From other things I've read, probably vitamin D deficiency form lack of sunlight is a factor. Of course, there can be too much of a good thing too... Intermitt

From other things I've read, probably vitamin D deficiency form lack of sunlight is a factor. Of course, there can be too much of a good thing too... Intermittent sun exposure has also been blamed rather than frequent to promote tanning. Lack of exercise and stress from indoor jobs may play a part as well in increased cancer risk in general.