Regular exposure to sunlight, specifically ultraviolet B (UV-B), may reduce the risk of developing rheumatoid arthritis. These were the findings from a long-term study of more than 200,000 women, published in the Annals of the Rheumatic Diseases. The report speculated that vitamin D, which the body produces when exposed to sunlight, might protect women from arthritis.
The study estimated the levels of UV-B exposure using a detailed assessment based on latitude, altitude, and cloud cover, categorised as low, medium or high. It is measured in R-B units: for instance a count of 440 R-B units over 30 minutes is sufficient to produce slight redness in untanned white skin. Exposure was then estimated according to the US state of residence, and ranged from an annual average of 93 in Alaska and Oregon to 196 in Hawaii and Arizona. Likely estimates of UV exposure at birth and by the age of 15 were also included.
Those involved in the study were female nurses from two age groups: the first aged 30 to 55 were monitored from 1976 onwards; and the second group were aged 25 to 42 monitored from 1989. The UV-B effect was clearly evident in the study of more than 120,000 nurses in the older age group. Those with the highest levels of exposure were 21% less likely to develop rheumatoid arthritis than those with the lowest levels.
However no association could be found for UV-B exposure among the younger group of nurses. The researchers suggest that this group of women might have been more savvy about the potential hazards of acquiring a tan, and remarked that "differences in sun protective behaviours, for example greater use of sun block in younger generations, may explain the disparate results. Our study adds to the growing evidence that exposure to UV-B light is associated with decreased risk of rheumatoid arthritis. The mechanisms are not yet understood, but could be mediated by the production of vitamin D and attenuated by use of sunscreen or sun avoidant behaviour."
This backs up the findings of other studies, showing a link between geography and the risk of rheumatoid arthritis as well as other autoimmune conditions, including type 1 diabetes, inflammatory bowel disease, and multiple sclerosis.