In Tamraz v. Lincoln Electric Company, et al the 6th Circuit held that an expert could not stack speculation about mechanisms upon unseen and undetected lesions in the plaintiff to get from manganese exposure to his Parkinson's disease. Furthermore, the court ruled that you can't reasonably arrive at a causation opinion using what's erroneously called a differential diagnosis (it's more properly called simply a process of elimination) when you've no sound basis for ruling anything either in or out.
On the other hand, there's growing evidence that vitamin D deficiency is strongly associated with Parkinson's. Best of all, it makes sense. It looks like the enteric nervous system is first to be degraded in Parkinson's and that system is exquisitely sensitive to the gut mediated immune system which in turn is adversely affected by vitamin D deficiency. See: "Parkinson Disease: Could Sunlight Offer Protection from Parkinson Disease?"