Research Brief: Pooled Analysis Calls Smoking an ALS Risk Factor

As if there wasn’t already enough reasons not to light up, a new study in the February Archives of Neurology reports that smoking boosts one’s risk of amyotrophic lateral sclerosis (ALS) by a factor of 1.4. The study, led by Hao Wang at the Harvard School of Public Health, considered more than a million people from five different prospective studies, 832 of whom had ALS. The younger people started smoking, the higher their risk of this motor neuron disease.

Smoking has long been considered a possible ALS risk factor (Kamel et al., 1999). However, recent results are conflicting. Some studies say smoking is a risk factor (Binazzi et al., 2009; Gallo et al., 2009); others say it is not (Schmidt et al., 2010; Okamoto et al., 2009), and some say it is, but only in women (Alonso et al., 2010; Alonso et al., 2010).

In the current work, Wang and colleagues pooled data from five long-running prospective studies: the Nurses’ Health Study, the Health Professionals Follow-up Study, the Cancer Prevention Study II Nutrition Cohort, the Multiethnic Cohort, and the National Institutes of Health—AARP Diet and Health Study. The authors note that, unlike studies that recruit participants at ALS clinics, this method is likely to represent a more complete spectrum of people with ALS. Using death records, the researchers determined that 832 of the 1.1 million participants in the studies likely died of ALS. Those who smoked were more likely to die of the disease. There was no significant gender effect.

The authors suggest that nitric oxide or other cigarette ingredients might damage nerves or cause oxidative stress, which is often linked to ALS. Given that the earlier a person tried that first cigarette, the higher his or her risk, one possibility is that early smoking starts a cascade of events in susceptible individuals, wrote study coauthor Eilis O’Reilly of the Harvard School of Public Health in an e-mail to ARF.

I don’t think this closes the debate, added coauthor Marc Weisskopf, also at Harvard, in another e-mail, noting there are still further questions about how smoking and ALS relate. Conversely, Carmel Armon, who in 2009 concluded smoking is a clear risk factor for ALS (Armon, 2009) and was not involved in the current work, wrote in an e-mail that he sees no reason for further research. There has to be a point where we accept the facts as they are, he wrote.

Wang H, O’Reilly EJ, Weisskopf MG, Logroscino G, McCullough ML, Thun MJ, Schatzkin A, Kolonel LN, Ascherio A. Smoking and risk of amyotrophic lateral sclerosis: a pooled analysis of 5 prospective cohorts. Arch Neurol. 2011 Feb;68(2):207-13. Abstract

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