The finding could be useful in pointing toward new directions in treatment since it reinforces earlier research suggesting a connection between the brain chemical dopamine and Parkinson's.

Dopamine, a message transmitter, is low in patients with Parkinson's.

Parkinson's results when brain cells that produce dopamine die. It is a progressive disease that occurs in roughly 1 percent of the over-65 population. Current treatments may temporarily alleviate the symptoms -- including the characteristic shaking that eventually can lead to paralysis -- but there is no known cure.

In the study of 365 Parkinson's patients and 317 unafflicted family members, those with the disease were about 40 percent less likely to have ever been smokers or coffee drinkers.

Scientists do not understand the relationship between dopamine, Parkinson's, coffee and cigarettes.

"Smoking and caffeine may modify underlying genetic susceptibilities that exist in families with Parkinson's disease, but further work is needed to see how this interaction ultimately plays out," said study co-investigator Mark A. Stacy, MD, associate professor of medicine and director of the Duke Movement Disorders Center.

Another neurologist who worked on the study, Burton L. Scott, MD, PhD, associate professor of medicine, suggested that perhaps people who have higher levels of dopamine in their brains are more likely to enjoy caffeine.

Smoking cigarettes and drinking excessive amounts of caffeine pose health risks of their own, cautioned Scott, and should not be adopted as a way to avoid developing Parkinson's disease.

Pesticide exposure has also been strongly linked to Parkinson's risk.