Childhood Sun Exposure Linked to Reduced MS Risk

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http://www.medscape.com/viewarticle/560747




Childhood Sun Exposure Linked to Reduced MS Risk CME/CE
News Author: Caroline Cassels
CME Author: Désirée Lie, MD, MSEd
Disclosures

Release Date: August 1, 2007; Valid for credit through August 1, 2008 Credits Available

Physicians - maximum of 0.25 AMA PRA Category 1 Credit(s)™ for physicians;
Family Physicians - up to 0.25 AAFP Prescribed credit(s) for physicians;
Nurses - 0.25 nursing contact hours (None of these credits is in the area of pharmacology)



August 1, 2007 — A new study of monozygotic twins suggests sun exposure in childhood may reduce the risk for multiple sclerosis (MS) by almost 60%.

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.

Clinical Context
According to the authors of the current study, MS is an autoimmune disease of the central nervous system with substantial concordance of 20% among monozygotic twins, and environmental determination is also suggested by the risk according to greater latitude and the effect of migration on risk. According to the authors, the genetic effect of twinning diminishes as latitude decreases, and sun exposure may be a mechanism of protection against risk, with UV exposure exerting an immunosuppressive effect by reducing natural killer cell activity and activating vitamin D, suppressing the production of cytokines associated with MS activity. In 1 study, according to the authors, Tasmanians with 2 to 3 hours of youthful sun exposure per week on average were found to have nearly 60% reduction in MS risk compared with those without sun exposure.

This is a study within the International Twin Study of North American twins who were diagnosed with any 1 of several chronic diseases including MS. For this study, 1 member of each twin monozygotic pair had symptomatic MS, and the relative sun exposure of the twins in childhood was determined retrospectively to asses the association between youthful sun exposure and MS risk.

Study Highlights
Twins were recruited via annual advertisement in North America to capture pairs in which at least 1 member had physician-diagnosed MS.
416 monozygotic twin pairs were identified with 1 member diagnosed with MS, and discordancy for MS was verified by contact with the unaffected twin.
The ratio of female to male pairs was 3:1.
For 94.6% of affected twins, diagnosis occurred between age 15 and 50 years.
Diagnosis was self-reported, and 2% were of cases were found to be not MS by early medical record review.
The average interval between diagnosis and study entry was 10 years, and after entry subjects were followed up for an average of 13 years.
Each twin was asked about 9 categories of sun exposure and to specify whether he/she or the co-twin spent more time outdoors in childhood "during hot days," "during cold days," "during summer," "spring," "winter," and "fall" and which twin spent more time "suntanning," "going to the beach," and "participating in team sports."
Measures of sun exposure were comparative rather than absolute, as substantial recall bias was expected for absolute recall.
Only pairs who were in agreement about the twin with greater sun exposure were included for analysis.
A sun exposure index (SI) was calculated for each twin member, which represented the total number of responses for which the twin's exposure exceeded that of the co-twin.
The pairwise difference in SI was then calculated by subtracting the SI value of the affected twin from that of the unaffected co-twin, thus generating a range of SI differences from -9 to 9.
Twins were categorized by place of birth into those born in Canada and states north of latitude 41º north, and those with at least 1 grandparent of Scandinavian or Celtic origin were considered of European ancestry.
Twins diagnosed with MS before the age of 29.3 years were considered to have been diagnosed early.
To reduce respondent bias, only responses prior to 1993 were analyzed, as sun exposure was not considered a factor in the etiology of MS at that time.
Of 193 disease-discordant pairs with both twins completing the questionnaire, 114 pairs considered themselves identically sun-exposed with respect to the 9 exposures.
79 pairs were discordant in at least 1 of the 9 exposures and were included in analysis.
Most twins who were analyzed were female (83%), born in the Northern states (43%), and of European ancestry (16%).
47% were diagnosed before age 30 years.
Most twins had kept in contact at least once per month.
The risk for MS decreased with greater sun exposure, as determined by each of the 9 categories.
Significant associations were seen for spending more hours outdoors during spring (odds ratio [OR], 0.25), during hot days (OR, 0.40), while suntanning (OR, 0.40) and while at the beach (OR 0.42).
The protective effect of sun exposure was seen among female but not male twin pairs.
Other exposures were independent of the MS diagnosis including those considered etiologically suspect (such as fish consumption) at the time of the study.
Overall, affected twins had less sun exposure than unaffected co-twins.
Increased risk for MS was associated with a lower SI value.
A 1-unit increase in the SI difference was associated with a 25% reduction in risk for MS (P = .004), and this linear trend persisted after adjustment for birth place, age at diagnosis, sex, and North European ancestry.
The association was not affected by genetic susceptibility, childhood infection, incidence of infectious mononucleosis, personal smoking, diet, and age at menarche.
Amount of twin contact did not affect the association.
Pearls for Practice
Increased childhood exposure to the sun, as determined by exposure to sun-related activities, is associated with lower risk for subsequent MS.
The reduction in risk for MS associated with childhood sun exposure is dose dependent.



Medscape Medical News 2007. ©2007 Medscape



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[Bill]

For those of you in regulated states where they have placed restrictions on people under XX age for tanning, have you forwarded this to your representatives?