Originally Posted by johnt

I've not done one of these geographical associations for some time. I find them useful not so much as proof, which of course they aren't - there is no rigorous statistical analysis, nor does it take into account any of a myriad of confounding issues - but to kick-out obviously flawed hypotheses.

Compare:
A. The geographical prevalence of PD in the US [1].
B. Zinc in the soil in the US [2].

Eye-balling the maps I suspect that there is a small negative association between the two. Thus, we can't throw out the hypothesis that low levels of zinc are linked to PD.

I would suspect that the causal mechanism by which zinc levels in the body associate with those in the soil is through the food chain and the water supply.