Keeping physically fit helps reduce the likelihood of falling sick with a cold, a new study published in the British Journal of Sports Medicine suggests. (Rick Wilking/Reuters)

Keeping physically fit helps reduce the likelihood of falling sick with a cold, a new study suggests.

The U.S. study, published in Tuesday's issue of the British Journal of Sports Medicine, was based on 1,002 adults up to age 85.

Participants at Appalachian State University in Kannapolis, N.C., reported on how often they got aerobic exercise equivalent to a brisk walk.

People who got aerobic exercise five days a week or more for at least 20 minutes had more than a 40 per cent reduction in illness days compared with those who averaged one day of exercise per week or less, said study author David Nieman, director of the university's human performance laboratory.

The severity of upper respiratory tract infections (URTI) like colds was also reduced 32 per cent between the least fit and most fit groups, the researchers found.

"Among the various demographic and lifestyle factors evaluated in this study of 1,002 men and women, perceived fitness and exercise frequency ranked second only to older age in the magnitude of reduction of days with URTI during the winter and fall seasons," the study's authors concluded.

Immune booster

In the study, participants were asked about their lifestyle, diet and recent stressful events, which can affect how well the immune system works.

But exercise not only reduces stress and helps people sleep better, the researchers said. Physical activity also causes more immune cells to circulate through the body, where they are available to kill any viruses, Nieman said.

"You just have a better capability to detect pathogens and kill them, and then that adds up over time to reducing illness rates," he said.

The actual effect of exercise on the immune system only lasts a few hours, which may be why the benefit was only seen in people who exercise at least five days a week.

The number of days with cold symptoms varied from an average of eight days in autumn to 13 in winter. Participants were tracked for 12 weeks.

The immune-boosting effect of exercise was in addition to its known benefits in fighting heart disease, stroke and cancer.

The work was funded by grants from Coca-Cola and Quercegen Pharmaceuticals.