TUESDAY, Feb. 5 (HealthDay News) -- Older women who've had regular exposure to sunlight may be less likely to develop rheumatoid arthritis, new findings indicate.

This beneficial effect -- which is believed to be due to ultraviolet B (UV-B) in sunlight -- was only evident in older women. This may be because younger women are more aware of the skin-related hazards of sunlight and take more steps to limit their exposure, the researchers said.

For the study, the investigators looked at about 235,000 participants who took part in two phases of the U.S. Nurses' Health Study. The first phase began in 1976 with nurses aged 30 to 55 and continued until 2008. The second phase began in 1989 with nurses aged 25 to 42 and continued until 2009.

By the end of the two phases, 1,314 of the women had developed rheumatoid arthritis, according to the study published in the current online edition of the journal Annals of the Rheumatic Diseases.

The nurses' UV-B exposure was estimated based on data from the states where they lived while taking part in the study. Likely estimates of their UV-B exposure at birth and by age 15 were also included.

Among women in the first phase of the Nurses' Health Study, those with the highest estimated levels of UV-B exposure were 21 percent less likely to develop rheumatoid arthritis than those with the lowest levels.

However, no such association between UV-B exposure and rheumatoid arthritis risk was seen among women in the second phase. These women were younger than those in the first phase and may have been more aware about the dangers of too much sun exposure and avoided it, the study authors suggested.

"Our study adds to the growing evidence that exposure to UV-B light is associated with decreased risk of rheumatoid arthritis," concluded Dr. Elizabeth Arkema, of the department of epidemiology at Harvard School of Public Health, and colleagues.

But even though the researchers found an association between greater estimated exposure to UV-B light and lower risk of rheumatoid arthritis in the women in the first phase of the Nurses' Health Study, the finding did not prove that there was a cause-and-effect relationship.

It's not known how UV-B exposure might reduce the risk of rheumatoid arthritis, but it could be due to the skin's production of vitamin D in response to sunlight, the study authors suggested in a journal news release.

More information

The American Academy of Family Physicians has more about rheumatoid arthritis.