Regular exercise may help reduce the misery of the flu by boosting the immune system’s response to the virus, a study in mice hints.

Studies have suggested that people who exercise moderately suffer fewer and less severe colds and flu infections than couch potatoes do - while exhausting workouts may increase a person’s vulnerability to these infections.

In theory, the benefits of moderate exercise may stem from its effects on immune defenses. Research has found that exercise boosts activity in various parts of the immune system that help limit a viral attack or help clear an invading virus from the body more quickly.

In the new study, researchers found that when they had a group of mice regularly run on a treadmill over 3.5 months, the animals developed less-severe symptoms when infected with the flu virus, as compared with mice not subjected to the rodent workouts.

In addition, mice that exercised right before flu infection, but not regularly over the preceding months, also showed some protection against severe symptoms - which in mice means dampened appetite and weight loss.

Those benefits, however, were only apparent in the couple days after infection, whereas regular long-term exercise reduced flu symptoms over the whole course of infection.

Dr. Marian L. Kohut and colleagues at Iowa State University in Ames report the findings in the Journal of Infectious Diseases.

Whether the findings translate to humans is unknown. But they offer possible clues as to why regular exercisers have been found to suffer fewer or at least less-severe respiratory infections.

Compared with their non-exercising counterparts, mice that exercised showed lower levels of various inflammatory substances in their lungs soon after being infected with the flu virus. Those levels stayed lower in mice that exercised regularly, whereas they rose over the next several days in mice subjected to just one workout.

The regularly exercising mice also showed lower concentrations of virus in their lungs early on.

These findings, Kohut’s team writes, suggest that exercise boosts “early innate antiviral defenses” - though the exact mechanisms remain unclear.

For now, the results offer one more potential reason to get regular moderate exercise.

SOURCE: Journal of Infectious Diseases, November 1, 2009.