Sunlight may lower women’s risk of rheumatoid arthritis
A study has found that women who are exposed to lots of sunlight are less likely to have rheumatoid arthritis than women who live in less sunny areas. Though this is interesting research, we have to be cautious about these findings.
BMJ Group News
What do we know already?
If you have rheumatoid arthritis, your immune system, which normally fights infection, attacks the lining of your joints. This makes your joints swollen, stiff, and painful.
Previous studies have shown that people living in parts of the world where there is little ultraviolet (UV) light from the sun have a higher risk of rheumatoid arthritis than people who live in sunnier regions. Doctors think that UV light can affect the immune system, either directly, or by causing the body to produce larger amounts of vitamin D.
At the moment, this is just a theory. We still don’t fully understand the relationship between UV light, vitamin D, and the immune system, and we don’t know how this might affect people’s risk of rheumatoid arthritis. There have been very few studies looking at this.
In the largest study so far, researchers studied two groups of women: one group of 106,000 aged 30 to 55 in 1976, and a second group of 116,000 aged 25 to 42 in 1989. Over the next 20 years the researchers estimated how much exposure to UV light these women had, based on the US state they lived in, and recorded whether the women developed rheumatoid arthritis. From this, they worked out if there was a link between the amount of sunlight exposure the women had, and their risk of rheumatoid arthritis.
What does the new study say?
Among women in the older group, the more sunlight women were exposed to, the lower their risk of rheumatoid arthritis. Women who had the most exposure to sunlight had a 21 percent lower risk of getting rheumatoid arthritis than women exposed to the least sunlight.
In the older group of women, around 7 in 1,000 women with the most sunlight exposure developed rheumatoid arthritis over the 20 years of the study, compared with 12 in 1,000 with the least sunlight exposure.
But in the younger group, women exposed to the most sunlight were no less likely to get rheumatoid arthritis than women with the least sunlight exposure. In that group, there was no difference in the number of women who got rheumatoid arthritis over the course of the study.
The researchers think that sunlight exposure didn’t have the same effect on younger women because of a difference in attitudes and knowledge about sun exposure. This may have meant that younger women were more likely to use sunscreen or avoid prolonged sunbathing.
How reliable is the research?
This is a large study, and the first to follow women over time to see if their amount of sun exposure affected their risk of rheumatoid arthritis.
The researchers used a measure, UV-B flux, which took into account where women lived, the altitude, latitude, and average daily cloud cover. This is a reasonably accurate measure of sunlight exposure, but it doesn’t take into account things like how much time women normally spent outdoors, whether they travelled to sunnier places, or whether they changed how much sunscreen they used. It’s also possible that some other factor, unrelated to sunlight exposure, could have affected the women’s behaviour or environment, and that this affected their risk of rheumatoid arthritis.
The results of course only apply to women. Also, the researchers restricted the study to Caucasian women only, so we don’t know if these findings apply to women from other races or ethnic backgrounds.