Is moderate exercise good or bad for your immune system?

Moderate exercise, meaning sessions of roughly up to 45 minutes, benefits your immune system. Several studies show that people who are regularly moderately active get fewer infections than people who barely move. This effect has been demonstrated most clearly in older adults and people with a chronic condition. Moderate exercise also has an anti-inflammatory effect: it influences signalling molecules in the blood that keep the immune response on track. That appears to be an important mechanism behind the broader health benefits of physical activity.

Intense, prolonged exertion reverses this picture. More than an hour and a half of high-intensity exercise temporarily suppresses several immune cells (such as white blood cells and lymphocytes). That dip lasts from three to twenty-four hours after the exertion. For occasional long training sessions this is not a major problem for most healthy people, but in elite athletes who train too hard for weeks on end the immune problems can persist. Respiratory infections are even one of the most frequently cited reasons for missed training days among this group.

Whether heavy exercise in itself raises the risk of infection is less certain. Sleep deprivation, stress, travel and nutritional deficiencies go hand in hand with the exercise itself during intensive training programmes. Disentangling those factors from the effect of exercise alone is difficult, and the debate on this point has not yet been settled.

In addition, several smaller studies suggest that even a single session of moderate exercise can improve the response to a vaccination in vulnerable groups. This evidence is still limited, but it fits the broader picture that moderate activity gives the immune system a boost.