How Exercise Affects Your Immune System

We’ve been told for many years that our bodies are wide open to infection in the hours (or even days) following long or intense workouts. We’ve also been told that competing in endurance events like marathons and obstacle races practically guarantees we’ll catch a bug before we even hit the post-race showers.

However, this idea that exercise suppresses our immune system may actually be a myth. In fact, the relationship between exercise and immunity is pretty complex. Here’s what you need to know.

Exercise And The Immune System

The challenge with talking about exercise and the immune system is that there are a lot of unanswered questions.

“We have a decent understanding of how exercise affects the immune system, but not a great one,” says Josh Clark, C.P.T., a strength and conditioning coach at LoHi Athletic Club in Denver. “Anybody who says they understand it is lying to you.”

How Exercise Affects Your Immune System Over Time

What we do know: Exercise has been shown to strengthen your immune system over time. In fact, according to a 2016 Medicine and Science in Sports and Exercise study, even low levels of physical activity are associated with a 10 percent lower risk of bacterial infections.

Exercise boosts circulation of anti-inflammatory cytokines (proteins secreted by the immune system) and other disease-fighting cells. Over time, this makes your immune system better at hunting down foreign pathogens (like bacteria and viruses), Clark says. It also ups its inflammation-fighting abilities.

Your Immune System Immediately After A Workout

Despite our sense of the long-term benefits, we still don’t quite know how a single bout of exercise affects our immune system in the following hours.

The current leading theory: the “open window” hypothesis. It suggests that our bodies are especially vulnerable to infection in the days following exercise—especially long, intense cardio.

According to a 2018 Frontiers in Immunology review, this theory is based on research from the 1980s and 1990s that found that acute bouts of intense exercise led to a temporary decrease in some antibodies and blood immune cells. Many researchers took this as a sign that the immune system was compromised.

However, as one 2007 Journal of Applied Physiology review notes, research has yet to directly link this post-exercise immune depression and an increased rate of illness in athletes.

Some experts think it’s time to rethink the open window hypothesis altogether. “There is no reliable evidence to suggest that exercise—even of an extreme nature—has a detrimental effect on the competence of the immune system,” say review authors James E. Turner, Ph.D. and John P. Campbell, Ph.D.

In fact, they believe acute exercise may actually make your immune system more responsive to potential threats. “The latest and best evidence suggests a heightened state of immune surveillance after exercise,” say Turner and Campbell. “In this state, cells are redistributed to places other than the blood to look for damaged or infected body cells.”

Your Immune System After An Endurance Race

Still, some experts point to research on upper respiratory tract infections (URTIs) after endurance events (like marathons) as proof that the open window exists.

Trouble is, URTIs are often self-reported, and not always confirmed by lab tests. (A prime example: This 1983 South African Medical Journal study, which suggested runners developed more URTIs after running a marathon.)

Read More: Are You Doing Too Much HIIT?

Unfortunately, without lab tests, it’s impossible to know how many runners truly had URTIs following a marathon. (Their symptoms could have had non-infectious causes, like allergies or asthma, for example.) “If people do get an infection, it’s probably due to their attendance at a mass participation event, where lots of people (and their bugs) gather,” say Turner and Campbell.

Plus, other factors also increase your odds of infection, Turner and Campbell say. A few common ones: chronic stress, sleep-deprivation, and/or insufficient fuel.

When Exercise Can Harm Your Immunity

One case in which exercise can truly sucker-punch our immunity: overtraining syndrome.

Overtraining syndrome, which typically affects competitive athletes, occurs when exercise stresses pile up faster than we recover from. The result: a long-term decrease in performance that may or may not be accompanied by other symptoms, according to a joint statement from the European and American Colleges of Sports Medicine. (Some of those symptoms can include mental fatigue and decreased immunity, marked by increased incidence of sickness and infection.)

Unfortunately, it can be hard to recognize overtraining. “There’s no gold standard for diagnosing overtraining,” Clark says.

As with other aspects of the exercise-immune system relationship, the mechanisms involved in overtraining syndrome are unclear. However, one theory—called the “cytokine tissue-trauma” hypothesis—suggests that excessive training causes a buildup of pro-inflammatory cytokines. This buildup eventually leads to immune system depression, sickness, and symptoms like loss of appetite, lethargy and depression, according to a 2012 Acta Clinica Croatica review.

The Bottom Line

We still have a lot to learn about how exercise affects the immune system. However, the idea that we’re extra vulnerable in the hours after exercise could be on its way out.

Still, there may be such a thing as too much exercise. In principle, just as chronic psychological stress can disrupt our bodies, so can performing high levels of exercise for weeks or months on end without adequate recovery. “Again, though, the evidence for this is limited,” say Turner and Campbell.

Luckily for most gym-goers, overtraining is typically more relevant to competitive athletes. “If you have a nine-to-five job, I can almost guarantee you’re not training as hard as an athlete,” Clark says.

If you do suspect you’re overdoing it in the gym, cut your training volume in half (but keep the intensity the same) for one week. For example, if you typically back squat 100 pounds for four sets of 10 reps (40 reps total), do three sets of seven reps (21 reps total) using that same weight.

Otherwise, check in with a certified coach and consult with your doctor.

References & Further Reading

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