# The Immune System and Intense Training: What Research Shows

> Intense training taxes the body - including the immune system. What the so-called open window phenomenon describes and which nutrients are discussed in sports nutrition.

Intense training poses particular challenges for the immune system. While moderate physical activity strengthens the body's defenses, extreme exertion can lead to a temporary weakening. It is crucial to understand how these mechanisms work and what measures athletes can take to support their immune function.

## Moderate Exercise Strengthens the Body's Defenses

Regular, moderate aerobic exercise, defined as 150 minutes or more per week at 60-75 % of maximum oxygen uptake (VO2max), has a **positive influence on the immune system**. This type of activity promotes the circulation of immune cells, improves macrophage function and increases the activity of natural killer cells. Studies show that moderately active people suffer from upper respiratory tract infections less often than inactive individuals.

## The "Open Window" After Extreme Exertion

After very intense or prolonged efforts, such as marathons or Ironman races, a phenomenon known as the "open window" can occur. During this phase, **immune parameters are temporarily altered**: the number of natural killer cells and certain T lymphocytes in the blood can drop, and secretory immunoglobulin A (sIgA) in the mucous membranes can be reduced. At the same time, proinflammatory cytokines and cortisol rise. Although the exact clinical significance of an actual immune suppression is still debated, the **risk of upper respiratory tract infections** is elevated during this recovery phase.

## Essential Nutrients for Immune Function

Several nutrients play a key role in supporting the immune system, especially under training stress. **Vitamin D** is of great importance here, since deficiency is common among athletes and is associated with an increased risk of infection. It contributes to the normal function of the immune system. **Vitamin C and zinc** are also essential, as they are depleted more heavily in athletes through sweat losses and increased oxidative stress. An adequate intake of these micronutrients is important for maintaining immune defenses.

## Sleep and Glutamine as Immune Modulators

**Sleep** is a fundamental regulator of immune function. During sleep, proinflammatory cytokines are cleared, antibody production is normalized and T cell memory is consolidated. Even a single night with less than six hours of sleep can significantly increase the risk of catching a cold. **Glutamine**, an amino acid, serves as an important energy source for immune cells. After exhausting efforts, plasma glutamine levels can drop, which has been linked to an increased risk of infection during the recovery phase.

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## Frequently Asked Questions

**Why is moderate training good for the immune system?**
Moderate training improves the circulation of immune cells, increases the activity of natural killer cells and optimizes macrophage function, which strengthens the body's defenses and lowers the risk of infection.

**What is the "open window" phenomenon?**
The "open window" describes a temporary phase after very intense or long efforts in which certain immune parameters are altered and the risk of upper respiratory tract infections may be elevated.

**What role does vitamin D play for athletes?**
Vitamin D is crucial for the normal function of the immune system. A deficiency, which is common among athletes, is associated with an increased risk of infection, since vitamin D receptors are present on key immune cells.

## Sources

Gleeson M (2007), Journal of Applied Physiology - Immune function in sport and exercise.
Castell LM et al. (1996), European Journal of Applied Physiology - Does glutamine have a role in reducing infections in athletes?
Nieman DC & Wentz LM (2019), Journal of Sport and Health Science - The compelling link between physical activity and the body's defense system.

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