Skip to content
Skip to content
Muscle & StrengthHub

Nutrition6 min read

The open window hypothesis: does hard exercise weaken your immune system?

Many runners catch a cold after a marathon. The open window hypothesis says hard exercise briefly suppresses the immune system for hours afterwards, letting infections in. Newer research suggests the picture is more complicated, and that regular exercise strengthens immunity overall.

By Updated

Share this guide

A tired runner in a foil blanket holding a mug of tea on a park bench.

The short answer

The open window hypothesis claims hard exercise suppresses immunity for 3–72 hours afterwards. Researchers now argue that immune cells mostly move into tissues rather than disappear, and that illness after big events is more likely linked to crowds, travel, poor sleep and stress (Campbell and Turner, 2018). Regular exercise is linked to fewer infections (Nieman and Wentz, 2019).

  • Regular moderate exercise is linked to fewer respiratory infections (Nieman and Wentz, 2019).
  • Carbohydrate during long exercise reduces the stress hormone response (Nieman and Wentz, 2019).

What is the open window hypothesis?

Why do I get sick after a marathon?

  • Crowds at the start, on transport and at the finish spread viruses.
  • Travel, poor sleep before the race and stress all affect immunity.
  • Some symptoms, such as a sore throat, can come from breathing hard in cold or dry air rather than infection.

How to protect your immune system after a hard workout

  • Fuel long sessions: carbohydrate during exercise lessens the stress hormone response.
  • Sleep well, especially in the week before a race.
  • Wash hands, avoid sharing bottles and keep away from people who are ill around big events.
  • Build training gradually and take easier weeks.

Carbohydrates, the immune system and exercise

Eating carbohydrate during long, hard exercise reduces the rise in stress hormones and some short-term immune changes (Nieman and Wentz, 2019). Whether that lowers infection rates is less clear. Plan carbohydrate for long sessions with the race nutrition planner.

People also ask

Does exercise weaken your immune system?

Regular exercise is linked to better immune health. Very hard events cause short-term changes, but whether they raise infection risk is debated.

Why do I get a sore throat after a marathon?

Breathing hard in cold or dry air can irritate the throat; it is not always an infection (Campbell and Turner, 2018).

Does carbohydrate during exercise help immunity?

It lessens the stress hormone response to long exercise, but it has not clearly been shown to prevent infections (Nieman and Wentz, 2019).

How long is the open window after exercise?

The theory proposed 3–72 hours, but researchers now question whether the window really exists.

Was this guide helpful?

Scientific References & Evidence

  1. Campbell, J.P. and Turner, J.E. (2018) 'Debunking the myth of exercise-induced immune suppression: redefining the impact of exercise on immunological health across the lifespan', Frontiers in Immunology, 9, p. 648. doi: 10.3389/fimmu.2018.00648. Available at: https://doi.org/10.3389/fimmu.2018.00648
  2. Nieman, D.C. and Wentz, L.M. (2019) 'The compelling link between physical activity and the body's defense system', Journal of Sport and Health Science, 8(3), pp. 201-217. doi: 10.1016/j.jshs.2018.09.009. Available at: https://doi.org/10.1016/j.jshs.2018.09.009

Read next

Educational information for adults aged 18 and over. It does not diagnose, treat or manage any medical condition. If you have a health condition, or symptoms that worry you, speak to your doctor or a health professional.