How Much Exercise Does It Take to Boost Immunity?

Exercise can boost our immune system by so much that we can reduce the number of symptom days by 25 to 50 percent. Imagine if there was some drug that could do that—it would rake in billions, whereas, exercise is free and tends to only have good side effects.

It doesn’t even take much of a workout to get results. Studies find that if you let kids run around for just six minutes, the levels of immune cells circulating in their blood can increase by 50 percent, as they pour from the bone marrow and redeploy throughout the body. Even moderate exercise can also boost IgA production, protective antibodies that coat all of our moist membranes. Compared to a sedentary control group, those who performed aerobic exercises for thirty minutes three times a week for twelve weeks had a 50 percent increase in the levels of IgA in their saliva and reported significantly fewer flu-related symptoms. Salivary IgA levels are associated with lower overall mortality risk over time—particularly cancer mortality—perhaps as a proxy for immune function more generally, and you can boost those levels with exercise.

Exercising may also boost the activity of natural killer cells, our immune soldiers that focus on eliminating both tumor cells and virus-infected cells. Here are five different types of cancer growing in a petri dish, mostly leukemias and lymphomas. Draw blood from people and drip on some of their natural killer cells into the dish, and they start killing off the cancer cells. Ok, but that was blood taken from someone before exercise. Draw blood after 30 minutes of cycling and drop on the same number of natural killer cells, and they each kill cancer about 60 percent better. This may be one of the reasons why exercise seems to help both prevent cancer and improve cancer survival.

Might it improve vaccination responses? If aerobic exercise gets your immune cells to spill out from your bones, can’t you just slouch on the couch all year then just jump up and take a brisk walk right before getting the jab? Studies to this effect performed on young adults had inconsistent results, for example, improving the antibody response to a flu vaccine in women but not men, or to a meningitis vaccine in men but not women.

Against half doses or weaker vaccines, acute exercise interventions appeared to work better, suggesting perhaps a ceiling effect, whereby their immune responses were normally so robust exercise might not improve them further. But, maybe it would work more consistently in older adults? Sadly, 45-minute moderate intensity aerobic exercise right before flu or pneumonia vaccinations didn’t seem to help in older adults. Even two months of 45-minute moderate intensity daily activity—six weeks before and two weeks after vaccination—appeared insufficient to make a difference. Eccentric resistance exercise, for example, slowly lowering heavy dumbbells after bicep curls or lateral raises to inflame the muscle right before the vaccine goes in also doesn’t seem to be a viable strategy.

What about exercise to reduce infection risk directly? In a study entitled “Moderate exercise protects mice from death due to influenzas virus,” mice exercised for 20 to 30 minutes four hours after flu virus exposure and then each of the next three days had nearly twice the survival rate (18 percent dying compared to 57 percent dead in the sedentary group). There’s nothing so dramatic in the human literature, but based on a study that followed more than a thousand adults through flu season, those who exercised even just a few days a week were sick about 40 percent fewer days than sedentary individuals. Randomized controlled interventional trials back this up. For example, one study found that while elderly, sedentary women randomized to a mild range of motion and flexibility calisthenics control group had a 50 percent chance of getting an upper-respiratory illness during the fall season, those randomized to begin a half-hour-a-day walking program dropped their risk down to 20 percent. For conditioned athletes, though, the risk was just 8 percent. Exercising could potentially make our immune systems more than five times better at fighting infection.

In a year-long study, sedentary, postmenopausal women randomized to 45 minutes of moderate-intensity exercise five days per week were experiencing three times fewer colds by the end of the study compared to the control group, who were instead randomized to weekly stretching sessions, even though the exercise group only ended up exercising around four times a week. A similar study of younger overweight women found that 15 weeks of daily walking cut the number of days sick with upper respiratory tract infections symptoms in half, though this was due to cutting the duration of each episode in half rather than cutting down on the frequency of illness.

In terms of more serious infections, randomizing older men and women to one-hour exercise classes twice a week for two years did not, unfortunately, cut down on the risk of coming down with pneumonia. Although yoga training can improve lung function in the elderly, it has yet to be shown to help prevent pneumonia either. There was, however, a treatment trial of yoga breathing exercises for pulmonary tuberculosis performed in India. Compared to a mindfulness meditation, “breathing awareness” control group, those randomized to the yoga group cleared their active infections faster. Even “laughter yoga” might help.

The very evolution of laughter is thought to have been as an antidote to stress, the release of nervous energy. Within 60 minutes of people watching a comedy video (complete with “Gallagher’s classic Sledge-O-Matic finale”), levels of the stress hormone cortisol in their bloodstreams were cut by more than half. Since cortisol acts as an immunosuppressant, which is why cortisol-like steroids like prednisone are used for inflammatory autoimmune diseases, the drop in cortisol with laughter may explain why those laughing heartily at a humorous video had improvements in natural killer cell function compared to those randomized to a control group watching tourism videos. (Replicating these results today might be difficult given the choice of comedic stimulus: Bill Cosby.)