Mexico: no association of sunlight exposure and MS

Down Mexico-way sun exposure is complex trait and difficult to interpret in relation to MS. #MSBlog #MSResearch

“This smallish study in Mexico found no particular association between sun exposure, or lack of sun exposure, and MS. It is a pity they didn’t use skin damage as  the metric for assessing sun exposure. A scoring technique using silicone skin casts has been used extensively in Australia. Could sun exposure be more important in childhood  than adolescence? A Norwegian study has shown this in the past. What is clear that once someone develops MS they seem to withdraw from going out. Why? Could it be because of  heat sensitivity? Lack of mobility? Economic factors? Unsightly injection site reactions from DMTs? Low mood and social isolation? I would be interested in finding out.”

“Does this study tell us something about MS? Yes, sun exposure and vitamin D are only one factor underlying the parthenogenesis of MS. Other factors may be more, or less important. Working out how these interact with each is other in the causal chain is critical. I am hoping that there is one pivotal or critical factor that if removed breaks the chain and prevents MS. The other factors apart from a simple outdoor activity metric need to be considered, for example covering-up, sun block usage, atmospheric pollution and cloud cover and skin type (dark-skinned people need more sun to make vD than light-skinned people). This study needs to be confirmed using objective measure of sun exposure. Is it the sun that is more important than vD? Some researchers believe that vD supplements are not a replacement for sun exposure. There is evidence that UV light affects the immune system by targeting lymphocytes that traffic through blood vessels in the skin. More on this next time.”

Is this an outdoor activity?

Epub: Espinosa-Ramírez et al. Sunlight exposure and multiple sclerosis in a tropical country. Neurol Res. 2014 Jan 12:1743132813Y0000000307.

Objective: We analysed past and current sun exposure in MSers as compared with matched controls in Mexico, a country with tropical climate.

Methods: In a case-controlled study that include 83 MS patients and 166 matched controls, we inquired about sunlight exposure on two different periods: during adolescence and during the immediate past 5 years. Indicators were: exposure on quotidian and weekend outdoor activities with direct sunlight contact as expressed on frequency by mean number of days, daytime (morning, noon, afternoon), number of hours, visits to sunny places, and use of sunblocking agents. Additional elements were socioeconomic status, skin colour, and antecedent of varicella infection during childhood.

Results: MS patients showed a larger proportion of white skin. MS patients had more sunlight exposure during adolescence (80% versus 60%, P = 0·002); this tendency prevailed on current indicators (46% versus 30%, P = 0·02). However, current exposure on weekends (10% versus 22%, P = 0·02) and visits to the beach (64% versus 98%, P = 0·002) were lower in MS than in controls. 

Discussion: Mexico gets more sunlight through the year than areas with high incidence of MS; nevertheless, its prevalence has greatly increased over the last decades, making it a relevant emerging disease. Our results indicate that in a tropical country, there is no association between sunlight exposure and the risk to develop MS, given the immunological effects of sunlight exposure either through UV radiation or vitamin D metabolism.

About the author

Prof G

Professor of Neurology, Barts & The London. MS & Preventive Neurology thinker, blogger, runner, vegetable gardener, husband, father, cook and wine & food lover.

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