Call it a flimsy silver lining to a noxious blue cloud: Long-term smokers have half the risk of Parkinson's disease that nonsmokers do, according to a new report.
In 12,000 people studied, those who smoked the most—the equivalent of at least a pack a day for 60 years—had the lowest risk. And after smokers stubbed out their last butts, the protective effect faded.
Cigarette, cigar, and pipe smoking appear to offer similar anti-Parkinson's benefits, according to the report in the July Archives of Neurology.
Author Beate Ritz of the University of California, Los Angeles characterizes the amount of Parkinson's protection provided by smoking as moderate. "Never-smokers have about a twofold higher risk of Parkinson's disease than ever-smokers," she says.
However, because Parkinson's disease is fairly rare—only about 60,000 new cases are diagnosed each year in the United States—and because smoking causes cancer and heart disease, "nobody would ever recommend smoking in order to prevent Parkinson's," Ritz emphasizes.
Ritz and her colleagues compiled data from 11 epidemiological studies dating back to 1960. The studies included about 3,000 Parkinson's patients and 9,000 healthy people. Ritz' team recorded the smoking history and Parkinson's status of each subject.
For 2 decades, researchers have speculated that tobacco prevents Parkinson's. The new report, though, "is extremely convincing," says Maryka Quik of the Parkinson's Institute in Sunnyvale, Calif.
The protective effect was seen in every age group except those diagnosed at age 75 or later. Ritz speculates that the difficulty of diagnosing Parkinson's in older patients may explain that result.
As for how smoking may prevent the disease, "nicotine is the likely suspect," says study coauthor Harvey Checkoway of the University of Washington in Seattle.
Robert L. Copeland Jr. of the Howard University College of Medicine in Washington, D.C., agrees. He points to studies in his lab and elsewhere showing that nicotine protects neurons that generate dopamine, a key signaling molecule in the brain.
Parkinson's symptoms appear after patients lose 70 to 80 percent of their dopamine-making neurons. Several small studies have tested nicotine patches and gum for symptom relief in people who already have the disease. But by then, it may be too late for nicotine to do much good, says Quik. Such trial results have been equivocal.
Ritz offers other possible explanations. Some 4,000 chemicals pollute tobacco smoke, and any of those alone or in combination with nicotine may contribute to the anti-Parkinson's effect, she says.
Some aspects of the data lead Ritz to speculate that the explanation isn't chemical at all. Instead, there may be some fundamental difference in susceptibility to nicotine addiction between people who develop Parkinson's and those who don't. "It could be a difference in the dopamine system in brain," Ritz says. Dopamine is important both in Parkinson's disease and in addiction.
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