PD shares something with many modern afflictions.It is idiopathic (origin unknown), widespread both geographically and across class lines, affects widely distributed body systems (seemingly) without rhyme or reason, comes on slowly, and is generally maddening.

It is tempting to blame environmental toxins and other "obvious suspects", but all fail to explain more than a part of the puzzle.

But this hypothesis is subtly different in that it blows right by the question of original cause and addresses the issue as a disruption of the body's ability to repair itself.

In light of that approach, consider the following-
1) PD is first noted in London approximately one lifetime after a large number of people have abandoned a life of working in the sunlight (repair systems working) for one of both Vit D depletion and increased exposure to destructive forces.
2) There is an increase in PD among farmers with the highest rate among the Amish. This would seem to make no sense. However, anyone from a farming background will tell you that you never see a farmer working in the sun without a long sleeved shirt once he is out of his teens. This is because of the factor of protection from irritating substances and because it is cooler.
3) PD is overwhelmingly a white man's disease. It is true that pale skin produces more Vit D than dark skin and paradox rears its head once again - unless dark skin also is a marker for more efficient use of Vit D itself. Do similar racial disparities exist for, say, MS?
4) The lowest rates of PD are nearest the equator and the highest near the poles.
5) One of the studies that I posted bemoaned the variance among individuals and the difficulty in determining the proper dosing with Vit D, so even growing up together doesn't mean a common fate.

And so on....
There is a lot of room in this tent.