Chinese study connects past smoking habits to increased risk for ALS
Former smokers face higher odds, though total lifetime exposure showed no trend
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- A Chinese study links past smoking to a higher risk of ALS.
- Former smokers were nearly 80% more likely to have ALS.
- Further research is needed, especially in Asian populations.
People with a history of smoking may face a higher risk of developing amyotrophic lateral sclerosis (ALS) than non-smokers, according to a large study conducted in China.
While the findings align with previous research linking smoking to ALS, researchers found no consistent evidence that heavier or longer smoking exposure further increased that risk.
They noted that “these findings highlight disease-specific effects of smoking and the need for further research in Asian populations.”
The study, “Smoking and the risk of neurodegenerative diseases in a Chinese case-control study,” was published in BMJ Neurology Open.
ALS is caused by the progressive loss of motor neurons — the nerve cells that control voluntary muscle movement — leading to worsening muscle weakness over time. While smoking is considered a risk factor for ALS and can exacerbate symptoms, most supporting studies were conducted in Europe and the U.S., leaving its impact on Asian populations less clear.
Examining smoking habits in China
To investigate this connection, researchers conducted a hospital-based case-control study among people with neurodegenerative conditions at the West China Hospital of Sichuan University in China.
The study enrolled participants with newly diagnosed neurological conditions, including 430 people with ALS, 523 individuals with Parkinson’s disease, and 271 with multiple system atrophy (MSA), a form of atypical Parkinsonism. A control group of 1,033 individuals without neurological diseases was included for comparison.
Trained interviewers assessed participants’ smoking history through standardized, in-person interviews. Participants were classified as “ever-smokers” if they had smoked at least one cigarette daily for six consecutive months or longer. Former smokers were defined as those who had quit more than a year prior to the interview, while current smokers were those still smoking or who had quit within the past year.
Participants with ALS had a mean age of 55.7 years, and 57.4% were men. More than half (53.7%) had never smoked, whereas 28.8% were current smokers and 17.2% were former smokers. Among those with a smoking history, the average duration was 31.6 years, totaling an exposure of 32.5 pack-years. Former smokers had quit for a mean of nine years.
Pack-years measure lifetime cigarette consumption, where one pack-year equals smoking one pack per day for a year, with a pack equaling 20 cigarettes. For instance, smoking one pack a day for 20 years equals 20 pack-years.
Additionally, most participants with ALS reported not drinking alcohol (62.6%), or tea or coffee (65.6%) and had never been exposed to pesticides (58.8%). However, 39.8% did report high levels of pesticide exposure.
Risk patterns and exposure findings
Most participants also engaged in moderate to high levels of physical activity starting at age 20 and had a normal body mass index, a measure based on height and weight.
Compared with never-smokers, ever-smokers and current smokers as a whole did not show a significantly higher risk of ALS. However, former smokers were nearly 80% more likely to have ALS. The researchers found no clear association between the total amount of smoking over a lifetime and overall ALS risk.
Additionally, people who had stopped smoking within the previous four years showed a trend toward nearly twice the risk of ALS compared to current smokers.
The researchers wrote that these findings “suggest positive associations” between smoking and ALS, but emphasized that the lack of “clear exposure-response relationships” means the evidence is not conclusive.
Similar patterns appeared for MSA, where ever-smokers and former smokers faced a higher likelihood of developing the disease than never-smokers. In contrast, both ever-smokers and current smokers had a lower risk of Parkinson’s disease compared to non-smokers.
The researchers noted that these results are “consistent with findings from previous studies in both Asian and non-Asian populations…. For ALS and MSA, we found evidence suggestive of positive relationships with cigarette smoking, but no clear exposure-response relationships were observed.”
The study’s limitations include its case-control design, which relied on participants’ recall of their smoking history. Furthermore, self-reported daily cigarette consumption did not account for shifts in smoking habits over time, which may have led to misclassification of exposure.
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