Lack of exercise is a well-known risk factor for conditions such as cardiovascular disease, diabetes and certain forms of cancer. But some research also suggests that physical inactivity should be considered a risk factor for respiratory infections caused by the common cold, influenza, pneumonia and COVID-19. 

Studies show that regular exercise, of moderate intensity, is associated with a decreased risk of respiratory infections. More severe outcomes with COVID-19 also have been linked to low physical activity.

The latest study, published Tuesday, found that children with higher levels of daily physical activity are less susceptible to upper respiratory tract infections, such as the common cold. For every 1,000 average daily steps taken, children experienced 4.1 fewer days of respiratory symptoms. Children who played sports for at least 3 hours each week also tended to have fewer respiratory infections.

The study, which involved 104 children ages 4-7, did not establish a direct cause-and-effect relationship because it was observational. But the researchers had a few theories for their findings. Higher levels of physical activity reduce levels of inflammatory cytokines, which are associated with chronic inflammation and disease. They also promote stronger immune system responses. 

Another possibility is that additional immunity comes from small extracellular vesicles – small, cell-derived particles – released by the muscles after exercise, the researchers said. 

Still, health officials emphasize that vaccination and other preventive measures, such as frequent hand-washing, wearing a mask in high-risk settings, drinking fluids and getting enough sleep, are the most effective ways to reduce the risk of respiratory infection.

Too much exercise may have the reverse impact on the immune system. Some studies have suggested that too much physical activity increases the risk of infection. In one study, exercise stress was linked to an increase in influenza severity and risk of death. 

Another study found that 90 minutes of high-intensity endurance exercise can make athletes more vulnerable to illness for up to 72 hours after their workouts. Some researchers theorize that during intense physical exercise, the body produces hormones that may temporarily lower immunity. Additional research found that intense exercise before or during a respiratory illness can worsen the infection. 

National exercises guidelines advise that adults get 150 minutes of moderate physical activity each week. Moderate activities include walking briskly, recreational bicycling, gardening and vigorous housecleaning. The American Lung Association says that aerobic and muscle-strengthening activities can strengthen the lungs and tone breathing muscles, such as the diaphragm.