Jan 04, 2023
5 mins read
Over the holiday break, I had time to kick back and watch some movies and TV shows. As usual, the workshop gathered to watch Elf and The Santa Clause. Then I downloaded season 1 of Yellowstone to see what everyone has been talking about. I also watched Still Alice, a story about a professor with early onset Alzheimer's disease. In one scene, a doctor explains that cognitive decline can be rapid once the condition is diagnosed, especially in people with highly developed intellect. He explains that high cognitive ability makes it possible to compensate for cognitive decline and mask early symptoms. Eventually, the ability to compensate is outstretched by the neuro-degenerative effects of the disease, and a rapid decline in function ensues.
The movie reminded me of a paper I read before the break. It dealt with Parkinson’s disease and had a similar message, suggesting that physical activity delays the clinical manifestations of Parkinson’s disease. Today’s issue of Shop Talk discusses that paper and serves as an entrée into upcoming issues that also deal with Parkinson’s disease.
For those approaching this topic for the first time, here is a little background.
Parkinson’s disease is bad. It’s a progressive neurological disorder with no known cure. It occurs when cells in the brain stop making dopamine, a chemical neurotransmitter important for movement, motivation, and pleasure. As dopamine levels decline, people with Parkinson’s disease develop tremors and muscular rigidity. They move more slowly and less accurately than normal and may experience pain, fatigue, sleep disturbances, urogenital dysfunction, and memory problems.
Physical activity reduces the risk of Parkinson’s disease, but we don’t know why. It could be due to reverse causation, a phenomenon wherein people with Parkinson’s disease reduce their activity prior to diagnosis. Another possibility is neuroprotection wherein exercise protects dopamine-producing cells from damage and death. Others have proposed a motor reserve hypothesis that argues that people who are physically fit can sustain a larger amount of brain damage before the onset of overt symptoms.
In their 2020 paper published in Journal of Parkinson’s Disease, Tomas Olsson and his colleagues report evidence in favor of the motor reserve hypothesis. Their work was done in Sweden and has a uniquely Swedish vibe. The study involved nearly 200,000 cross country skiers who participated in the Vasaloppet – the largest long distance ski race in the world. Race participation served as a proxy for physical activity because the authors reasoned that people participating in a 30, 45, or 90 km ski race were physically well-trained. (I tend to agree.) Using the Swedish National Patient Registry, they compared the skiers to a sample of nearly 200,000 non-skiers from the general population. Both groups were followed for 20 years. The authors tallied the number of people in each group who were diagnosed with Parkinson’s disease and noted the age at which the diagnosis was made.
Until the age of 49, skiers were significantly less likely than non-skiers to be diagnosed with Parkinson’s disease. However, once the skiers hit 50, they were as likely as non-skiers to have a Parkinson’s diagnosis. These observations are consistent with a motor reserve hypothesis, wherein individuals who exercise experience as much brain pathology as the general population but take longer to manifest clinical signs of disease.
What’s the upshot? To ski or not to ski? I say ski, and the Swedes probably agree. It won’t protect your brain cells, but you’ll feel better for longer. And what else is there to do in the 97 days of January in the Northern hemisphere? Oh right… TV and movies.
That’s all for now. We’ll be back soon with more on Parkinson’s disease. Meanwhile, you can dig into this topic by checking out the resources below. If you like what we’re doing, please forward this issue to a friend and check out our archives. If you have a moment, tell us what you think. We love hearing from you.
1. Read the whole article on skiing and Parkinson’s disease.
Olsson TT, Svensson M, Hållmarker U, James S, Deierborg T. Delayed Clinical Manifestation of Parkinson's Disease Among Physically Active: Do Participants in a Long-Distance Ski Race Have a Motor Reserve? J Parkinsons Dis. 2020;10(1):267-274. doi: 10.3233/JPD-191762. Erratum in: J Parkinsons Dis. 2021;11(1):373. PMID: 31609700; PMCID: PMC7029367.
2. Here are some links to Elf, The Santa Clause, Still Alice, and Yellowstone.
3. For an introduction to Parkinson’s disease, check out Parkinson’s 101 at the Michael J. Fox Foundation.
4. For the evidence nerds, here are some thoughts on study design.
The Olsson article is a self-described prospective observational study. Prospective means that data were collected after the study was designed. Observational means that the researchers did not intervene; they just watched.
We might also categorize the study as a non-inception cohort design where individuals with and without a risk factor of interest (i.e., physical activity) were identified and followed to see who developed the outcome of interest (i.e., Parkinson’s disease). It might seem strange to call physical activity a risk factor, but that’s the term we use to describe any prognostic indicator regardless of whether it is thought to be detrimental or beneficial.
Observational studies have several strengths. Because researchers need not intervene, it is feasible to examine large samples like the 400,000 Swedes described here. We need large samples to detect small effects and generalize results beyond a sample. Large samples are less feasible when we choose to intervene. Imagine if Olsson’s team set out to train 200,000 participants for a ski race, as one might do in a randomized controlled trial. That study would be labor intensive, costly, and down-right onerous.
But the feasibility of an observational study comes at a cost, usually in the form of experimental control. Olsson’s team could not control for fitness level. And we don’t know what the skiers and non-skiers did over the 20-year follow-up. For example, skiers might have become sedentary after the race, and non-skiers might have been involved in other, equally rigorous forms of exercise. Or maybe not. There’s no way to know.
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