Exercise and Immune Health: Finding the Right Balance

The J-Curve Relationship Between Exercise and Immunity

A long-standing model in exercise immunology describes the relationship between exercise load and infection risk as a J-shaped curve: sedentary behaviour is associated with a baseline (moderate) infection risk, regular moderate exercise is associated with a lower infection risk than being sedentary, but very prolonged, high-intensity exercise (especially without adequate recovery) is associated with a temporarily elevated infection risk above baseline — hence the "J" shape, dipping down before curving back up at the extreme end.

This model has been refined over time — more recent research suggests the risk increase at the high-intensity end is smaller and more context-dependent than originally proposed, and is most clearly seen after single bouts of extremely prolonged endurance exercise (such as marathon or ultramarathon racing) rather than regular hard training in well-recovered athletes. Still, the core practical takeaway holds: moderate, consistent exercise is broadly immune-supportive, while extreme, under-recovered training load is not.

Based on: Nieman DC. "Exercise, infection, and immunity." Int J Sports Med. 1994;15 Suppl 3:S131-S141. PubMed ↗

Overtraining and Upper Respiratory Infection Risk

Endurance athletes completing very long events (marathons and beyond) have shown a temporarily elevated rate of upper respiratory tract infections in the days following the event, compared to non-participating runners of similar fitness — often referred to as the "open window" theory, where a short period of temporarily altered immune function follows an extremely demanding single bout of exercise. Chronic overtraining without adequate recovery — repeated hard sessions without sufficient rest — is separately associated with persistent fatigue, elevated resting heart rate, and greater susceptibility to minor illnesses over time.

Supporting Immunity Through Sleep and Nutrition

Exercise doesn't operate on immune function in isolation — sleep and nutrition are at least as influential, and often the deciding factor in whether training load translates to better or worse immune resilience:

• Sleep: Consistent, adequate sleep (see our How Much Sleep Do You Need? guide) is one of the most well-supported factors for immune resilience, and insufficient sleep compounds the immune-suppressive effects of hard training.
• Adequate calorie and carbohydrate intake around long or intense sessions: Training in a significant energy deficit, especially around prolonged endurance sessions, has been linked to greater immune disruption than the same training performed with adequate fuelling.
• Protein intake: Adequate protein supports the immune system's own protein-dependent components (like antibodies) alongside its role in muscle recovery. See our Protein Calculator to check your target.
• Managing overall life stress: Chronic stress independently suppresses immune function (see our Stress and Weight guide for the cortisol mechanism), compounding the effect of hard training if both are elevated simultaneously.

Recommended Moderate Exercise Guidelines

For general immune-supportive benefit, the standard population physical activity guidelines apply — roughly 150 minutes of moderate-intensity aerobic activity per week, which aligns with the "regular moderate exercise" portion of the J-curve associated with lower infection risk than being sedentary. This is not a ceiling on healthy training volume for well-conditioned individuals, but a reasonable target for anyone specifically prioritising immune health as a primary goal, alongside consistent sleep and adequate nutrition.

If you notice a pattern of frequent minor illnesses alongside high training volume, elevated resting heart rate, and persistent fatigue, that combination is worth treating as a signal to add recovery time — see our Recovery Between Workouts guide and check your Resting Heart Rate Calculator trend rather than pushing through.

Based on: Campbell JP, Turner JE. "Debunking the Myth of Exercise-Induced Immune Suppression: Redefining the Impact of Exercise on Immunological Health Across the Lifespan." Front Immunol. 2018;9:648. PubMed ↗