The findings come out of the long-running Nurses' Health Study, which includes only women, but previous research suggests that exercise may also protect men against the chronic skin condition, characterized mostly by inflamed, scaly patches.
As many as 7.5 million Americans have psoriasis, according to the National Psoriasis Foundation, which says it's the most common autoimmune disease. Men and women are equally affected. Previous research has linked higher body mass index, or BMI, family history of psoriasis, alcohol use, and smoking to the risk of psoriasis.
In the new study, scientists followed nearly 87,000 female nurses for 14 years. None of them had been diagnosed with psoriasis at the beginning of the study. Over the course of the study, the nurses completed three detailed questionnaires about physical activity and were asked to report whether they were ever diagnosed with psoriasis. A total of 1,026 women said they were diagnosed during the study period and provided survey information about their physical activity.
Compared with no vigorous physical activity, vigorous exercise -- the equivalent of 105 minutes of running at a 6-mile-per-hour pace every week -- was associated with a 25% to 30% lower risk of psoriasis. The association remained significant after accounting for BMI, age, smoking, and alcohol use. The researchers say theirs is the first study to investigate the independent association between physical activity and psoriasis.
"The intensity of the exercise is the key," says researcher Abrar Qureshi, MD, MPH, vice chair of dermatology at Brigham and Women's Hospital and assistant professor at Harvard Medical School.
Only running and performing aerobic exercise or calisthenics were associated with a reduced risk of psoriasis. Other vigorous activities, such as jogging, playing tennis, swimming, and bicycling, were not. The researchers speculate that the highly variable intensity of the latter group of activities might account for the lack of an association with a lower psoriasis risk.
More than a decade ago, Siba Raychaudhuri, MD, reported that male and female psoriasis patients who exercised were likely to have less severe disease. "Walking was protective also," says Raychaudhuri, a rheumatologist at the University of California, Davis. He says he was "a little bit surprised" that Qureshi did not find that to be the case but added that "this study is more elegant than ours" because it collected more detailed information about exercise intensity.
Qureshi's team speculates that the lower risk of psoriasis in women who exercised vigorously might be due to a reduction in system-wide inflammation. Vigorous exercise also might be protective against psoriasis because it decreases anxiety and stress, which are tied to new cases and exacerbations of the disease, the researchers say.
"A good amount of data show that emotional stress reduction is good for psoriasis reduction," Raychaudhuri says.
Other Possible Explanations
Exposure to ultraviolet light is a psoriasis treatment, so time spent outdoors exercising, and not the exercise itself, might have explained the lowered risk of the disease, Qureshi says. But his study found that women who ran for only an hour a week had a significantly reduced risk of developing psoriasis than women who spent at least four hours walking outside at an average pace.
Chris Ritchlin, MD, MPH, a University of Rochester rheumatologist, calls Qureshi's findings "very interesting." Still, Ritchlin says, while exercise is known to be associated with reduced inflammation, "is there something about people who are really athletically inclined that we're not thinking about that would prevent them from getting psoriasis?"
Qureshi says that could be the case, which is why his study needs to be replicated. "You have to interpret the results cautiously because it is a single study," he says. "It is certainly possible that the women who exercise more are just more health-conscious. There could be other factors that could protect them from developing psoriasis."
Qureshi's study appears online in the Archives of Dermatology.