Increased ultraviolet (UV)-B exposure reduces the risk for rheumatoid arthritis (RA) in women, US researchers suggest, but the drive to protect against skin cancer in recent years may have negated this benefit.
The investigators identified a change in the relationship between UV-B exposure and RA from the 1976 Nurses' Health Study (NHS) of 106,368 women aged 30-55 years to the 1989 NHSII of 115,561 women aged 25-42 years.
Specifically, NHS results showed that women in the highest tertile for exposure over long-term follow-up (>117 Robertson-Berger units×10-4 [RB], median 164 RB) had a 21% lower risk for RA than women in the lowest tertile for exposure (=109 RB, median 104 RB), after adjusting for confounding factors including age, race, smoking, parity, menopause, and income. The risk was 16% lower for women in the middle tertile (median 113 RB).
By contrast, UV-B exposure had no significant impact on the risk for RA among participants of NHSII, report Elizabeth Arkema (Harvard School of Public Health, Boston, Massachusetts) and co-authors.
"Our study adds to the growing evidence that exposure to UV-B light is associated with decreased risk of RA," the researchers write.
"The mechanisms are not yet understood, but could be mediated by cutaneous production of vitamin D and attenuated by use of sunscreen or sun avoidant behavior," they say, noting that women in the later NHSII study are more likely to have followed advice to protect against the sun than NHS participants.
There was no significant difference in the NHS and NHSII trends when UV-B exposure was calculated for each woman at time of birth or at age 15 years, and the researchers note that the majority of women did not move out of their state between birth and age 30 years. "Therefore, it is difficult to determine whether the effect we observed is due to exposure during childhood or during adulthood," Arkema et al comment in the Annals of Rheumatic Diseases.
In all, 933 NHS and 381 NHSII participants were diagnosed with RA during follow-up. UV-B exposure was determined from latitude, altitude, and cloud cover of each participant's state of residence. The 20-year absolute risk for RA for women in the highest and lowest tertiles for UV-B exposure was 0.7% and 1.2%, respectively, compared with 0.5% and 0.5% for the NHSII study.
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