Determining the effect of sun exposure and physical activity on the incidence of primary progressive multiple sclerosis

Background: The onset of primary progressive multiple sclerosis (PPMS) can be triggered by sun exposure and physical activity. The present study aimed to determine the risk of sun exposure and physical activity in PPMS development. Methods: The case-control study recruited PPMS cases and healthy controls from the general population from September 2019 to September 2020 in Tehran, Iran. The selection of sex-matched controls from the same source population of cases was performed. Sun exposure was assessed in terms of exposure time in two age groups of 13-19 years and over 20 years in winter and summer. Results: This study examined 146 PPMS cases and 294 controls. The sun exposure in summer in the age group 13-19 was negatively associated with the incidence of the disease (OR=0.68 (CI=0.57-0.82)). This relationship was also observed in the groups of men (OR=0.55 (CI=0.40-0.74)) and women (OR=0.70 (CI=0.53-0.91)). Exposure to sunlight in winter in the age group 13-19 was negatively associated with the incidence of the disease (OR=0.28 (CI=0.19-0.42)). In the group over 20 years, sun exposure was negatively associated with disease incidence in summer (OR=0.49 (CI=0.36-0.66)). Exposure to sunlight on average in winter in the age group over 20 years was negatively associated with the incidence of the disease (OR=0.14 (CI=0.07-0.28)) which in the male group OR=0.12 (0.04-0.32)) and women (OR=0.06 (CI=0.02-0.21)) were seen. Regarding physical activity, only physical activity above 4000 MET per week in the group of women was negatively associated with the incidence of the disease (OR=5.30 (CI=1.05-26.59), but in other groups, this negative relationship was not observed. Conclusion: Exposure to sunlight in winter and summer had a negative relationship with the incidence of the disease. Physical activity was only negatively associated with the incidence of the disease in very high doses and women.