Moreover, colds also appeared to be less severe for those in better shape. Among those who felt the fittest, the severity of symptoms dropped by 32 percent and by 41 percent among those who exercised most, the researchers note.
One limitation of the study was a lack of adjustment for all variables that might affect the outcome, such as exposure to cold germs at work or from children in the home, the researchers noted.
But the study did account for a variety of factors, including age, body mass index and education. And after taking those factors into account, the researchers found that being older, male, and married reduced the frequency of colds. However, the most significant factors (besides being older) were perceived fitness and the amount of exercise a person got, Nieman's group found.
Nieman said one explanation for the finding could be that exercise activates the immune system at a higher rate than normal and causes immune cells to attack viruses. "Exercise gets these cell circulating around the body; they engage the enemy and deal with them," he said.
This effect happens each time you exercise, and then the immune system returns to normal until you exercise again, Nieman said, adding, "Any aerobic exercise should give you these immune benefits."
Infectious disease expert Dr. Marc Siegel, an associate professor of medicine at New York University, agreed that "exercise plays a major role in immune response."
However, Siegel added that people who are physically fit may report fewer sick days because they are "more macho." Perceived wellness may counter feelings of feeling ill, he noted.
But the effect is not purely psychological, Siegel added. "It's a combination of psychological and physical factors," he said.
Siegel noted tha
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