Exposure to ultraviolet-B and risk of developing rheumatoid arthritis among women in the Nurses’ Health Study

Ann Rheum Dis doi:10.1136/annrheumdis-2012-202302
Elizabeth V Arkema1,2,
Jaime E Hart3,4,
Kimberly A Bertrand1,3,
Francine Laden1,3,4,
Francine Grodstein1,3,
Bernard A Rosner3,5,
Elizabeth W Karlson2,
Karen H Costenbader2
1 Department of Epidemiology, Harvard School of Public Health, Boston, Massachusetts, USA
2 Division of Rheumatology, Immunology and Allergy, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, Massachusetts, USA
3 Channing Division of Network Medicine, Department of Medicine, Brigham and Women's Hospital and Harvard Medical School, Boston, Massachusetts, USA
4 Department of Environmental Health, Harvard School of Public Health, Boston, Massachusetts, USA
5 Department of Biostatistics, Harvard School of Public Health, Boston, Massachusetts, USA
Correspondence to Dr Elizabeth V Arkema, Department of Epidemiology, Harvard School of Public Health, 677 Huntington Avenue, 9th Floor, Boston, MA 02115, USA; earkema at post.harvard.edu
Received 4 July 2013; Revised 3 January 2013; Accepted 4 January 2013; Online 4 February 2013

Objective To examine the association between ultraviolet-B (UV-B) light exposure and rheumatoid arthritis (RA) risk among women in two large prospective cohort studies, the Nurses’ Health Study (NHS) and the Nurses’ Health Study II (NHSII).

Methods A total of 106 368 women from NHS, aged 30–55 years in 1976, and 115 561 women from NHSII, aged 25–42 in 1989, were included in the analysis. We identified women with incident RA from the start of each cohort until 2008 (NHS) and 2009 (NHSII). Cumulative average UV-B flux, a composite measure of ambient UV exposure based on latitude, altitude and cloud cover, was estimated according to state of residence and categorised as low, medium or high. Estimates of UV-B at birth and age 15 years were also examined. We used multivariable-adjusted Cox proportional hazards models to estimate HR and 95% CI.

Results 1314 incident RA cases were identified in total. Among NHS participants, higher cumulative average UV-B exposure was associated with decreased RA risk; those in the highest versus lowest category had a 21% decreased RA risk (HR (95% CI); 0.79 (0.66 to 0.94)). UV-B was not associated with RA risk among younger women in NHSII (1.12 (0.87 to 1.44)). Results were similar for UV-B at birth and at age 15.

Conclusions These results suggest that ambient UV-B exposure is associated with a lower RA risk in NHS, but not NHSII. Differences in sun-protective behaviours (eg, greater use of sun block in younger generations) may explain the disparate results.

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