Women who are exposed regularly to sunlight could reduce their risk of developing rheumatoid arthritis, researchers reported last night.
According to a study that tracked the health of more than 221,000 nurses in the USA, exposure to specifically ultraviolet B (UVB) mitigated against the risk of developing the condition.
The benefits were only evident among older women, however, because, the authors surmise, younger women tend to use more sun block.
The study, published in the latest edition of the Annals of the Rheumatic Diseases, examined two phases of the US Nurses’ Health Study (NHS), the first of which tracked the health of more than 120,000 nurses between 1976, when they were aged between 30 and 55, and 2008.
The second (NHSII) tracked the health of a further 115,500 nurses between 1989, when they were aged between 25 and 42, and 2009.
A UV-B flux, a composite measure of UVB radiation, based on latitude, altitude, and cloud cover, was used to measure exposure. The flux is measured in R-B units: a count of 440 R-B units over 30 minutes, for example, is sufficient to produce slight redness in untanned white skin.
Exposure was then estimated according to the US state in which the participants lived, which ranged from an annual average of 93 in Alaska and Oregon to 196 in Hawaii and Arizona.
Over the study period, 1,314 women developed rheumatoid arthritis and among the nurses in the first study cohort, those with the highest levels of exposure to UVB were 21% less likely to develop rheumatoid arthritis than those with the least.
But there was no such association for UV-B exposure among women in NHSII. This is possibly because these younger women were more wary about the potential hazards of exposing their skin to the sun, suggest the authors.
“Differences in sun protective behaviours (eg, greater use of sun block in younger generations) may explain the disparate results,” they explain.
The findings back up other studies, which have demonstrated a link between geography and the risk of rheumatoid arthritis as well as other autoimmune conditions, such as type 1 diabetes, inflammatory bowel disease, and multiple sclerosis, say the authors.
“Our study adds to the growing evidence that exposure to UV-B light is associated with decreased risk of rheumatoid arthritis,” say the authors. “The mechanisms are not yet understood, but could be mediated by the cutaneous production of vitamin D and attenuated by use of sunscreen or sun avoidant behaviour.”
Arkema E, Hart J et al. Exposure to ultraviolet-B and risk of developing rheumatoid arthritis among women in the Nurses’ Health Study. Ann Rheum Dis. 2013;0:1–6. doi:10.1136/annrheumdis-2012-202302 [abstract]
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