Current or past tobacco smokers are at lower risk of developing Parkinson’s disease than lifelong non-smokers, a long-term study of doctors in the U.K. reports.
How smoking might protect a person against Parkinson’s, however, remains poorly understood. The many known health risks of tobacco smoking also far exceed “any protective effect,” its researchers caution.
The study, “Tobacco smoking and the risk of Parkinson disease A 65-year follow-up of 30,000 male British doctors,” was published in the journal Neurology.
Smoking is a well-known risk factor for cardiovascular diseases and many cancers, among other ills. But with Parkinson’s disease, it appears to exert a protective effect.
Previous studies have suggested that Parkinson’s incidence is inversely associated to smoking, with one study reporting that smoking was associated with a 60% lower risk of Parkinson’s.
But the link between smoking and Parkinson’s incidence is not well understood, as data on smoking habits are usually collected after disease onset, which is susceptible to bias. The number of Parkinson’s cases and length of follow-up duration is also limited, making conclusions difficult.
Researchers at the University of Oxford followed male doctors over 65 years to investigate the risk of Parkinson’s disease associated with smoking habits, the amount of tobacco smoked, and the duration of effects among ex-smokers.
Their analysis included 29,737 doctors, all members of the British Medical Association. Data on smoking habits were collected in seven self-completed questionnaires from 1951 to 1998.
Participants were asked to classify themselves as current tobacco smokers, ex-smokers, or those who have never smoked. Questions addressed their smoking habits: age at starting to smoke, amount smoked daily, and type of tobacco.
Researchers observed that the percentage of current smokers declined markedly over the study. At its start in 1951, about 67% of doctors (age 65 to 69) were smokers. This number dropped to 8% by 1998, the last year of follow-up. The percentage of those classified as cigarette smokers also decreased, from 63% in 1951 to 33% in 1998.
Specific cause of death was monitor through November 2016.
Between 1951 and 2016, participants were followed on average for 35 years. Of 25,379 deaths registered from 1961 to 2016 (the first 10 years of follow-up were excluded), 283 were attributed to Parkinson’s disease.
The crude death rate from Parkinson’s was lower for current smokers (30 per 100,000 person-years) compared to non-smokers (46 per 100,000 person-years). Of note, crude death rate is the total number of deaths of residents in a specified geographic area, divided by the total population for the same area.
Next, researchers modeled how smoking habits were associated to Parkinson’s risk at the start of follow-up (baseline model, the year 1951) and then periodically after each survey questionnaire (the updated model).
Results showed current tobacco smoking was inversely linked to Parkinson’s risk at baseline and in the updated model.
At baseline (1951) doctors classified as current smokers had a 30% lesser risk of PD compared to those who never smoked. In the updated model, current smokers had a 40% lower disease risk compared with never smokers.
Parkinson’s risk was also found to depend on the daily amount smoked. A statistically significant inverse correlation (higher daily amount, lower risk) in both baseline and updated models was seen between the amount of daily tobacco smoked and Parkinson’s incidence.
Doctors who quit smoking for 10 or more years had a 14% lower Parkinson’s risk than those who had never smoked; those who had quit smoking for nine or few years had a 29% lower risk.
The mechanisms underlying the observed inverse relationship are still not fully understood. Some argue that nicotine may have neuro-protective properties by stimulating the release of dopamine — the brain chemical lacking in Parkinson’s patients — but other tobacco compounds may also play a role.
The researchers caution that “the adverse effects of smoking on risks of vascular and respiratory diseases, neoplasms [cancers], and other noncommunicable diseases for which tobacco is the chief risk factor greatly exceed any protective effect of current tobacco smoking on the risk” of Parkinson’s.
Doctors who died of causes other than Parkinson’s did so at younger ages, a mean of 77, than did those who died of Parkinson’s (a mean age of 82), the study noted.
“The present report demonstrates a causally protective effect of current smoking on the risk of PD [Parkinson’s disease],” the researchers concluded, adding that future studies to understand the mechanisms underlying such a protective effect “may be worthwhile and may contribute to a better etiologic understanding of PD.”
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