn, the edge of tanlines.
Jbee