Exercise in moderation was found to be key to the findings.
Older women who regularly engaged in exercise had about half the risk of coming down with a cold than similar women who didn't get a regular workout, a new study shows.
Writing in the November issue of the American Journal of Medicine, researchers at the Fred Hutchinson Cancer Center in Seattle report that even by getting about 30 minutes of exercise five days a week, the previously sedentary women were able to achieve an immune-system boost.
"This adds another good reason to put exercise on your to-do list, especially now that cold season is here," said Cornelia Ulrich, senior author of the study, which involved 115 overweight, postmenopausal Seattle-area women.
Americans come down with some 1 billion colds a year, making the viruses a leading cause of doctor visits and lost days from work and school, according to the National Institute of Allergy and Infectious Diseases. Adults report suffering two to four colds a year, on average.
In the study, women were randomly assigned to either the moderate exercise program (45 minutes a day, five days a week) or a once-weekly 45-minute stretching session.
Overall, the stretchers experienced about twice as many colds during a 12-month period as did the exercisers, and among women who reported at least one cold, the stretchers tended to report colds more frequently. However, there was no significant difference between the two groups in the frequency of other types of upper-respiratory infection, such as the flu, or episodes of allergies.
"The enhanced immunity was strongest in the final quarter of the yearlong exercise intervention," Ulrich said. "This suggests that when it comes to preventing colds, it's really important to stick with exercise long-term."
However, she also emphasized that exercise in moderation -- such as 30 to 45 minutes of brisk walking each day -- is key. Other studies have shown that excessive, exhaustive exercise can deplete the immune system and increase the risk of colds and other infections.
Walking was the most common form of exercise for the women, accounting for about half their workout time at gyms and three-quarters of home-based exercise. And even though the women didn't do quite as much as suggested, the average of 30 minutes they did put in "was enough activity to boost immune function in the long run," Ulrich said.
"It's been shown that just a 30-minute walk can increase levels of leukocytes, which are part of the family of immune cells that fight infection."
The main goal of the study, funded by the National Cancer Institute, wasn't focused on colds, but sought to assess the impact of physical activity on markers of breast-cancer risk. And in fact, the women in the exercise group did achieve significant reductions in weight, total-body fat and abdominal fat.
The researchers also note that while their yearlong look was the longest known to have been done yet on the impact of exercise on resistance to infections, it may actually take people longer than a year before their immune systems fully respond to a workout program.