Sunlight Wards Off RA?

Elizabeth V. Arkema, M.D., of Harvard School of Public Health and Brigham and Women’s Hospital in Boston, along with colleagues, is shedding light on the role of sunlight in rheumatoid arthritis (RA). Her study found that regular exposure to sunlight—specifically ultraviolet B (UV-B)—may reduce the risk of developing the disease.

The researchers used data from the U.S. Nurses Health Study (NHS), the first of which has tracked the health of more than 120, 000 nurses since 1976, when they were aged between 30 and 55, until 2008. The second (NHSII), has tracked the health of a further 115, 500 nurses since 1989, when they were aged between 25 and 42, until 2009.

The team went beyond using geography alone to quantify likely levels of UV-B exposure; they utilized a more sensitive assessment, known as UV-B flux, which is a composite measure of UV-B radiation, based on latitude, altitude, and cloud cover. Exposure was then estimated according to the U.S. 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.

Over the study period, 1, 314 women developed RA. Among nurses in the first NHS cohort, higher cumulative exposure to UV-B was associated with a reduced risk of developing the disease. Those with the highest levels of exposure were 21% less likely to develop rheumatoid arthritis than those with the least, the analysis showed.

This supports 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. But the authors of this study found no such association for UVB exposure among women in NHSII, who were younger than those in the first NHS, and might have been more savvy about the potential hazards of acquiring a tan.

“Differences in sun protective behaviors (e.g., greater used of sun block in younger generations) may explain the disparate results, ” stated the authors in the February 4, 2013 news release.

Dr. Arkema told OTW, “We were most surprised to learn that a higher average UV-B exposure was associated with a 21% decreased risk of rheumatoid arthritis in an older cohort of women, but not in the younger cohort. This is the first study using area-level UV-B measurements to examine this association, and these results may be considered preliminary. Future studies should examine dose and time of exposure and take into account use of sunscreen and time spent outdoors.”