Running and the Immune System: Moderate Training Boosts Immunity vs Overtraining Suppresses It

Introduction

After the COVID-19 pandemic, Taiwanese runners’ attention to immunity has increased significantly. Many people choose running precisely because they’ve “heard it can boost immunity,” yet they catch colds frequently during high-intensity training, and even collapse before or after race events. This seemingly contradictory phenomenon has a clear immunological and physiological explanation.

The Basic Division of Labor in the Immune System

The human immune system is divided into two major branches:

• Innate Immunity: Rapid, non-specific defense, including natural killer cells (NK cells), neutrophils, and macrophages, responding within minutes to hours after infection.
• Adaptive Immunity: Specific responses from T cells (cell-mediated immunity) and B cells (antibody/humoral immunity), requiring 1–2 weeks to develop.

Exercise training affects these two systems differently, and the effects are highly dependent on training intensity and duration.

The J-Curve: The Relationship Between Exercise Volume and Infection Risk

Exercise scientist David Nieman proposed the famous “J-Curve Hypothesis”:

• Sedentary individuals: Immune function is at a low baseline, with moderately elevated infection risk.
• Regular moderate-intensity exercisers (running 150–300 minutes per week, Zone 2–3 intensity): Lowest infection risk, 20–40% lower than the general population.
• Very high-volume athletes (e.g., marathoners in the pre-race buildup, ultramarathon runners): Infection risk rises significantly, especially after races.

The Open Window Theory

After high-intensity or prolonged (>90 minutes) exercise, immune function temporarily declines for 3–72 hours—this period is called the immune open window. Mechanisms include:

• Stress hormones (cortisol, adrenaline) are secreted in large amounts, suppressing T cell and NK cell activity.
• Salivary secretory immunoglobulin A (sIgA) concentrations drop significantly, weakening upper respiratory mucosal defenses.
• Although neutrophil counts increase, their bactericidal capacity (Oxidative Burst) decreases.

For Taiwanese runners, if they immediately expose themselves to crowded MRT environments after running 42 km at the Taipei Marathon, their infection probability is significantly higher than before the race.

Immune-Enhancing Mechanisms of Regular Moderate-Intensity Training

In contrast, long-term, regular moderate-intensity running produces cumulative immune-enhancing benefits:

• Increased NK cell activity: NK cell counts temporarily rise after each exercise session, and long-term accumulation raises resting values.
• M2 macrophage polarization: Promotes an anti-inflammatory phenotype, reducing chronic low-grade inflammation (a common mechanism underlying many chronic diseases).
• Improved gut microbiota: Regular aerobic exercise promotes gut microbiome diversity, benefiting gut immunity.
• Lower chronic stress hormone levels: Reduced baseline cortisol, alleviating immune suppression.

Practical Recommendations

1. The 72 hours after a race are a high-risk period: Avoid crowded public transport and large gatherings; wear a mask when going out (a habit that is especially useful for Taiwanese people).
2. Add a taper week after high-intensity weeks: Schedule an easy week every 3–4 weeks to allow sIgA levels to recover.
3. Supplement with vitamin C, vitamin D, and zinc: There is moderate-strength evidence supporting their protective effects during the post-exercise immune open window, particularly vitamin D, which is commonly deficient in Taiwan’s winter (reduced sunlight).
4. Sleep is the best immune recovery agent: Sleep deprivation (<6 hours) doubles infection risk. A common mistake among Taiwanese runners is maintaining very late bedtimes even after high-intensity weeks.
5. Watch for overtraining syndrome: Persistent fatigue lasting more than 2 weeks, elevated resting heart rate, and declining performance are signs of prolonged immune system stress. Reduce training immediately and consult a sports medicine physician.

Conclusion

Running is an effective tool for enhancing immunity, but only when training volume is moderate and recovery is adequate. The J-curve tells us that what benefits health most is not the most training, but “just the right amount” of training—a reminder worth keeping in mind at all times for Taiwanese runners pursuing performance goals.

Related Reading

• Immune Suppression from Cycling Overtraining: A Guide to Cold Prevention and Training Volume Adjustment
• The Immune Window After Training: Quantifying the Risk of Upper Respiratory Infections After Intense Exercise
• The Post-Exercise Immune Window: How to Maintain Immunity Under High Training Volume
• Exercise and Immunity: Moderation Is Best—A Coach’s Guide to the J-Curve and That “Open Window”