The positive outcomes of UVB exposure was demonstrated in mostly older women, possibly because younger females are educated on the dangers of sunlight and protect themselves with sunscreen, the authors suggest.
The authors drew their conclusions from two phases of the U.S. Nurses' Health Study (NHS). The first phase followed the health of over 120,000 nurses starting in 1976, when they were between the ages of 30 and 55, until 2008.
The second phase (NHSII) followed the health of an additional 115,500 nurses starting in 1989, between the ages of 25 and 42, until the year 2009.
Instead of depending on geography to quantify likely rates of UVB exposure, the investigators chose to use a more careful examination, something called UV-B flux - a composite calculation of UVB radiation consisting of cloud cover, latitude, and altitude.
UV-B flux is measured in R-B units: a count of 440 R-B units of more than 30 minutes, for instance, is enough to cause a subtle sunburn on white skin.
Development of RA Differed By Age & GeographyAccording to the U.S. state of residence, exposure was estimated and ranged from 93 in Oregon and Alaska, to 196 in Arizona and Hawaii, on average. UV exposures at birth and at the age of 15 were also estimated and included in the study.
Throughout the duration of the study, 1314 women developed rheumatoid arthritis. Among nurses in the NHS group, higher cumulative exposure to UVB was linked with a decreased risk of developing the disease.
Those nurses with the most elevated rates of exposure were 21 percent less inclined to develop rheumatoid arthritis than those with the least.
Previous research has established an association between geography and the risk of rheumatoid arthritis, in addition to other autoimmune conditions such as:women living in northeastern states in the U.S. are more likely to develop rheumatoid arthritis. The authors found a link between vitamin D deficiency and rheumatoid arthritis.
Therefore, women living in states such as Vermont, Maine, and New Hampshire were seen to have higher rates of rheumatoid arthritis because they were exposed to less sunlight.
During NHSII, no link for UV-B exposure was seen. These particular women were younger than those in the first NHS, and therefore might have been more aware of the possible dangers associated with tanning.
The authors explained, "Differences in sun protective behaviours (eg greater use of sun block in younger generations) may explain the disparate results."
They also note that it is not yet understood at what stage of life the protective effect of UV-B happens. Most study participants did not change houses between birth and adolescence, and there was no difference seen in the outcomes for these points.
The authors concluded:
"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 cutaneous production of vitamin D and attenuated by use of sunscreen or sun avoidant behavior."
In contrast to the current study, one completed earlier this month suggested that vitamin D did not help improve arthritis of the knee. Adults who had osteoarthritis in their knees and took vitamin D supplements, saw no improvement in cartilage loss or pain relief.
Written by Kelly Fitzgerald