Researchers say previous studies have suggested that cigarette smoking may be a risk factor for ALS, also known as Lou Gehrig’s disease, but the results have been conflicting or involved only a small number of participants.
ALS is a neurologic disease affecting the nerve cells in the brain and spinal cord that control many muscles throughout the body. The diseased nerve cells can no longer communicate with muscles, effectively leading to muscle wasting and weakness.
The study involved more than a million participants. Researchers found that current or former cigarette smokers were 42%-44% more likely to develop ALS than people who had never smoked.
More than 5,500 people each year are diagnosed with ALS in the U.S. There is no cure, and there are limited treatment options for the disease, which causes rapid muscle deterioration.
Researchers say the cause of ALS is unknown in about 90% of cases; environmental factors are thought to play a role in affecting a person’s risk.
The study looked at the relationship between cigarette smoking and ALS in five different long-term studies involving 1.1 million people, 832 of whom developed ALS.
The results showed current smokers were 42% more likely to be diagnosed with ALS and former smokers had a 44% higher risk.
Among current or former smokers, the risk of ALS increased as the age at which they started smoking decreased.
Although the risk of ALS increased by 10% for each increment of 10 cigarettes smoked per day and by 9% for each 10 years of smoking, these associations did not persist when the group of never smokers was excluded from the analysis.
“Significant trends in the risk of ALS were observed with the duration of smoking and the number of cigarettes smoked per day, but these trends were largely driven by the low ALS risk among never smokers,” write researcher Hao Wang, MD, PhD, of the Harvard School of Public Health, and colleagues, in the Archives of Neurology.
Researchers say further studies are needed to confirm this link between ALS and cigarette smoke.
But they say there are several possible ways in which cigarette smoking may increase the risk of ALS. For example, nitric oxide or other components of cigarette smoke may cause direct damage to neurons, and chemicals in tobacco may generate free radicals that may also damage cells associated with the disease.