The research, published in the Journal of Neurology by a team at Karolinska Institutet, drew on the Environmental Investigation in Multiple Sclerosis, or EIMS, a population-based case-control study that has been running in Sweden since 2005. The investigators analyzed data from 3,008 people newly diagnosed with multiple sclerosis and 5,773 matched controls drawn from the national population register. Crucially, the team restricted their analysis to participants whose disease onset, or corresponding reference year among controls, occurred within five years before study enrollment, a design choice intended to ensure that reported exercise habits genuinely preceded the disease rather than being shaped by its early, invisible stages.

Participants reported their leisure-time physical activity five years before inclusion, categorized into four levels ranging from largely sedentary living to sweat-inducing exercise at least three times per week. The lowest level corresponded to less than two hours per week of light activity such as walking or cycling, while the highest required at least thirty minutes of vigorous exercise three or more times weekly. Sun exposure over the same period was captured through a composite index combining sunbathing in Sweden, travel to sunnier countries, and use of sunbeds, with scores summed into a scale ranging from three to twelve.

At first glance, the results seemed to confirm what many earlier studies had suggested. In a model adjusting only for age, sex, and residential area, the most active participants had a 22 percent lower risk of multiple sclerosis than the least active, with an odds ratio of 0.78. A clear downward trend emerged across the activity levels, with each step up the activity ladder associated with roughly 8 percent lower odds of disease. For anyone who has followed the exercise-prevention literature, this is exactly the pattern one would expect to see.

But the picture changed dramatically once the researchers accounted for the full web of lifestyle and environmental factors that travel alongside physical activity. After adjusting for ancestry, educational level, smoking status, alcohol consumption, body mass index at age twenty, and sun exposure, the odds ratio for high versus low activity drifted to 0.94, a figure statistically indistinguishable from no effect at all. The per-level trend estimate similarly weakened from 0.92 to 0.97. In other words, much of what looked like protection from exercise appeared to be protection from something else that active people tend to have more of, most notably sunshine.

The stratified analyses revealed the study’s most intriguing finding. Among participants with low sun exposure, defined as a composite index score below six, high physical activity remained associated with a 24 percent reduction in multiple sclerosis risk, with an adjusted odds ratio of 0.76 and a statistically significant inverse trend across activity levels. Among those with high sun exposure, no such association existed; the odds ratio was a flat 0.97. Formal statistical testing confirmed that the interaction between physical activity and sun exposure was unlikely to be a fluke, with a p-value of 0.01, and model-predicted probabilities visualized the gradient clearly: the protective slope of exercise was steepest in the sun-deprived and vanished as sun exposure rose.

Skeptics might argue that physical activity is simply a proxy for time spent outdoors, and that sunshine, not exercise, does the real work. To test this, the team conducted a sensitivity analysis in a subgroup of participants recruited after 2015 who had provided detailed information on time spent outdoors during summer leisure and at work. Remarkably, the inverse association between physical activity and multiple sclerosis risk persisted essentially unchanged after additional adjustment for outdoor time, with the odds ratio per activity level moving only from 0.87 to 0.88. This suggests that exercise itself, or something closely tied to it beyond mere outdoor exposure, may genuinely matter for people whose sun exposure is otherwise low.

What biological mechanisms could underlie such a pattern? Regular exercise is known to exert systemic anti-inflammatory effects, influencing cytokine profiles, metabolic regulation, adiposity-related inflammation, and the signaling of myokines, molecules released by contracting muscle. These pathways intersect with the immune processes implicated in the autoimmune attack on the central nervous system that defines multiple sclerosis. Sun exposure, meanwhile, is the principal driver of vitamin D production and has well-documented immunomodulatory effects, and low sun exposure has previously been shown in the same Swedish cohort to raise multiple sclerosis risk both directly and indirectly. One plausible interpretation is that sunlight and exercise act on overlapping biological pathways, so that the marginal benefit of exercise becomes visible only when the sun-related pathway is underactive.

The authors are careful to emphasize the caveats. All exposures were self-reported retrospectively, opening the door to recall bias, and the interval between the reported activity period and clinical onset varied among participants. Reverse causation remains a serious concern, because multiple sclerosis is increasingly recognized to have a prodromal phase, sometimes years before diagnosis, in which fatigue, reduced exercise tolerance, pain, and subtle neurological changes may quietly reshape a person’s activity habits. The broad four-level activity categories also cannot capture every nuance of duration, intensity, and seasonal variation, and the sun exposure index is an imperfect stand-in for total ultraviolet radiation dose. The sun-exposure interaction, the team notes, should be considered exploratory until replicated.

Still, the study’s strengths are considerable: a population-based design, nearly 3,000 incident cases, high participation rates, and unusually comprehensive confounder data, including a check against residual confounding by population structure using genetic principal components. The takeaway for the public is not that exercise is useless against multiple sclerosis, but that its potential preventive role is context-dependent, most apparent in people with limited sun exposure and robust even after accounting for time outdoors. As the authors conclude, physical activity should be understood in relation to sun exposure, outdoor behavior, and the broader lifestyle patterns in which it is embedded, a reminder that in disease-prevention research, no single behavior operates in isolation.

Subject of Research: The association between leisure-time physical activity, sun exposure, and risk of developing multiple sclerosis

Article Title: Leisure-time physical activity, sun exposure, and multiple sclerosis risk: a population-based case–control study

Article References: Guo, Q., Olsson, T., Alfredsson, L., & Hedström, A. K. (2026). Leisure-time physical activity, sun exposure, and multiple sclerosis risk: a population-based case–control study. Journal of Neurology, 273(10), Article 623. https://doi.org/10.1007/s00415-026-14170-9

Image Credits: AI Generated

DOI: 10.1007/s00415-026-14170-9

Keywords: multiple sclerosis, physical activity, sun exposure, case-control study, epidemiology, autoimmune disease, vitamin D, outdoor activity, risk factors, Karolinska Institutet, disease prevention, Journal of Neurology