Where There's Smoke, There's Less Parkinson's Disease

LOS ANGELES -- Cigarette, pipe, and cigar smoking, and chewing tobacco as well, were all associated with a lower risk for Parkinson's disease in an analysis of pooled data from 11 studies.

LOS ANGELES, July 9 -- Cigarette smoking and other tobacco use appear to protect against Parkinson's disease, according to a pooled analysis.

The Parkinson's data stand in stark contrast to all of tobacco's baleful effects on the body, reported Beate Ritz, M.D., Ph.D., of the University of California at Los Angeles School of Public Health, and co-authors.

Data gathered in large cohort studies suggested that current smokers had a 77% lower risk for Parkinson's disease compared with never smokers, the investigators wrote in the July issue of the Archives of Neurology.

The protective effect of smoking increased with pack-years smoked, and decreased with years since quitting.

"Our data support a dose-dependent reduction of Parkinson's disease risk associated with cigarette smoking and potentially with other types of tobacco use, they wrote.. "Importantly, effects seemed not to be influenced by sex or education."

The lower risk of Parkinson's in tobacco users may be related to nicotine exposure, the authors suggested, although they noted that studies of nicotine alone in Parkinson's have been mixed.

The evidence from both case-control and cohort studies pointed to a protective effect of smoking against Parkinson's disease, the authors noted.

"Recent studies also suggested that Parkinson's disease risk is particularly low in active smokers with a long history of intense smoking; some even suggested dose-related risk reductions with increasing pack-years of smoking," they wrote. "This prompted speculation as to whether and how these observations might inform Parkinson's disease treatment and prevention."

To see whether demographic factors or type of tobacco use might modify the putative protective effect, the authors pooled data from eight U.S. case-control studies and three large cohort studies--the Nurses' Health Study, Health Professionals Follow-Up Study, and Honolulu-Asia Aging Study. The studies were conducted from 1960 through 2004.

The case-control studies contained data on a total of 2,328 patients with Parkinson's and 4,113 controls matched by age, gender, and racial/ethnic background. There were 488 patients with Parkinson's in the combined cohort studies, who were compared with 4,880 controls chosen from age- and gender-matched risk sets. The main outcome measure was incident Parkinson's disease.

"Our pooled-risk estimates for current and former versus never smoking suggest a similar sized risk reduction for Parkinson's disease among men and women but a stronger reduction in current than former smokers and in cohort studies compared with case-control studies," the investigators wrote.

In the case control studies, the odds ratio for Parkinson's disease among current versus never-smokers (men and women combined) was 0.53 (95% confidence interval, 0.44-0.63), and for former smokers was 0.76 (95% CI, 0.68-0.86).

In the cohort studies, the odds ratio for current vs. never smokers was 0.23 (95% CI, 0.15-0.36), and for former smokers was 0.64 (95% CI, 0.52-0.77).

When they looked at relationship of smoking duration and Parkinson's disease, they found that the risk went down as the number of pack-years smoked increased. Among women, nine or fewer pack-years of puffing was associated with an adjusted odds ratio of 0.93 (0.88-1.00), whereas 60 or more pack-years was associated with an odds ratio of 0.52 (95% CI, 0.31-0.89). A similar decrease in risk was seen for men.

The reductions in relative risk averaged 5%-8% for every 10 pack-years smoked. Conversely, risk rose again the longer that former smokers stayed abstinent.

"For smokers, we also observed inverse trends for quantity of cigarettes and duration of cigarette smoking separately, the investigators wrote. "For both sexes, a strong inverse dose-response trend was observed for the number of years from cessation of smoking to Parkinson's disease diagnosis. This did not change when we excluded never smokers from the comparison group such that the reference group consisted only of former smokers who had quit 25 years ago or more."

The authors found that the inverse association between smoking and Parkinson's risk was not influenced by education, but race and ethnicity appeared to play a role with heavy smoking white and Asian Americans, but not African-Americans or Hispanics, having lower risk for the disorder.

There was also evidence to suggest that in men pipe or cigar smoking, or both, and chewing tobacco use may be associated with lower risk for Parkinson's. Data on non-cigarette tobacco use among women were not available, the authors noted.

Although the investigators found a dose-response relationship between pack-years of smoking and Parkinson's disease, the inverse association was not seen for those older than at diagnosis, regardless of whether they restricted this analysis to smokers or included nonsmokers.