José Obeso
Professor of Neurology at the Faculty of Medicine of the CEU San Pablo University, Director of the Centro Integral de Neurociencias HM CINAC and a full member of the Royal National Academy of Medicine of Spain
"This is a vast study involving a very large population, carried out in China, with excellent clinical and laboratory methodology. The study examines a specific mechanism – one which has not, until now, been assessed in such depth and with such conclusiveness – relating to the role of carbon monoxide (CO) as a factor causally linked to the lower incidence of Parkinson’s disease (PD) amongst regular smokers.”
Is the research of high quality?
“The research is of excellent quality due to the number of participants, the rigorous inclusion criteria and definition of the different groups (regular smokers, former or occasional smokers, and never-smokers), the measurement of CO, and the detailed analysis of many potential associated factors that could influence the findings.”
How does it fit in with existing evidence?
“It unequivocally confirms the association between smoking and a lower risk of developing PD. It also confirms other known associations in the Caucasian population, such as the slightly higher incidence of PD in men than in women, the direct relationship between age and the risk of developing PD, etc. However, the most original and relevant finding is that they have identified, with statistical certainty, an inverse relationship between CO levels and the risk of developing PD. Thus, in the population that has never smoked, CO levels are the lowest in the entire study and the incidence of PE is highest.”
Could findings of this kind encourage people to take up smoking or prevent them from giving up?
“They certainly should not. As we often reiterate, this is scientifically valuable data that can help us understand the relationship with tobacco and a possible neuroprotective effect of CO, which should be explored in other ways.”
What are the implications of these findings for clinical practice?
“Essentially none. A study published in 2023 revealed that daily nicotine use did not lead to any improvement in Parkinson’s-related symptoms and, on the contrary, there was some deterioration.
Important: for those already suffering from PD, it makes absolutely no sense to start smoking, as this would almost certainly result in a deterioration in general health and neurological status. This must be understood without any doubt.
As the saying goes, ‘smoking kills’, and the adverse effects on general health are far more widespread and severe than any potential benefit in reducing the incidence of PD.”
Does the study have any significant limitations that need to be taken into account?
“Certainly. Although the sample is huge, there is a bias towards a mean age at which the incidence of PD is not yet very high. Regular, long-term smokers in China, in large numbers, are not yet that common, which may have an effect on the data. In any case, the inverse association described between CO and PE is very robust. A more significant limitation is that CO is a gas obviously associated with changes in the air caused by smoking, but chemically speaking, it is easy to understand that many other direct changes—involving other molecules—as well as indirect changes occur. Therefore, at this stage, it is not possible to confirm or establish a direct causal relationship between CO and a lower incidence of PE.”