One of the most commonly cited measures to protect ourselves from illness and strengthen our immune system is to exercise regularly. Is there any truth to this scientifically-speaking? Does exercise improve immunity in the way we hope?

The notion of exercise supporting immunity seems counterintuitive in some ways. Exercise naturally induces fatigue, which logically should make it harder for us to protect ourselves as bodily resources are shifted towards recovery, not defence. Anyone that has trained for any length of time will have, at one time, overdone it – and we all know how that goes. You feel tired, irritable and lack motivation. You feel like you want to sleep more and may even feel like you are coming down with an illness.

This phenomenon is supported by some scientific evidence, suggesting that acute bouts of intense exercise suppress the immune system by downregulating lymphocyte production and negatively impacting other essential hormones and messenger proteins. This led to the formation of what was initially called the ‘open window’ theory, positing that short term suppression of the immune system might create a window of opportunity in which you are temporarily more susceptible to illness.

The scientific evidence supporting this dates back to the 1980s and 1990s, however. Does more recent research support the suggestion that vigorous exercise suppresses immune function? Campbell and Turner examined this in a 2018 review paper, wherein they examined three mechanisms used to support the open window theory, namely:

  1. Increased risk of infection after an acute bout of prolonged and vigorous aerobic exercise
  2. Acute bouts of vigorous exercise leading to a temporary reduction to salivary IgA levels, leading to a higher risk of contracting opportunistic infections
  3. Transient decreases in peripheral blood immune cell levels in the hours following vigorous exercise

After reviewing all available literature around these three mechanisms, Campbell and Turner (2018) concluded: “…evidence implies that a reduction in the frequency and function of lymphocytes (and other immune cells) in [the] blood in the hours following vigorous and prolonged exercise does not reflect immune suppression”. You would struggle to be more definitive than that. They add that instead the observed effect “represents a state of immune surveillance and immune regulation”, meaning that immune cell activity is heightened, not reduced, as it attempts to recover. In other words, even if changes in specific markers of immunity are detected, this doesn’t mean that overall immune functioning is suppressed.

Campbell and Turner (2018) ultimately note that evidence supports that leading a physically active lifestyle and performing regular bouts of exercise reduces incidents of communicable and non-communicable diseases – suggesting, once and for all, that immune functioning is supported by exercising. Davison et al. (2016) came to a similar conclusion, writing “regular moderate [exercise] is particularly beneficial for immune enhancement and reducing the risk of infection”. 

Instead, it is inactivity and obesity that causes immunosuppression and an increased susceptibility to illness. Evidence suggests that fat cells and tissues play an important role in this. After all, fat tissue is anything but a dormant energy store, but rather an active endocrine organ that influences both cytokine activity and hormone production. All is not lost though – a 2013 paper from Huang et al. suggested that regular bouts of exercise can help to reverse the immunosuppressive effects of obesity, even without weight loss.

How much exercise do I need to do to support my immune system?

We have already established that exercise benefits the immune system. The question is, how much exercise do we need to do to have a positive impact on our immune system? 

It is at this stage that we need to introduce you to the concept of ‘hormesis’, a theory arguing that there is an ideal dose of bodily stress that our body will adapt to in the mid to long term to protect it from future bouts of stress. We apply a similar principle when building muscle, challenging them with heavy weight to cause growth which helps us better deal with the same weight in the future. 

Staying with this analogy, we know that we can provide too little muscular stress, wherein our body doesn’t detect an issue, and does not trigger muscle growth in response. We also know that doing too much causes us to stagnate, injure ourselves or even regress. There is a perfect sweet spot of “just enough” when it comes to challenging our muscles, and in the same way, there is a right amount of exercise we can do to best support our immune system.

How do I know I’m doing too little exercise?

Essentially, we are looking for the signs that we are under training here, the signs that indicate the stress we are putting our bodies under is not enough to cause adaptation or growth. These include:

  1. Exercise feeling easy, not leaving us out of breath or in need of rest
  2. Not improving over time
  3. Having no indication of exercising, no soreness, aching etc.

How do I know I’m doing too much exercise?

Now, we are looking for indicators of overtraining, signs that indicate that the stress we are putting our bodies under is more than we can recover between bouts of exercise. These include:

  1. Extreme muscle soreness lasting for days, or over a week
  2. Pain indicating imminent injury, or preventing us from exercising
  3. Chronic mental fatigue and lethargy (could be related to other factors, though)
  4. Regression in performance relevant to the exercise you are doing