Getting regular physical exercise is one of the best things you can do for your body, but it could also lead to heightened risk of developing amyotrophic lateral sclerosis (ALS), according to new research.
ALS – aka motor neurone disease or Lou Gehrig's disease – is most widely known these days as the neurodegenerative condition that affected the late, great Stephen Hawking, but while there's still a lot we don't know about what causes this fatal disease, intensive physical activity looks to be a risk factor we should be aware of.
"We observed a linear association, which means that the risk appeared to increase with each increase in exercise level," says neurologist Leonard van den Berg from the University Medical Centre Utrecht in the Netherlands.
To assess the associations between physical activity and ALS – which have long been studied by researchers, but never definitively understood – van den Berg's team investigated the lifestyles of over 1,500 adults diagnosed with ALS in Ireland, Italy, and The Netherlands.
These people's data – collected by interviews and questionnaires – included information on their lifetime physical activity levels, along with their gender and things like their educational attainment, employment history, and smoking and alcohol intake.
These responses were compared with a control group of almost 3,000 healthy participants matched as closely as possible in demographics to the ALS participants.
When all the other factors were accounted for, physical activity levels stood out as a statistically significant risk factor affecting people's likelihood of developing the disease.
In the study, physical activity was measured in what's called Metabolic Equivalent of Task (MET) minutes, and the researchers found the higher the MET score, the greater the risk of developing ALS – with the finding consistent across physical activity levels for both leisure (sports and hobbies) or work (in occupations requiring physical exertion).
Per the data, the average heightened risk for physical activity in leisure time was 7 percent, and 6 percent for occupational physical activity, with the association being most evident in the Irish and Italian cohorts.
Noting that previous studies indicating this kind of result had found only strenuous activities showed an association with ALS, the researchers found even moderate recreational exercise demonstrated a link to the condition.
An increased risk of up to 7 percent might not seem like much, but the team says the overall risk can be as high as a 26 percent increase in risk when comparing a person who is more active than average with one who is significantly less active than average.
It's very important to note that due to the observational nature of this kind of research – which relied on participants' self-reporting and powers of memory – nobody is suggesting causation here.
But the researchers nonetheless think their result warrants further investigation, especially since we really have no firm explanation for how this association could be so prevalent, even if it seems subtle.
While the cause of ALS is unknown in the vast majority of diagnoses, up to 10 percent of cases are due to genetic inheritance. As to how physical activity might contribute to people's genetic predisposition to developing the disease, nobody's entirely sure.
While welcoming the study, other researchers are cautious that people may interpret the findings the wrong way.
"If you adopt a sedentary lifestyle, you're going to die much faster of heart disease than you will of ALS if you go out and exercise vigorously," physical rehabilitation researcher David Putrino from the Mount Sinai Health System, who wasn't involved in the study, told HealthDay.
"Under no circumstances should anyone stop exercising because of this study."
That point is especially pertinent when you consider how rare ALS is among the world population, compared to the multitude of conditions prevented or mitigated by healthy levels of exercise.
While the researchers behind the new study suggest the higher risk of ALS might be a "trade-off" for obtaining well-known benefits of staying physically active, in reality, until we know a lot more about what's behind this association, this isn't something most people should ever worry about.
"It is important to keep in mind that ALS is a relatively rare disease affecting around 2 in every 100,000 people," explains neuroscientist Tara Spires-Jones from the University of Edinburgh, who wasn't involved in the research.
"Physical activity protects us from much more common diseases including Alzheimer's, heart disease, diabetes and cancer, which together affect more than 10 million people in the UK today."
The findings are reported in the Journal of Neurology, Neurosurgery & Psychiatry.