The study "observed a significantly protective association with diabetes, but not obesity, on risk of ALS."
The study, published in JAMA Neurology, set out to examine the association between diabetes- and obesity-related hospital admissions and the risk of a diagnosis with ALS.
Marianthi-Anna Kioumourtzoglou, ScD, of the Harvard T.H. Chan School of Public Health in Boston, MA, conducted the study with coauthors, using data from Danish national registers for 3,650 patients diagnosed with ALS between 1982 and 2009.
The average age at diagnosis was 65.4 years and the patients were compared with 365,000 healthy controls. Some 9,294 patients were identified as having diabetes, 55 of whom were subsequently diagnosed with ALS.
The authors say the findings are in agreement with previous reports of a protective association between vascular risk factors and ALS. They conclude:
"We conducted a nationwide, population-based study and observed an overall protective association between diabetes and ALS diagnosis, with the suggestion that type 2 diabetes is protective and type 1 diabetes is a risk factor.
"Although the mechanisms underlying this association remain unclear, our findings focus further attention on the role of energy metabolism in ALS pathogenesis."
Rare but fatal disease
Motor neuron diseases are progressive neurological disorders in which motor neurons - the cells that control the essential voluntary muscle activity allowing us to speak, walk, breathe and swallow - are destroyed.
ALS is the most common of these conditions - also known as classical motor neuron disease or Lou Gehrig's disease (American baseball player Lou Gehrig died of the disease in 1941), it is ultimately a fatal disorder that disrupts signals to all voluntary muscles.
Background evidence cited in the paper says around half of patients with ALS - which is rare, having an incidence rate of between 1.5 and 2.5 for every 100,000 people in the population every year - die within 3 years of its onset.
ALS was the subject of appeals for fundraising and greater awareness in the summer of 2014 - when the ice bucket challenge swept through social media.
Several possibilities if the link is causal
The latest study is a retrospective, population-based one, looking back over data to find links, so it was not of a prospective design that might have proven cause-and-effect links.
The authors discuss the potential etiology of the link, however: "If the protective association with diabetes results from some causal association with an aspect of diabetes rather than as a result of correlation with something else, then several possibilities could be surmised."
Possibilities discussed include that diabetes medications may be protective against ALS, metabolic factors resulting from diabetes could have a role, or that high concentrations of uric acid that have been associated with diabetes have also been linked to "lower incidence of other neurodegenerative diseases and prolonged survival in ALS."