In the realm of medical mysteries, few are as paradoxical and thought-provoking as the link between smoking and Parkinson's Disease. This connection, one of the most robust and perplexing in neuroepidemiology, defies conventional wisdom. While smoking is universally acknowledged for its detrimental health effects, its association with a significantly reduced risk of Parkinson's Disease presents a challenging contradiction for researchers and the public alike. As we delve into this counterintuitive phenomenon, we find ourselves at a crossroads of caution and curiosity, navigating the intricate interplay between a harmful habit and a potential neurological safeguard.
The protective enigma of smoking
The relationship between smoking and reduced risk of Parkinson's disease risk is not only surprising but also complex. Studies have consistently shown that smokers are less likely to develop Parkinson's disease compared to non-smokers, an observation that extends to various aspects of smoking, including how many cigarettes a person smokes and for how long. In investigations with identical twin pairs in which one had Parkinson's disease and the other did not, the twin without Parkinson's disease tended to smoke more than the twin with Parkinson's disease. Intriguingly, the risk reduction correlates with increased smoking dose and extends even to those who had smoking parents, suggesting a deep-rooted connection beyond mere coincidence.
Is it the nicotine?
Nicotine, a prominent component of tobacco, initially seemed a promising lead. Its prevalence in tobacco and psychoactive properties made it a likely candidate for reducing the risk of Parkinson's disease risk among smokers. However, two clinical trials, including the NIC-Parkinson's disease study, have painted a different picture. Despite nicotine's protective indications in laboratory studies, these clinical trials haven't shown a significant benefit in Parkinson's disease motor function. This suggests that other elements in cigarette smoke are potentially protective.
Other constituents of tobacco have a suspected role. At low concentrations, carbon monoxide demonstrates physiological functions that could hint at neuroprotection. However, protective effects of smoking are also seen when consuming smokeless tobacco (tobacco that's chewed, sucked or sniffled rather than smoked), making carbon monoxide less likely to be the culprit. Similarly, monoamine oxidase-B inhibitors found in smoke have been suggested as potential factors due to their role in Parkinson's disease treatment. These, together with the effect of smoking on the gut microbiome, an emerging area of interest in Parkinson's disease research, warrant further exploration.
Should we look at this backwards?
A critical question arises: what if we are viewing this association backward? Could it be that Parkinson's disease's biology affects smoking behaviour rather than smoking influencing the risk of Parkinson's disease? Studies suggest that patients with Parkinson's find it easier to quit smoking and need fewer nicotine substitutes to do so. This could be tied to the role of dopamine, a key neurotransmitter implicated in addiction, which is severely diminished in Parkinson's disease. If this theory holds, it might explain the observed lower smoking rates among Parkinson's disease patients.
While the potentially positive link between smoking and reduced risk of Parkinson's is intriguing, we can't avoid the overwhelming evidence of smoking's harmful effects. Smoking is a leading cause of numerous deadly diseases, including lung cancer, heart disease, and stroke. And then there's the way it ages your appearance, the wrinkles, the yellow teeth, bad breath and so on.
Researchers now face the challenge of unravelling how smoking's association with reduced Parkinson's disease risk could be leveraged without endorsing smoking itself. The focus is on isolating the protective elements within tobacco smoke and understanding their mechanism of action.
A Clue Hidden in Contradiction
Despite the undeniable harms of smoking, its strong association with lowered Parkinson's disease risk remains a critical clue in understanding and potentially combating Parkinson's disease. This paradoxical link, far from being an endorsement of smoking, stands as a reminder of the intricate and often unexpected ways in which lifestyle factors can intersect with disease. Watch this unravelling space.
Dr Victor Dieriks
Group Leader Synuclein Lab
Centre for Brain Research
The University of Auckland, New Zealand