Health

People with Parkinson's disease are less likely to be smokers and coffee drinkers than their healthy siblings, according to a study of family members. The finding adds to a growing body of evidence that some substance in tobacco might protect the brain against this devastating neurological disorder and sheds new light on coffee's effects on the disease.

Researchers say the study provides new evidence that the causes of Parkinson's vary. They also stress that the negative health effects of smoking far outweigh any protective effect the substance might have against this neurodegenerative disease.

Parkinson's disease, which generally strikes people over the age of 50, leads to a loss of coordination, dementia and may result in early death.

For decades, scientists have found evidence suggesting that smokers are less likely to develop this illness than non-smokers. But experts believe that genes can influence one's risk of developing Parkinson's, and the vast majority of these studies involved participants who were unrelated and therefore genetically dissimilar.

To control for this genetic variability, William Scott of the University of Miami in Florida, US, and his colleagues studied the smoking and coffee-drinking habits of Parkinson's patients and their family members.

Close relatives

They recruited 356 Parkinson's patients and 317 of their close relatives, such as siblings. All of the relatives had passed the age at which their family member with Parkinson's had first shown signs of the disease.

The team found that the Parkinson's patients were about 40% less likely to have ever smoked than their healthy relatives, and 50% less likely to be long-term smokers, with roughly 10% smoking for more than 30 years, compared with 15% of their healthy counterparts.

Of the healthy relatives, about 40% consumed an average of three or more cups of coffee a day over their lifetime. By contrast, those with Parkinson's were about 40% less likely to report drinking this much coffee.

Exactly why smoking and consuming coffee had an inverse association with Parkinson's disease remains largely unclear.

Dopamine link

One possible mechanism involves a signalling chemical in the brain called dopamine. The death of dopamine-producing cells in the brain appears to drive the progression of Parkinson's disease, and both smoking and drinking coffee can raise levels of the chemical.

Scientists have also found some evidence that a substance in tobacco smoke may boost dopamine levels by blocking the production of toxins that poison dopamine-producing cells (see Why the wicked weed wards off Parkinson's).

But Scott cautions that the relationship between Parkinson's and smoking may not be a simple one of cause and effect. The biology of certain people's brains who develop Parkinson's disease may simply make them less likely to become addicted to cigarettes, he says.

Smoking risks

Scott says the new research lends support to the idea that there are multiple ways that Parkinson's can strike.

A variety of different genetic mutations are associated with Parkinson's, and only some of them may have any kind of smoking link. For example, a Parkinson's-related gene mutation called NOS2A does not appear linked to the habit - patients with the mutation were just as likely as their close relatives to report regularly smoking and drinking coffee.

Scott emphasises that the results of his study should not give anyone an excuse to start smoking. He says that the well-established risks of smoking - such as developing lung cancer or having a heart attack - "absolutely" outweigh any potential protection it might offer against Parkinson's.

"And relative to lung disease and heart disease, Parkinson's disease is far less common," he adds.

Journal reference: Archives of Neurology (vol 64, p 576-580)

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