Introduction

Is immunity the overlooked pillar of performance? We have focused research and product development in the sports nutrition space on muscle recovery, endurance, energy metabolism, focus, and strength. Whether you are training for a marathon, coaching a team through the playoffs, or helping a client push through high-volume training, immune health plays a massive role in performance longevity. After all, you cannot set a personal record when you are sidelined with a cold. Athletes and active nutrition users invest heavily in hydration, macronutrients, muscle building, and recovery techniques, but when it comes to supporting immune health, it is usually an afterthought for this group. 

We adapt to stress each time we exercise – it is what makes us stronger, faster, and more resilient. Stress disrupts the body’s homeostasis – the body interprets this as a signal to adapt by building stronger muscles, improving cardiovascular capacity, enhancing energy metabolism, and strengthening the nervous system. Adaptation only happens when we allow the body to recover and rebuild – too much stress on the body often leads to chronic inflammation and suppressed immune function. The right dose of stress + adequate recovery will lead to positive adaptation. Too much stress + not enough recovery will lead to breakdown. 

Acute vs. Chronic Stress

Acute stress can temporarily strengthen immunity and increase the production of pro-inflammatory cytokines to enhance short-term defense. In contrast, chronic stress suppresses immune function through prolonged cortisol elevation, leading to inflammation and a higher susceptibility to infection and autoimmune disorders. Chronic stress alters T and B cell proliferation, antibody production, and cytokine signaling, disrupting immune balance.^[1]

During heavy training or intense competition, the body experiences a surge of physiological stress that can temporarily suppress immune function. This response is primarily driven by the activation of the hypothalamic-pituitary-adrenal (HPA) axis and the sympathetic nervous system (SNS), which release glucocorticoids (such as cortisol) and catecholamines (such as adrenaline and noradrenaline). Elevated cortisol suppresses pro-inflammatory gene transcription, reduces cytokine production, and promotes lymphocyte apoptosis—essentially dialing down immune cell activity. Meanwhile, catecholamines influence immune cell trafficking, cytokine secretion, and antibody synthesis, shifting immune priorities toward immediate stress adaptation rather than long-term defense. The combined effect of these hormonal changes leads to a transient suppression of cell-mediated immunity, altered Th1/Th2 cytokine balance, and a decreased inflammatory response. This state, sometimes referred to as the “open window” of immune vulnerability, leaves athletes temporarily more susceptible to infection following prolonged or exhaustive exercise bouts.^[2] 

The Open Window of Immune Vulnerability

The “open window” of immune vulnerability is a concept that describes a temporary period of depressed immune function following intense or prolonged physical activity. The timeframe lasts from a few hours to up to 24 hours. During this time, the body’s defenses are thought to be weakened, creating an “open window” during which viruses and bacteria have a greater opportunity to cause infection, most often in the upper respiratory tract. The term originated when researchers began observing that endurance athletes, particularly marathon runners, reported a spike in cold and flu-like symptoms the day following major races. The terminology has refined the idea over time, recognizing that trained athletes often adapt over time, but the concept remains foundational to protect immune function during heavy training cycles. So, can dietary supplements help with the theory? 

Ingredient Studies

Several studies have shown the benefits of dietary supplement intake related to immune function in athletes or active nutrition users. 

A 2021 gold standard study in healthy male marathoners took either a multi-stain probiotic or a placebo. Samples were collected at baseline, 24hrs pre-race, immediately post, and 1hr post. The study showed that the probiotic group had a lower incidence of upper respiratory tract infection (URTI) and lower symptom severity. In addition, the probiotic group showed lower IL-6 immediately post and higher IL-10 at 1H post, suggesting a quicker tilt toward anti-inflammatory resolution. 

Another 2021 two-part study examined how vitamin D status affects respiratory infections during intense military training. In the first part of the study, 21% of participants with D-sufficient levels in winter and those with adequate vitamin D levels were 40% less likely to develop upper respiratory tract infection than those deficient. Each infection caused roughly caused three missed training days. The second part of the study compared oral vitamin D supplementation with safe simulated sunlight over 12 weeks in the winter. Both strategies effectively restored vitamin D sufficiency in over 95% of the participants. Compared to placebo, vitamin D supplementation reduced peak URTI symptom severity by 15% and cut the total number of days with URTI by 36%. 

Another study of 278 healthy adults preparing for the 2017 Austin Marathon consumed a dairy beverage containing 250 mg of insoluble β-glucan, 250mg of soluble β-glucan, or a placebo for 45 days before, the day of, and 45 days after the marathon. Results showed that insoluble β-glucan significantly reduced total URTI severity and symptomatic days compared to placebo. The insoluble yeast B-glucan provided stronger immune protection against exercise-induced respiratory illness than soluble β-glucan, suggesting that β-glucan influences immunomodulatory effects in endurance athletes. 

Beyond probiotics, vitamin D, and β-glucans, several nutritional strategies can help reduce stress that widens the open window of the immune vulnerability concept. Carbohydrate timing plays a critical role during endurance sessions; 30-60g of carbohydrates per hour can help blunt spikes in IL-6 and cortisol, preserving immune cell function and shortening recovery time. Polyphenols, such as quercetin and catechins, have shown mixed results but promising evidence in reducing the number of URTIs in endurance athletes by modulating oxidative and inflammatory stress. Omega-3 fatty acids can help immune cells transition from activation to recovery without lingering inflammation. Finally, adequate protein intake, particularly with leucine-rich sources, contributes to immune cell regeneration and tissue repair post-exercise. 

While athletes and active nutrition users are chasing new personal records, our immune systems are quietly running its own marathon. Immune vulnerability reminds us that recovery is not just about muscle repair but giving our body a chance to defend itself. A tough training may temporarily leave the immune system weaker, but with the right nutrition and dietary choices, athletes can close that window faster. The strongest athletes are not just the ones who train the hardest, but also the ones who stay healthy to keep showing up.