Reacción a "Exhaled carbon monoxide has been linked to a lower risk of Parkinson’s disease"

The study is epidemiologically sound: a prospective design, over half a million participants and 12 years of follow-up. But for me, its significance lies not where it might seem at first glance. Confirming that smoking is associated with a lower incidence of Parkinson’s does not constitute a decisive contribution, because that observation has been on the table for decades. What is striking is the subgroup of people who had never smoked: even amongst them, higher levels of exhaled carbon monoxide are associated with fewer subsequent cases of the disease. And here, the measurement is objective, with adjustments made for foreseeable external sources, ranging from passive smoking to the domestic use of solid fuels.

However, an association does not imply a protective effect. Carbon monoxide could be involved in some protective mechanism or simply indicate other biological processes or exposures that do influence the risk. The study does not distinguish between the two.

The mechanism is plausible. We produce carbon monoxide endogenously via the heme oxygenase pathway, and experimental findings link it to the response to oxidative stress and to neuroprotective effects, reducing the aggregation of α-synuclein in animal models.

The message to the public must be unequivocal. These findings do not justify taking up smoking or delaying giving up smoking; the study itself finds, as might be expected, higher rates of lung cancer, cardiovascular disease and mortality amongst smokers. Nor do they justify deliberately exposing oneself to a gas that causes severe poisoning.

Furthermore, there remains a limitation that is easily overlooked. The fact that something is associated with a lower incidence of the disease says nothing about whether administering it would slow its progression in someone who has already been diagnosed. These are two distinct questions, and this research addresses only the first. Before discussing clinical applications, the finding would need to be replicated in other populations and its safety and efficacy demonstrated in controlled trials. This is a hypothesis worth exploring, but so far there is no reason to change clinical practice or tobacco prevention policies.