Illness and immune function
Evidence: moderate
The famous link between heavy running and getting sick is weaker than decades of repetition imply. Most of the evidence is observational and based on self-reported symptoms, and many athlete “colds” are not confirmed infections. The one firm rule is to avoid training with fever or systemic illness. The boring levers, sleep, hygiene, sensible load and eating enough, do more than any supplement.
Not medical advice
This is a general knowledge base, not medical or dietary advice. If you are injured, unwell or weighing up a supplement or a change to your diet, speak to a doctor, physiotherapist or registered dietitian who knows your situation.
The J-curve, and why it is contested
The long-standing model is the “J-curve”: moderate training lowers upper-respiratory-infection risk below sedentary levels, while heavy or prolonged exertion raises it (Nieman 1994). The supporting field data are real, for example marathon finishers reporting several times more infection symptoms in the week after a race than non-racing runners (Nieman et al. 1990), but they rest on self-reported symptoms, not confirmed infection.
The mechanistic basis has since been reframed. The “open-window” idea, that immune function is suppressed for hours after hard exercise, is now argued to be largely an artefact: the post-exercise dip in blood immune cells reflects those cells redistributing to other tissues for heightened surveillance, not suppression (Campbell & Turner 2018). The leading exercise-immunology groups’ consensus is deliberately cautious: infection susceptibility is multifactorial, and whether exercise itself raises risk, independent of travel, crowding, sleep loss, stress and low energy availability, is unresolved (Simpson et al. 2020). Compounding this, in a large share of athlete “infection” episodes no pathogen is found at all, so much of what gets counted as a cold may be allergy, asthma or airway irritation (Spence et al. 2007).
Deciding whether to train
The widely repeated “neck check”, train gently with above-the-neck symptoms (runny nose, mild sore throat) and rest with below-the-neck or systemic ones, is clinical lore with no trial backing, useful as a rough guide but no more (Ruuskanen et al. 2024). The one firm rule has a clear rationale: do not train with fever or systemic illness, because exertion during a viral illness that inflames the heart muscle can trigger dangerous arrhythmia. Such events are rare in absolute terms, and common-cold viruses are not the cardiac-risk ones, which is why the caution attaches to febrile, whole-body illness rather than a head cold. After any significant illness, rebuild gradually rather than resuming full load, since even a short layoff costs fitness (see return to running).
What actually lowers risk
The best-supported steps are unglamorous: adequate sleep, where short sleep measurably raises susceptibility to infection (Prather et al. 2015); hand hygiene and avoiding sick contacts; sensible training-load progression to avoid overreaching; and adequate energy and carbohydrate availability, since under-fuelling and low energy availability stress the system, and carbohydrate during long efforts blunts the stress-hormone response (Walsh 2018; Nieman 1998). Supplement claims are the most oversold part: vitamin C does not prevent colds in general (with a narrow exception under extreme physical stress), probiotic effects are small and inconsistent, and zinc evidence concerns treating a cold’s duration rather than preventing illness (see supplements that don’t hold up).