If you get casual sun exposure you may be decreasing your risk of rheumatoid arthritis (RA), according to research published in Annals of the Rheumatic Diseases.
In 2005, roughly 1.5 million people over the age of 18 had RA, according to the Center for Disease Control and Prevention. RA is a chronic inflammatory disease that causes inflammation of the joints and surrounding tissues, typically in the hands and feet. Although the cause of RA is unknown, it is considered to be an autoimmune disease.
The Vitamin D Council published a brief news story covering the study, but I would like to add some thoughts to the summary.
Elizabeth Arkema, PhD, and colleagues analyzed data from the Nurses’ Health Study (NHS), a prospective group of female nurses aged 30-55 years which began in 1976, and the Nurses’ Health Study II (NHSII), which included women aged 25-42 starting in 1989. The participants completed questionnaires every 2 years assessing diseases, lifestyle and health practices. The researchers analyzed UVB exposure from UVB flux data available by state across the US. Women were monitored in the study until 2008 for the NHS group and 2009 for NHSII.
A total of 221,900 women were included in the study, of which 1,314 had RA. The authors found the following:
- Among women in NHS, higher average UVB exposure was associated with a significant decrease in RA risk.
- Those in the highest UVB category had a 21% decreased risk of RA when compared with the lowest UVB exposure (p=0.005).
- There was no association between UVB exposure and risk of RA in NHSII.
Why was there an association in the NHS group and not the NHSII group, you ask? Well, UVB could have a protective effect for older-onset RA but not younger-onset RA. The difference could also be due to variation in sun exposure habits. “…the later birth cohort of NHSII participants (born between 1946 and 1964) were more likely aware of the dangers of sun exposure and, perhaps, had more sun-protective behavior making residential UV-B not as good a proxy for actual sun exposure in NHSII.”
Interestingly, a separate analysis found that UVB at the time of blood draw was a significant predictor of vitamin D status among women in NHS but not among those in NHSII.
The authors also point out that a majority of participants lived in the same state at birth and at age 30. Consequently, the researchers cannot determine whether the observed association is due to UV exposure during childhood or during adulthood.
The study confirms past research on the association between geographic location and RA risk, although this is the first study to use UVB data by state to measure UVB exposure. The authors call for future research to further investigate a vitamin D dose response relationship to RA. They also want to look into whether UVB exposure during childhood or adulthood is associated with a decreased risk of RA.
Do you know anyone with RA who lives at northern latitudes?
Arkema EV, Hart JE, Bertrand KA, Laden F, Grodstein F, Rosner BA, Karlson EW, Costenbader KH. Exposure to ultraviolet-B and risk of developing rheumatoid arthritis among women in the Nurses’ Health Study. Ann Rheum Dis. 2013.