It’s been suggested that prolonged and vigorous exercise weakens (or suppresses) our immune system and increases susceptibility to 'opportunistic' infection. Certainly, this was the conclusion that researchers were drawing from studies in the 1980s and 1990s.

But recent reviews of the research offer an alternative angle to this 'immunosuppressive' viewpoint as exercise has been found to boost our immune function and strengthen our resistance to illness and long-term disease, IF we find the 'sweet spot' when it comes to exercise duration and intensity...

Contents:

Exercise and immunosuppression - early research

Early exercise-illness studies conducted at the end of the 20th century asked runners whether they were experiencing any symptoms of an upper respiratory infection (valid symptoms included a runny nose, cough, or sore throat) after competing in a mass participation, long distance event.

Many of the participants confirmed that they were indeed suffering one or more of these symptoms, which led to the link between exercise and a greater risk of infection.

And so the seed for the argument that exercise has an ‘immunosuppressive’ effect on people was sown...

There was a fundamental limitation to these studies though. None of the self-reported ‘infections’ were confirmed by laboratory analyses. So, we can't be sure that they were genuine illness symptoms.

Additionally, more recent evidence suggests that most of the symptoms reported after exercise are not real infections but are caused by other factors, such as an allergy.

What these early observations did encourage was further research into what really happens to the immune system after strenuous exercise.

The impact of exercise on the immune system

During exercise, there's a dramatic increase in the number of immune cells found in the bloodstream. Just one example are the 'natural killer cells', which increase by 10-fold.

When exercise stops, the number of circulating immune cells decreases substantially, sometimes falling lower than pre-exercise levels and staying there for 3-72 hours, before eventually returning to ‘normal’.

At the time, this was believed to be further evidence of a true 'immunosuppressive' effect and the concept was even given a name: the ‘open-window theory’.

Image credit: ResearchGate ©.

Since its proposal, this concept has been widely accepted. But, strong evidence now challenges the idea and suggests that the decrease in immune cells has been misinterpreted.

The decline in immune cell numbers post-exercise doesn't mean they’ve been lost or destroyed (and therefore suppress the immune system), but we now know that the cells get redistributed - specifically to areas of the body which might have become infected. For example, redistribution to the lungs due to the increase in breathing rate and depth during exercise.

This alternative explanation suggests that an acute bout of exercise actually enhances our antibacterial and antiviral immunity by increasing immunity surveillance.

Exercise duration and intensity - finding the 'sweet spot'

Around the same time, another concept called the ‘J-curve’ of exercise immunity was established, which illustrates the relationship between training load and the associated level of infection risk.

The theory suggests that if you push your performance too far into overtraining, or even near it, then your risk of infection goes up significantly. Interestingly, it appears it can even exceed the risk associated with doing no exercise at all.

Most importantly, the J-curve highlights the existence of a ‘sweet spot’ and the relationship between an appropriate training load and a lower risk of infection.

In other words, athletes training moderately are better protected from coughs and colds than 'couch surfers'. But those training very, very hard run an increased risk of getting sick.

It’s thought that this is because a demanding training load creates physical damage in the body, which in turn causes a range of stress responses.

Too much stress and the body is less adept at fighting off infection.

Image credit: ResearchGate ©.

The modern view is that it’s not the act of participating in a strenuous bout of exercise which increases a person’s risk of infection, but rather the factors around it.

There’s a whole host of factors known to influence our immune function, including sleep disruption, temperature changes, fatigue, altered or inadequate diet, dehydration, psychological stress and/or environmental exposures.

The dangers of overtraining

It’s a misconception to label exercise as 'immunosuppressive'. There’s limited reliable evidence to support this viewpoint, and it’s far more likely that frequent, moderate exercise improves the competency of our immune systems.

Having said this, consistent overreaching in training will compromise your immune defences and should be considered highly counterproductive, particularly in times of high stress when your immune system may already be compromised.

6 ways to train and maintain a strong immune system

A weakened immune system can lead to poor health and lacklustre performance. So, keeping the immune system as strong as possible gives it the best chance of fighting off any infections you do come into contact with and means you can continue with consistent training in the long-term.

Here’s a few simple tips to adhere to in order to keep your immune system tip-top:

1/. Manage your overall training load. Regular exercise is essential for the mind, body and immune system but during periods of high stress (be that periods of stress at work, around big races or lots of back-to-back long haul travel) the risk of infection is greatest.
So, this is not the time to be pushing on or making any drastic changes to your training. The advice is to dial it back and don’t overload an already stressed system. Consider decreasing your total load from 'all guns blazing' to 75 or even 50 percent of your normal load.
You can still approach your training in a structured, scheduled fashion to help maintain motivation.

2/. When you do put in that hard interval session, make sure you fuel and hydrate effectively. The ultimate goal should be to avoid glycogen depletion, either through training or dieting.
Low energy availability and nutritional deficiencies are linked to an increased risk of infection by adding extra strain to the body (check out this paper published in 2004).
Prioritise a high-quality diet during times of increased susceptibility and if you do decide to do a longer, harder training session then make sure you eat and drink something soon afterwards (and throughout if warranted and feasible).

3/. Get enough sleep. A good 8 hours or so of quality shut-eye every night is a cornerstone of solid recovery and recuperation for all athletes. Recovery should be made a priority and be as big a part of your training regime as the training - so don’t skimp on sleep.

4/. Manage psychological stress and anxiety. Studies have shown that increased stress induced by an upcoming race can predispose an athlete to increased risk of infection.
The same is true with life stresses, anxiety and worry. Whether we’re conscious of it or not, we all absorb some level of stress (and some more than others), so being mindful of taking some time to mentally de-stress is key: go for a walk, meditate, read a book, stick your favourite Netflix series on.
Whatever it is that helps your mind relax, do it.

5/. Monitor signs of overtraining. It’s a great idea to record things like morning resting heart rate (ithlete is excellent for measuring heart rate variability), mood, performance and motivation in a training diary. If you record these metrics accurately and learn to read your body's signs, this kind of information can act as an early warning system for when you’re starting to get tired and run down, giving you the opportunity to back off in advance of getting ill.

6/. The final tip is an obvious one - minimise your exposure to potential sources of infection (especially at times when the immune system is likely to be compromised!). Doing this primarily involves a lot of relatively simple and repetitive ‘common sense’ safeguards such as…

  • Avoiding contact or proximity to those who are obviously ill (runny noses, coughs, sneezing)
  • Regular hand washing and good personal hygiene
  • Avoiding sharing cups, water bottles and cutlery
  • Steer clear of crowded areas such as public transport during rush hour

Being sensible in your approach to training around times of increased susceptibility could pay dividends by enabling consistency in the long run.

To use a well-coined phrase - "train smart".

Further reading