I forgot to include a summary in my previous post, so here it is:

If I've understood johnt correctly, his hypothesis is based on the following premise:
"The Braak hypothesis says that PD starts in the gut, before spreading to the brain. It is reasonable therefore to suggest that the dopaminergic neuron loss is higher in the gut than in the brain, ...".

If I have understood reference [1] correctly (see previous post), PD researchers (up until 2014 at least) can find no evidence of dopaminergic neuron loss in the gut.

As always, I'm happy to be corrected.