Childhood Sun Exposure Linked to Reduced MS Risk

Investigators at the Keck School of Medicine at the University of Southern California, in Los Angeles, compared sun-exposure levels in 79 pairs of identical twins where only 1 twin had MS. They found the twin with the disease reported less childhood sun exposure than the twin without MS.

“Sun exposure appears to have a protective effect against MS. Exposure to ultraviolet rays may induce protection against MS by alternative mechanisms, either directly, by altering the cellular immune response, or indirectly, by producing immunoactive vitamin D,” said study author Thomas M. Mack, MD, in a statement from the American Academy of Neurology.

The study is published in the July 24 issue of Neurology.

Significant Associations

Data for the study were derived from the International Twin Study, a large registry of North American twins diagnosed with any of several chronic diseases, including MS.

As part of a comprehensive 60-page questionnaire exploring possible etiologic factors, the twins ranked their sun exposure before 1993 in relation to each of 9 childhood sun-exposure activities. Each twin was asked whether he/she or the other twin spent more time outdoors during hot days, cold days, spring, summer, winter, and fall and which twin spent more time suntanning, going to the beach, and playing team sports as a child.

For the purposes of the study, only twins who were discordant for disease and sun exposure were included in the analysis. Of the 79 pairs of twins, most were female who were born in the northern United States and of North European ancestory.

According to the study, although the number of discordant pairs was limited, significant associations were seen for all 9 sun exposures addressed in the questionnaire and ranged from an odds ratio of 0.25 to 0.57.

Intriguing Finding

Overall, affected twins had less sun exposure than their unaffected twin, and the authors report they could find no factor, including childhood infection, incidence of infectious mononucleosis, personal smoking, diet, and age at menarche (for female twins), that might confound the observed protective effect of past sun exposure on MS risk.

The authors note that a similarly strong protective effect of childhood sun exposure was recently reported in a 2003 case-control study of prevalent MS cases conducted in Tasmania (van der Mei IA et al. BMJ. 2003;327:316), where children averaging 2 or more hours of sun exposure per week had a nearly 60% reduction in MS risk compared with their counterparts who received less than 2 hours per week of sun exposure.

The current analysis also found the protective effect of sun exposure was seen only among female twin pairs. This finding is "intriguing" and may represent a vitamin D–mediated sex-specific immunomodulation. However, the authors note that given the small number of male twins involved in the study, this finding should be interpreted with caution.

  “Our findings note the importance of sun exposure among people with identical genetic risk for MS. High priority should be given to research into how sun exposure reduces MS risk if we are to unravel the mystery of what causes MS,” said Dr. Mack.

Neurology. 2007;69:381-388.