From Sean Paajanen,
Your Guide to Coffee / Tea.
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A Hawaiian study published in the Journal of the American Medical Association (May 2000), has shown that of 8,000 Japanese/American men, those who drank 3+ cups of coffee per day were 5 times less likely to develop Parkinson's disease. The study took place over a 30-year period.

The theory is that caffeine is reducing the amount of neuro-transmitters produced by the brain, transmitters that may actual do damage to surrounding brain tissue. The actual action of caffeine in the brain is not known. It may also interfere with uptake of other transmitters, allowing the levels of dopamine to increase.

The polyphenols in green tea may also have a preventative effect with regards to Parkinson's disease. These chemicals act within the brain to improve the flow of dopamine between portions of the brain. Parkinson's is caused by a lack of dopamine in one part of the brain, and these polyphenols could prevent that depletion.

Studies with Parkinson's and polyphenols have been conducted in labs with mice, but there have been no actual tea-drinking studies done with humans.

References
Parkinson Disease: Is Starbuck's the Treatment?
Green Tea Protects Against Parkinson's
Green Light for Green Tea
Caffeine May Protect Brain from Parkinson's
Caffeine May Prevent Parkinson's