Article In Brief
A longitudinal study of long-distance skiers supports the concept of “motor reserve”—that those in unusually good shape physically have a reservoir of motor function that can delay the clinical appearance of Parkinson's disease symptoms.
A study of almost 200,000 long-distance skiers over 21 years found that they have a lower incidence of Parkinson's disease (PD) compared with people in the general population but there was a smaller difference in PD incidence over time.
The findings, reported in the January 10 issue of the Journal of Parkinson's Disease, lend further credence to the idea of “motor reserve”—that those in unusually good shape physically have a reservoir of motor function that can delay the clinical appearance of PD symptoms.
Independent experts said the results should be interpreted with caution, however, due to the smaller group sizes for individual age groupings and the lack of analysis for certain factors related to PD risk.
The findings stem from observations of skiers participating in the annual Vasaloppet cross-country skiing competition, in which almost 16,000 skiers race about 19, 28 or 56 miles in northern Sweden, the majority of them exercising at least four hours a week more than the general population.
Study Design, Findings
People with severe disease, such as cardiovascular or respiratory disorders, were excluded from the study to reduce the bias due to people not being healthy enough to participate in the race. Researchers did not adjust for confounders such as smoking and diet because of a lack of precise data on those factors. The median follow-up was 10 years, but many of the subjects were followed for much longer.
Researchers at the Lund University in Sweden found that skiers had a 29 percent lower incidence of PD than controls who were matched according to age group (in five-year intervals), sex, and region of residency (p = 0.005). To protect against reverse causation—in which those with prodromal PD are less likely to participate in the race in the first place—researchers did another analysis excluding those with a PD diagnosis within five years of racing. This analysis showed a lower incidence of PD among the skiers, but the difference was weaker and not statistically significant (p = 0.087).
There were also differences in PD incidence for individual age groupings—in those 59 to 69 and 69 to 100—but the overall differences were not statistically significant for those groups.
Researchers found a difference in PD incidence at earlier time points, but this converged with longer follow-up, said Tomas Deierborg, PhD, associate professor of neuroinflammation at Lund University.
Researchers looked particularly closely at skiers who had raced between 1991 and 2000 because they had the longest follow-up. They found that they had consistently lower PD incidence beginning at about five years of follow-up, but the incidence began to converge with that of non-skiers at about 15 years of follow-up. The skiers in this long follow-up group had a 25 percent lower cumulative PD incidence (p=0.035).
They reported differences in PD incidence among both men and women (p=0.027 for men; p=0.04 for women).
This convergence at longer follow-up is consistent with the idea that it is a motor reserve that is largely responsible for the lower PD incidence in those who are more physically fit, Dr. Deierborg said.
“We observe that the cumulative incidence rates between Vasaloppet participants and controls converge with time and age. This allows us to speculate that the negative association between exercise and PD may be more due to a motor reserve delaying PD-diagnosis without actually affecting the underlying pathology,” he said. If exercise conferred a neuroprotective effect, researchers said, this convergence likely would not have been seen.
He drew an analogy to cognitive reserve in dementia, in which education levels have been linked to lower age-adjusted prevalence of Alzheimer's dementia, even though those with more education have no less AD pathology.
“For the individual patient, it further underscores the importance of staying physically active throughout life so that one can better cope with the inevitable frailties and diseases of old age. If a person is physically active, it may be possible to maintain mobility for longer, despite the pathological changes of PD in the brain,” said Tomas Olsson, MD, a PhD student in neuroscience at Lund and the paper's primary author.
“It could also give some information for understanding the pathogenesis of PD. If the motor reserve hypothesis is true, then exercise does not have a major effect on the core pathology of PD, that is dopaminergic cell death, but is related to other compensatory mechanisms.”
Karin Wirdefeldt, MD, PhD, associate professor of neuroepidemiology at Karolinska Institute who has studied exercise and PD incidence, said the study adds to previous evidence that physical activity lowers PD risk and “highlights a very interesting hypothesis about motor reserve.”
But when it comes to the temporal relationship—a larger difference in PD incidence early in follow-up versus convergence in incidence after longer follow-up—Dr. Wirdefeldt advised caution because of a smaller number of events by age group and the lack of statistical significance for the hazard ratios when broken down by age groups.
“The authors did not perform any formal statistical test to test whether the magnitude of association differed by length of follow-up or age group,” she added.
Another caveat is that Vasaloppet participation is being used as a proxy for exercise, but it is only one aspect of physical activity and other types of physical activity might also be important, as shown in other studies, she said.
“As the authors themselves mention, the Vasaloppet participants differ from the general population also regarding other characteristics than physical activity—such as diet and comorbidities—which makes it difficult to disentangle exactly what drives the association,” she said.
Still, she said, the “findings point at possibilities for prevention—for example, in individuals with increased risk such as relatives of PD patients. If future studies can prove that physical activity also slows disease progression, and also define what type and intensity of physical activity is needed, we will have an additional tool in clinical practice, i.e. interventions to increase physical activity.”
Ergun Y. Uc, MD, professor of neurology specializing in movement disorders at the University of Iowa, said that the findings are interesting but echoed Dr. Wirdefeldt's observation that similar findings have been documented before. One study he noted assessed data from the National Institutes of Health AARP Diet and Health Study cohort and found that those who reported higher levels of exercise had lower PD occurrence in later years, with a dose-dependent relationship seen.
“This article makes a similar case, but in a subgroup of people,” Dr. Uc said.
He cautioned that it is not known how much physical activity those in the control group were getting, and noted that skiers had a higher education level, with 76 percent more of those in the skier group having university-level or similar schooling, making it possible that non-skiers were not as able to afford to ski, but could have engaged in other physical activities.
For their part, the researchers said that adjusting for age, sex, and education level did not alter their results.
Dr. Uc said: “The study adds incremental information to the field (and) emphasizes again the importance of being physically active as a potential general mechanism against neurodegenerative disease. It doesn't show superiority of skiing against other physical activities, per se.”
Rodolfo Savica, MD, PhD, associate professor of neurology and epidemiology at Mayo Clinic in Rochester, MN, said that an important point in the study was that it seems that at younger ages there was a reduction in the risk but as the age progressed the incidence difference was less impressive.
A point that he called “crucial” was that there was no clear analysis on additional risk factors that can contribute to PD risk, such as smoking and diet, that would be especially important for athletes in training.
“It is possible that some particular pattern of personality that is driving people to be more active and at the same time less likely to have Parkinson disease,” Dr. Savica added, while at the same time there could be “personality patterns that are making people with Parkinson's disease less willing to participate in the long-distance race, but are more likely to participate in more dangerous activities, as it has been shown in many studies.”
Drs. Deierborg, Olsson, Wierdefeldt, Savica, and Uc had no disclosures.