Sunlight May Lower Rheumatoid Arthritis Risk
Baku, February 7 (AZERTAC). Getting more exposure to the sun — specifically, to sunlight's ultraviolet-B (UVB) rays — may lower a woman's risk of developing rheumatoid arthritis, according to a new study published online in Annals of the Rheumatic Diseases. The study is the first to look specifically at UVB exposure and rheumatoid arthritis risk, and its findings may mean that UVB rays, and the vitamin D they cause the body to produce, may protect against the disease.
Rheumatoid arthritis is a condition in which the immune system attacks the body's joints, causing pain, inflammation, and, if left unchecked, damage to joints and reduced mobility. Although rheumatoid arthritis affects an estimated 1.5 million Americans, mostly women, medical experts do not know what causes it. Genes are thought to play a role, but so are environmental factors. Past studies have found a higher risk for rheumatoid arthritis among people living in the northeastern United States, an association that led researchers to wonder if less exposure to sunlight was the reason for the higher risk.
Researchers at Harvard School of Public Health and Brigham and Women's Hospital in Boston recently set out to test this hypothesis. For their investigation, the researchers used data from the Nurses' Health Study (NHS), a long-term epidemiological study that followed thousands of female nurses throughout the United States for several decades. During the NHS, participants completed questionnaires about their health status and dietary and lifestyle habits at the beginning of the study and every two years after that.
For the Annals study, the researchers looked at two different cohorts of participants in the NHS: an older group of 106,368 women who had been followed from 1976 to 2008, and a younger group of 115,561 who had been followed from 1989 to 2009. They estimated the women's exposure to UVB rays based on the states they lived in and an average UVB score for each state that took into account latitude, altitude, and cloud cover. Then the researchers checked to see how many of the participants received a diagnosis of rheumatoid arthritis during the study period.
Over the course of the study, 1,314 women in both groups developed rheumatoid arthritis. After adjusting for other factors that affect the risk of rheumatoid arthritis, such as smoking, the researchers found that among the older group of NHS participants, women who had a higher estimated UVB exposure were less likely to develop the disease. In women with the most UVB exposure, the risk of developing rheumatoid arthritis was decreased by 21 percent.
As epidemiological research, the study could not prove a causal relationship between UVB exposure and rheumatoid arthritis risk, cautioned Karen H. Costenbader, MD, a senior author of the study and researcher in the Department of Rheumatology, Immunology and Allergy at Brigham and Women's Hospital. Nor could it explain why UVB exposure seemed to have a protective effect.
"We hypothesize that it's either through vitamin D, or perhaps through another mechanism," Dr. Costenbader says. UVB exposure causes the body to produce vitamin D, which Costenbader noted has immunoregulatory properties and has generated a lot of recent research interest in rheumatoid arthritis and other autoimmune diseases like multiple sclerosis.
A finding that particularly interested the researchers is that the link between UVB exposure and rheumatoid arthritis risk could be seen only in the older group of women. In the younger group, greater UVB exposure did not appear to protect against rheumatoid arthritis. The researchers speculated that the younger women may have taken greater measures to decrease their exposure, such as wearing sunscreen.
"Our relationship with the sun has changed," Costenbader says. "It used to be that sunbathing was encouraged." But the younger group of women began the NHS study at a time of greater awareness of the risk of skin cancer from too much sun exposure.
A potential weakness of the study is that the researchers did not have data on individual women's actual sun exposure and instead had to rely on state averages to calculate likely exposure. In addition, the researchers focused on Caucasian women, so the results may not apply to women of other racial backgrounds.
Until more is known about a possible link, Costenbader emphasizes the importance of protecting oneself from too much exposure to UVB rays. "Rheumatoid arthritis is still a rare disease," she explains. "Exposure to UVB is strongly related to skin cancer, which unfortunately is not that rare. Weighing all the risks in the balance, wearing sunscreen is still very advisable."
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