Total job- and leisure-related physical activity over a lifetime is not a risk factor for amyotrophic lateral sclerosis (ALS), but heavy physical activity on the job is associated with a nearly two-times greater risk of ALS, a study in Germany shows.
Also, ALS patients reporting pre-diagnosis and current moderate physical activity showed the best survival, while both inactivity and regular, vigorous activity were linked to shorter survival times, suggesting that activity intensity influences disease course.
“The good news is that our results provide further evidence that physical activity you get off the clock, like walking and bicycling, does not appear to be a risk factor for ALS,” Angela Rosenbohm, MD, of Ulm University in Germany and the study’s first author, said in a press release.
“However, we did find an increased risk when we looked at intense physical activity that happens during work hours, for example, in occupations like farmer, steelworker or mason,” Rosenbohm added.
“While we did not prove this type of physical activity is a cause of ALS, the association could be because of repetitive motion on the job, or it could be due to other factors like exposure to chemicals or pollution,” Rosenbohm said.
The study, “Life Course of Physical Activity and Risk and Prognosis of Amyotrophic Lateral Sclerosis in a German ALS Registry,” was published in the journal Neurology.
Whether regular, vigorous exercise is an environmental risk factor of ALS has long been a topic of debate among scientists.
Professional athletes are known to develop ALS at a higher rate and younger age than the general public, and vigorous exercise can increase oxidative stress and neuronal toxicity — hallmarks of neurodegenerative diseases.
Oxidative stress refers to cellular damage associated with an imbalance between the levels of reactive molecules and cells’ production of antioxidants to neutralize them.
However, studies assessing exercise history in ALS patients show conflicting results, mainly due to differences in how they quantify exercise, the duration and type of exercise included, and the use of patient-completed questionnaires that always have a risk of inaccurate recall of past exercise.
Now, researchers in Germany set out to shed light on this potential link by assessing total lifetime physical activity, as well as lifetime physical activity during leisure time and while on the job, in both ALS patients and healthy people (used as controls).
They also investigated whether pre-diagnosis and current physical activity influenced ALS’ course.
The study involved 393 people who recently had been diagnosed with ALS — and who were participating in the ALS Registry Swabia, in the South of Germany — and 791 age- and sex-matched healthy people.
Participants completed a questionnaire in which they were asked to estimate the amount and intensity of physical activity at both work and leisure at 20, 30, 40, 50 and 60 years of age. Physical activity was divided into light activity, including walking and cycling, and intense, sweaty activity, including sports or heavy physical work.
Scoring with METs
The questionnaire scores were expressed as metabolic equivalents (METs), a validated measure of lifetime sport practice that quantifies energy expenditure. It considers each activity as the number of calories spent per body weight per hour.
For each hour of light activity, researchers assigned three METs, while each hour of intense activity corresponded to eight METs.
Results showed that total physical activity up to five years before the questionnaire was not associated significantly with ALS development, as both people with and without the disease showed a mean of 17–18 MET hours per week over their lifetimes.
Notably, compared with healthy individuals, ALS patients reported less physical activity at present and up to five years before the questionnaire, suggesting the neuromuscular disease may start years before symptoms appear, Rosenbohm said.
In addition, a significantly greater proportion of ALS patients reported jobs involving heavy physical activity, compared with controls (22% vs. 13%), reflecting a link between heavy activity on the job and a nearly two times higher risk of ALS.
People reporting light activity on the job showed no increased risk of ALS.
Similar results were obtained after adjusting for potential influencing factors, such as age, sex, and smoking, further strengthening these findings that may reflect exposures to other work-associated factors.
Rosenbohm emphasized, however, that given ALS’s rarity, people with jobs involving heavy physical activity still have a low risk of developing the disease.
Moreover, the researchers found that current and pre-diagnosis physical activity was associated with survival, with inactive patients and those with the highest activity levels showing the shortest survival.
Highest activity levels corresponded to 25 MET hours per week, which was the equivalent of biking or walking about five hours per week.
Particularly, inactive patients lived for a mean of 15.4 months after diagnosis, while hyperactive patients lived for 19.3 months. Best median survival (29.8 months) was seen among patients reporting moderate activity — 10.5 MET hours/week, the equivalent of biking or walking for two hours a week — after adjusting for potential influencing factors.
These findings suggest that physical activity intensity “may be a disease modifying factor with an unfavorable outcome in sedentary and hyperactive behavior,” the researchers wrote.
“While we did find that higher activity levels may negatively affect survival rates in people newly diagnosed with ALS, so may getting no activity at all,” Rosenbohm said, adding that “the message here is that moderate exercise is still best, even after symptoms of the disease begin.”
The researchers noted, however, the study was still limited by the possibility of inaccurate recall by patients regarding type and intensity of physical activity over their lifetimes.
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