Several studies have shown that smokers have a lower risk of developing Parkinson’s disease. A new study shows that it’s how many years of smoking a person has under their belt - rather than how much they smoke every day - that matters.

“Smoking is bad for you and no one should advocate smoking just for prevention of Parkinson’s,” Dr. Honglei Chen of the National Institute of Environmental Health Sciences in Research Triangle Park, North Carolina, one of the study’s authors, emphasized in comments to Reuters Health. But the findings could help researchers who are trying to figure out the underlying cause of the disease, Chen added. “Ultimately it’s going to take a multidisciplinary approach to understand this question.”

Chen’s team looked at 305,468 men and women aged 50 and older enrolled in the National Institutes of Health-American Association of Retired Persons Diet and Health Study. Over 10 years of follow-up, 1,662 developed Parkinson’s disease, or about one-half of one percent.

The researchers found that the more years that a person had smoked, the lower their risk of Parkinson’s disease. For example, past smokers who had smoked at least a pack a day for less than 10 years were 4 percent less likely to develop Parkinson’s disease than non-smokers; the risk was 22 percent lower in those who’d smoked for 10 to 19 years; 36 percent lower with 20 to 29 years of smoking; and 41 percent lower with 30 years or more.

The risk of developing Parkinson’s disease did not change based on how many cigarettes a person smoked each day.

When the researchers looked back at study participants’ early smoking behavior, they found that the people who developed Parkinson’s disease were actually less likely to be smokers at a given age than those who didn’t develop Parkinson’s, and were able to quit sooner; this raises the possibility that Parkinson’s patients were somehow less vulnerable to the addictive effects of smoking, the researchers say.

Chen and his colleagues argue that nicotine or other chemicals from tobacco are unlikely to offer a useful way to treat Parkinson’s disease. Studies, they note, have shown that smoking does not slow the progression of disease once Parkinson’s develops nor does it reduce the risk of death from the disease.

“Although smoking is associated with a lower risk of Parkinson’s, it’s not associated with survival of patients after they’ve got the disease,” Chen told Reuters Health.

But in an editorial accompanying the study, Drs. Beate Ritz and Shannon L. Rhodes of the UCLA School of Public Health argue that nicotine or other potentially nerve-protecting substances “might be appropriate for study” as treatments.

“Additionally,” they ask, “of the 4,000+ cigarette-smoke-derived chemicals, are there other choices that would minimize toxicity and maximize benefit?”

SOURCE: Neurology, March 16, 2010.