Parkinson’s disease is significantly less common among people who smoke cigarettes. It’s a paradox that has perplexed researchers for decades, drawing speculation that tobacco smoke somehow protects the brain from developing the neurodegenerative condition.
A prospective study on more than half a million adults across China has linked the carbon monoxide produced during combustion with the reduction in Parkinson’s, lending support to clinical trials exploring the molecule’s potential as a treatment.
Parkinson’s disease is a condition characterized by tremors, muscle rigidity, and problems with balance. Caused by the destruction of neurons in parts of the brain responsible for fine-tuning motor control, it is a disease that progressively worsens over time.
While researchers have made tremendous headway in diagnosing the disease early and delaying symptom onset, a cure remains elusive, and a detailed understanding of the mechanisms behind the disease still has too many gaps.
Ever since the German medical scientist Franz Hermann Müller published solid evidence of a link between smoking and lung cancer in 1939, the health risks of puffing on tobacco have only become clearer. There seems to be barely any part of the body that isn’t negatively affected in chronic smokers, from the lungs to the heart to the gut to the brain.
So discovering that smokers are less likely to develop a serious neurological impairment that affects millions worldwide has left medical experts scratching their heads.
There’s been no shortage of hypotheses. Could a predisposition to smoking promote behaviors that reduce Parkinson’s risk? Perhaps raised nicotine levels shut down processes that would typically create the perfect conditions for Parkinson’s to develop?
There’s a good reason to suspect nicotine may be involved. The neurotransmitter acetylcholine works in balance with dopamine – a neurotransmitter produced by the neurons that are lost in Parkinson’s disease. And nicotine happens to stimulate the very same receptors that also respond to acetylcholine.
Yet nicotine isn’t the only bioactive chemical that smokers soak up in large concentrations.
Carbon monoxide is another two-faced toxin with a curious mix of deadly and therapeutic effects. By hijacking oxygen’s seat on red blood cells, it can suffocate our tissues in high enough doses. Yet our bodies also generate small amounts of this simple compound to signal immune responses and protect cells from oxidative stress.
“Carbon monoxide has anti-inflammatory effects and in the brain helps to counteract cell stress,” explains Toni Pitcher, a researcher in Parkinson’s epidemiology from the University of Otago in New Zealand, who wasn’t involved in the study.
“There is the potential that individuals with higher internal levels of carbon monoxide are more able to fight off inflammatory processes and better maintain brain health.”
This alone led to preliminary clinical trials in animals exploring the use of carbon monoxide in treating conditions like Parkinson’s.
Despite this potential, evidence linking the chemical and the neurodegenerative disease is limited. So researchers from the University of Oxford led an investigation to establish whether carbon monoxide can help explain smoking’s unexpected shielding effect.
After confirming that chronic smoking was associated with a lower risk of Parkinson’s disease, they found that higher levels of carbon monoxide measured on the breath of people who had never smoked also correlated with a reduced risk.
The evidence is far from a smoking gun, constrained by numerous limitations. Most of the non-smokers were women, for example, leaving room for a sex bias in the results. It’s also an observational study, making it hard to establish cause and effect.
It’s also vital to keep the overall risks in mind. What may be gained in reducing the risk of Parkinson’s by a small amount is vastly outweighed by the increased risk of a large number of other health problems. Smoking is not the solution it appears to be.
Yet the study goes some way to explaining the link, which could help us better understand not only why Parkinson’s develops, but how we can safely delay or even prevent its onset in those at most risk.
“The challenge now is to replicate these findings in independent cohorts and to better understand how carbon monoxide counteracts the pathology associated with Parkinson’s,” says Pitcher.
“Study authors refer to a registered clinical trial testing the safety of low-dose carbon monoxide in people with Parkinson’s, highlighting its therapeutic potential.”
Source: Scimex
This research was published in JAMA.
