A weak link only between a person’s current smoking study and greater risk of amyotrophic lateral sclerosis (ALS) was seen by scientists in a U.K. study, with no evidence that lifetime smoking exposure affects such risk.
The researchers believe that this weak association is likely a “false-positive,” and there is no “strong evidence to support smoking as a risk factor.”
Their study, “UK case control study of smoking and risk of amyotrophic lateral sclerosis,” was published in Amyotrophic Lateral Sclerosis and Frontotemporal Degeneration.
Cigarette smoking has been proposed as a risk factor for developing ALS and accelerating disease progression. But it remains unclear which aspects of smoking put patients at greater risk.
Researchers set out to examine multiple variables of smoking, including whether ALS patients are former or current smokers, the number of cigarettes smoked per day, their smoking duration, number of packs smoked per year, and time since they stopped smoking.
They created a score — the comprehensive smoking index (CSI) — which considered smoking duration, intensity, and time since cessation to estimate lifetime smoking exposure. To date, no such score has been used to study associations between smoking and ALS.
Their study looked at 202 ALS patients, diagnosed between 2008 and 2013, whose data was available at the Motor Neurone Disease Association of England, Wales and Northern Ireland. Two hundred healthy individuals, matched for sex, age, and geographical location, were used as controls.
The two groups were similar overall, except for educational status and alcohol use, both of which were lower in ALS patients.
An initial analysis found no association between any of the smoking variables and ALS risk. But after adjusting for a number of smoking variables — age at time of taking the survey, sex, level of education, current smoking status, and alcohol initiation — the team found that being a current smoker (smoker at the time of the survey) increased by 3.6 times the likelihood of ALS.
However, that association was weak, and a statistical analysis used to counteract the problem of multiple comparisons suggested that this is a false positive. No other variable was found to be associated with the odds of having the disease, not even lifelong smoking exposure.
While the study was only powered to detect large effects of smoking and ALS risk, such as an increase of 80% or more in that risk, “we do not find strong evidence to support smoking as a risk factor, even using lifetime smoking exposure as measured by the CSI,” the researchers concluded.
Prior studies reporting an association between smoking and risk of ALS may have been biased, they added. These studies identified people with ALS from death certificates, which may over-represent smokers in this group due to a shorter life expectancy.
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