WASHINGTON - A long-term moderate exercise program can reduce the risk of colds among older women, U.S. researchers said on Thursday.

In the first randomized clinical trial to investigate the impact of moderate physical activity on the common cold, researchers from the Fred Hutchinson Cancer Research Center found that post-menopausal women who worked out regularly had about half the risk of colds as those who did not exercise.

"There has always been this anecdotal evidence, and some small studies, suggesting that with moderate exercise you can improve your immunity," said Cornelia Ulrich, lead author of the study published in the American Journal of Medicine.

"Our study ... is the first time that a rigorous trial showed that the number of colds can be affected by exercise," she said in an interview.

The study involved 115 overweight, post-menopausal women who had not been exercising before the trial.

The group was divided in two, with half the women assigned to undertake a moderate exercise program of 45 minutes a day, five days a week. The other half were told to take part in once-weekly, 45-minute stretching sessions.

The exercisers were told to do moderate physical activity such as walking on a treadmill, cycling on a stationary bicycle or rapid walking outside.

Over the course of a year, the women filled out questionnaires every three months to report the number of times they had allergies, colds or other problems.

The study found that over 12 months, the risk of colds decreased modestly in exercisers and increased modestly in the group of stretchers.

The researchers found that the ability of moderate exercise to ward off colds seemed to increase over time. In the last three months of the study, the group of women who were only stretching were three times as likely to catch a cold as those who were exercising regularly.

The study did not reach any conclusions about the benefit of stretching but said that regular cardiovascular exercise was most beneficial.

"With regards to preventing colds, it seems you really have to stick with exercise long term," Ulrich said.

The results were seen as important in understanding the health benefits of exercise, Ulrich said.

"It may apply also to other age groups, it may apply to men," she said. "In the past, immune studies have been quite consistent among men and women. I wouldn't expect that to be different."