Reacción a "A study analyses the relationship between quitting smoking and the risk of developing Parkinson's disease"

The study is of good quality. The design is adequate and the methodology is generally correct. Neurology is a very good journal and publishing there generally implies that the article is well done.

It was already well known that smoking less was associated with a higher relative risk of Parkinson's disease. In fact, it had been suggested as a possible protective factor against the disease, associated with the effect of nicotine. But the truth is that the reason for this is unknown. What was suspected, and in fact the authors also mention this, is that in people at risk of Parkinson's disease or who are in a very early pre-diagnostic phase, there is already a dopamine deficit and, therefore, also a deficit in the reward circuit and addiction mechanisms. In other words, it is easier for them to quit smoking because they already have biological deficits related to the disease (as I said, dopamine deficiency), not because quitting smoking increases their risk. In favour of this hypothesis is the fact that what modifies the risk is whether or not they quit smoking, not how much they have smoked.

[As for possible limitations] Fundamentally, it is a retrospective study and, as in all epidemiological studies of this type, it is not possible to control for all the genetic and environmental factors that may influence risk. In any case, the authors partially counteract this by including a huge sample and controlling for the most well-known factors (age, physical activity, blood pressure, cholesterol, etc.).

Beyond these statistical nuances, what is clear is that continuing to smoke increases mortality (and certainly morbidity as well). Regardless of the underlying cause of why people who quit have a higher incidence of Parkinson's disease, smoking is bad for your health.