Immune resistance during exertion
In plain language
At a major tournament a simple cold can make the difference between winning and elimination. Top athletes train for years, but around competitions they run extra risk: travel, stress, heavy load and sleep deprivation temporarily weaken the defences. This is called the ‘open window’: a short period after intense exertion in which the immune system is less alert.
The key to protection often lies not in a pill, but in the gut. About 70% of your immune cells are located around the gut lining, and your microbiome trains those cells continuously. A diverse, balanced microbiome makes you more resilient; a disturbed flora increases susceptibility. What's more, a healthy gut also protects the lungs and skin via the gut-lung and gut-skin axes. Those who manage recovery, sleep and nutrition well keep that window smaller — and stay more resilient at the decisive moment.
The science behind
Moderate exertion strengthens the defences, but very heavy or accumulated load can cause a temporary immune dip — the classic ‘open window effect’, with an increased chance of respiratory infections in the hours to days afterwards. Travel, jet lag and stress amplify this by disrupting the circadian rhythm and raising cortisol. Reviews in sports immunology describe how athletes in the run-up to major tournaments report more infections than non-athletes.
The gut is central here. A large part of the immune cells are located around the gut lining, and the microbiome trains those cells continuously. SCFAs stimulate regulatory T cells that keep inflammation in check; a diverse microbiome increases resistance to pathogens, while dysbiosis goes hand in hand with a leakier gut wall and higher susceptibility. Importantly: a balanced microbiome protects not only the gut but also the lungs and skin, via the gut-lung and gut-skin axes (Cell Host & Microbe). In this way the gut connects recovery with infection resistance.
The common thread is co-metabolism: whether an athlete falls ill through the open window or stays resilient arises from the interplay of DNA (inflammation and immune sensitivity), microbiome, metabolites (SCFAs, inflammation markers) and lifestyle (sleep, recovery, nutrition, travel) together. Taking probiotics blindly helps little; it's about supporting the whole in a targeted way. This reading is supportive and observational — not a medical diagnosis.
Illustration & CosmoTalks
How your body adapts to load, travel and pressure — five episodes.
- S6·E1 — Resilience is a trainable trait
- S6·E2 — Why one person adapts faster
- S6·E3 — Your internal clock sets your limits
- S6·E4 — Immunity as a silent performance factor
- S6·E5 — Adaptation: the art of adjusting
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See the books →Scientific references
• Marsland BJ., et al. (2015) — The gut–lung axis in respiratory disease. Cell Host & Microbe.
• Clarke SF., et al. (2014) — Exercise, microbial diversity and immunity. Gut.
• Van Gucht P. — HOST (COSMO trilogy): gut, immunity and resilience.
Frequently asked questions
- What is the open window effect?
- A short period after very heavy exertion in which the immune system is less alert and the chance of respiratory infections in particular temporarily rises.
- Why do athletes fall ill more often around competitions?
- Travel, stress, sleep deprivation and heavy load pile up. Together they disrupt the rhythm and temporarily weaken the defences.
- What does my gut have to do with immunity?
- A large part of your defences sits around the gut lining. Your microbiome trains those cells and, through the gut-lung and gut-skin axes, protects the lungs and skin too.
- Does simply taking probiotics help?
- Usually not blindly. It's about supporting the whole picture — recovery, sleep, nutrition and microbiome — in a targeted way. This is supportive, not medical advice.
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