There was an interesting Danish study recently published that suggested a significant association between gut function and the incidence of Parkinson's Disease. The study found that those people who had had their vagus nerve severed in the past had a significantly lower incidence of developing the disease.

This may suggest a possible link between gut health and resultant pathological effects on the brain. Further evidence that you should consider the health of your gut and indeed the type of bacteria that lives within it (your 'microbiome') as of the utmost importance to your general health and longevity.