Epidemiological evidence that physical activity is not a risk factor for ALS

To elucidate whether physical activity (PA) and sport increase the risk of developing amyotrophic lateral sclerosis (ALS), a literature review of epidemiological studies was conducted according to the Meta-analysis of Observational Studies in Epidemiology guidelines. Six databases (Pubmed, Scopus, ScienceDirect, IngentaConnect, Refdoc and the Cochrane database) were searched to April 2014. Experts were asked to identify studies in press. Studies of interest were examined for their level of evidence and synthetized using Armon's classification for exogenous risk factors for ALS. Of 37 epidemiological works included in the review, two (5.5%) provided class I evidence, and five (13.5%) class II. Others offered evidence of class III (n = 8, 21.6%), IV (n = 16, 43.2%) and V (n = 6, 16.2%). Results were stratified according to type of exposure: (1) PA related to sport and work (n = 14), (2) soccer and American football (n = 9), (3) occupation (n = 12), (4) proxies of PA (n = 2). Among articles which considered "PA related to sport and work", two class I studies and one class II study concluded that PA is not a risk factor for ALS. This evidence establishes (level A) that PA is not a risk factor for ALS. As regards "occupational related activity" a level of evidence of U was obtained (it is unknown whether the professional category "physical worker" is a risk factor for ALS). Football/soccer may be considered as a possible risk factor for ALS (level C) and there is a need for further research taking into account the numerous confounding factors that may arise in this field.