Older women who exercise regularly are up to three times less likely to catch a cold than those who don't work out, a U.S. study suggests.

Anecdotal evidence and smaller studies have suggested that moderate exercise may stimulate the immune-system cells that fight colds, but Cornelia Ulrich of the Fred Hutchinson Cancer Research Center in Seattle and her colleagues were the first to explore the link in a randomized clinical trial.

The researchers, who report in the November issue of the American Journal of Medicine, randomly assigned 115 sedentary, postmenopausal women who were overweight or obese to two types of exercise programs for 12 months.

Half of the women did moderate exercises such as brisk walking for 45 minutes per day, five days a week, while the rest did a stretching session for 45 minutes once a week.

All of the participants filled out questionnaires every three months about the number of times they had allergies, colds and other infections. They were taught how to differentiate between the upper respiratory tract infections.

To gain the benefits of moderate exercise for warding off colds, it seemed people had to stick with their workouts for the long term.

In the final three months of the study, the risk of colds among stretchers was more than three times higher than that for the exercisers, the researchers report in the November issue of the American Journal of Medicine.

About 48 per cent of stretchers reported having at least one cold over the 12-month study compared with 30 per cent among the exercisers.

"Although we did not find an effect overall on upper respiratory tract infections, our study suggests that moderate-intensity training can reduce the risk of colds in postmenopausal, nonsmoking, overweight or obese women," Ulrich said.

"This finding is of clinical relevance and adds a new facet to the growing literature on the health benefits of moderate exercise."

The researchers said they expect the results would apply to other age groups and to men, but said they couldn't guarantee it since they didn't study those groups.