Cigarette smoking tied to greater risk of ALS, especially for women
Meta-analysis supports reported ties between tobacco use and disease
Cigarette smoking, either currently or in the past, significantly increases the likelihood of developing amyotrophic lateral sclerosis (ALS), particularly for women, according to a pooled meta-analysis of 32 studies.
A risk of the neurodegenerative condition was higher in current smokers, and it increased in a non-linear manner with more intense smoking — meaning, it rose continuously up to a certain level of smoking, and then stabilized or decreased slightly with additional packs per year.
“One of our most pivotal findings was the dose-response analysis, revealing an inverted U-shaped curve,” Yun Hak Kim, a professor at Pusan National University in South Korea and the study’s senior author, said in a university news release.
“This curve highlights that the risk associated with smoking isn’t a linear progression. Instead, it peaks and then starts to decrease or plateau, suggesting that the risk of ALS is influenced by the intensity of smoking,” Kim added.
Cigarette smoking is among reported ALS risk factors
The study, “Association of smoking with amyotrophic lateral sclerosis: A systematic review, and meta-analysis, including dose-response analysis,” was published in the journal Tobacco Induced Diseases.
ALS is a neurodegenerative disorder that primarily affects the voluntary motor nervous system — the nerves responsible for controlling voluntary movements, such as walking, talking, and swallowing.
Cigarette smoking has been extensively studied as a potential environmental risk factor for ALS. However, while some studies indicate smoking increases ALS risk, others find no link between tobacco and ALS whatsoever.
A need exists for “a more thorough comprehension of the potential causal link between smoking and ALS,” the scientists wrote.
When faced with conflicting outcomes from different studies, a meta-analysis — the combination of results from multiple separate studies to gain greater statistical power — can help in reaching an overall understanding of the evidence.
Researchers conducted such a meta-analysis on 32 studies, published by scientists worldwide between 1981 and 2022, that investigated a relationship between smoking and ALS.
Four were cohort studies, and 28 were case-controlled. The cohort studies examined the occurrence of ALS in smokers versus nonsmokers. Case-control studies measured the frequency and amount of smoking among those with (cases) and without (controls) ALS, matched by sex and age. Follow-up periods ranged from seven to 28 years.
25% higher risk for female smokers, after adjusting for age, education, etc.
After pooling data from all 32 studies, there was a statistically significant 14% increase in the likelihood of ALS among current and past smokers. When the comparison was adjusted for factors that may influence the results, including sex, age, education, and socioeconomic status, the likelihood was still higher, at 12%.
When past smokers were excluded from the calculations, the risk of ALS rose to 28% among current smokers, with and without statistical adjustments.
Looking at patients by sex, researchers found that women smokers faced a 20% increased chance of developing ALS and a 25% higher risk after adjustments. There was no significant relationship between smoking and ALS among men.
“We have identified a significant association between smoking and ALS in females, while no significant association was observed in males,” the scientists wrote. This sex difference, they added, might be due to men being “more exposed to other potential risk factors for ALS, such as pesticides or organic solvents, during their occupational activities.”
Being a heavy smoker, up to a certain point, further elevates risk
Five studies included information on dose-response — the number of cigarettes smoked and odds of ALS — as measured by pack-years. A pack-year describes how many cigarettes have been smoked in a lifetime, with a pack equal to 20 cigarettes. For example, 20 pack-years equals smoking one pack a day for 20 years or two packs a day for 10 years.
Data showed a significant increase in ALS risk with increasing pack-years, which appeared to peak at about 30 pack-years and then declined through 50 pack-years. The team noted that this result might have been influenced by the limited number of higher pack-year studies, with these results mostly attributed to one study.
“Our study showed that there is a positive relationship between smoking and the risk of ALS,” the researchers wrote.
The quality of the selected studies then was assessed with a scale that describes studies as “good,” “fair,” or “poor” based on the selection of study groups, how well these groups can be compared, and how smoking exposure or outcomes were determined.
Among the 28 case-control studies, eight were rated as “good” and 20 as “poor,” while among cohort studies, two were rated as “good” and the other two as “fair.” Researchers said the higher “poor” rating among case-control studies was because the “assessment of exposure was not based on medical records or blinded interviews, and the response rate was either not mentioned or inconsistent between case and control group.”
There were no signs of publication bias, which occurs when studies with positive findings are more likely to be published.
“Our study provides evidence of a positive relationship between smoking and the risk of ALS,” the team wrote. “To mitigate the risk of developing ALS, discontinuing smoking, which is a modifiable risk factor, may be crucial.”