To set the context for this thread:
"In humans, magnesium showed a protective effect against PD in a Japanese population (OR 0.33, 95% CI 0.17–0.73 for highest quartile >312.9 mg/day)"[1].

I take this to mean that a person in the upper 25% of the population, measured by magnesium consumption, has about one third the risk of having PD compared to the rest of the population. Two caveats: the association may not be causal; OR, odds ratio, has some strange properties which make it unstable, I'm treating it as though it approximates to RR, relative risk.

[1] is well worth a read for anyone interested in the relationship between diet and PD.

Reference:

[1] BioMed Research International Volume 2015 (2015), Article ID 672838,
"Dietary Factors in the Etiology of Parkinson’s Disease"
Zeynep S. Agim and Jason R. Cannon
http://www.hindawi.com/journals/bmri/2015/672838/

John