How Much Exercise Harms Your Immune System?

I'm looking at you, marathoners and triathletes. While you're out there building superhuman endurance and making the rest of us feel bad, are you also beefing up your immune systems? Or does becoming an Ironwoman actually weaken your body's defenses?

It may depend on how you're exercising. Researchers in Taiwan compared two types of exercise, the names of which might reveal the researchers' own feelings toward hitting the gym: "Acute Severe Exercise" (ASE) and "Chronic Moderate Exercise" (CME). In medicine, "acute" is something that comes on quickly and is over soon, as opposed to a chronic illness. The flu, say, as opposed to mono.

The subjects were 13 males between the ages of 20 and 24. Though young and otherwise healthy, they weren't in shape; the subjects had been getting less than one hour a week of exercise for at least the past six months. At the beginning of the study, all 13 subjects underwent "acute" exercise, cycling at increasing levels of difficulty until they reached exhaustion.

Afterward, five subjects became controls. They were told to continue not exercising for the next four months. Twice during that period, they showed up for another bout of ASE, so researchers could make sure that their bodies and their exercise abilities were staying the same. Meanwhile, the other eight subjects began two months of "chronic" exercise. They worked out five days a week for 30 minutes. The moderate intensity of their workout was defined as a percentage of the work they'd been able to do during ASE. After two months, the exercisers were also instructed to stop exercising. They spent two more months getting no exercise at all. In each month of the study, they also did an ASE test so researchers could see how their bodies' response to severe exercise was changing.

Outwardly, the effect of consistent (excuse me, chronic) exercise on the bodies of formerly sedentary people was unsurprising. After two months of training, the CME subjects had lost weight, lowered their resting heart rates, and increased their endurance. Then they stopped exercising. After the two-month "detraining" period, subjects' weights and heart rates had returned to their original levels, though the work they could do in the ASE task was still elevated, showing a lasting effect on their fitness. The control subjects did their job well, staying the same during the four months.

But what the researchers were interested in was the inner changes in their subjects; namely, changes to white blood cells called neutrophils. These are key players in the immune system, responding to the site of infection in the body and attacking any invaders they find. Neutrophils are short-lived cells, committing cell suicide (called apoptosis) after only a few days in the bloodstream. If these white blood cells are too enthusiastic about offing themselves, it can weaken the immune system.

Neutrophil death may be linked to the abundance of oxygen-containing molecules that react with everything around them, harming structures inside the cell. Since extreme exercise can increase the amount of these harmful "reactive oxygen species" in the body's tissues, the researchers wanted to know how exercise affected neutrophils. They drew blood from their subjects periodically, both at rest and after their ASE trials, and removed the neutrophils for analysis.

They found that "acute severe exercise" did, in fact, accelerate neutrophil suicide. It also increased the amount of reactive, oxygen-containing molecules in the cells.

"Chronic moderate exercise," on the other hand, appeared to slow down the death of neutrophils. After two months of regular exercise, subjects' white blood cells were showing less oxidative stress and slower apoptosis. Even after subjects spent the following two months not exercising, the effect lingered.

In a final twist, the positive effects of consistent exercise seemed to counteract the harmful effects of extreme exercise. After the acute exercise task, subjects who'd been exercising regularly did not show the same damage to their neutrophils that they had at first. But after two sedentary months, the protective effect had begun to fade.

What does all this mean for the marathoner or the Ironwoman? Unfortunately, since the subjects were all men, the study says very little about women of any kind. But for the young, previously sedentary males involved, the study suggests that sudden, exhausting exercise accelerates the death of certain immune cells. Consistent and moderate exercise, on the other hand, prolongs these cells' lives. It also buffers the damaging effect of occasional extreme exercise. And when you stop exercising, the positive effects of your old routine linger, at least for a little while.

The researchers point to other studies that have shown a connection between sudden, extreme exercise and upper respiratory tract infections. In this study, we can't see the effect that various rates of neutrophil death had on subjects' immune systems as a whole. When neutrophil death was accelerated after acute exercise, were subjects truly more vulnerable to infection, or did the immune system compensate somehow for neutrophil loss? In subjects who got regular exercise and prolonged the lives of their neutrophils, was the immune system strengthened? Does keeping these short-lived cells alive for longer necessarily help prevent infection, or could it create a burden for the body?

Overall, the authors think the evidence is in favor of consistent and moderate exercise. For patients whose immune systems are impaired by HIV or chemotherapy, regular exercise might provide a boost. This study suggested that consistent exercise counteracts the negative effects of extreme exercise--at least some of the effects. But to stay on the safe side, the authors recommend that you avoid "acute severe exercise" like, well, the plague.


Syu, G., Chen, H., & Jen, C. (2011). Severe Exercise and Exercise Training Exert Opposite Effects on Human Neutrophil Apoptosis via Altering the Redox Status PLoS ONE, 6 (9) DOI: 10.1371/journal.pone.0024385