Consistent Moderate Daily Exercise Benefits Immune System 

Editor's Note: Dr. Jeffrey I. Messer serves as department chair and faculty in exercise physiology for the Exercise Science Department at Mesa Community College in Mesa, Arizona. Dr. Messer holds a Ph.D. in exercise physiology from Arizona State University in Tempe, Arizona.

By Dr. Jeffrey I. Messer for DyeStat/RunnerSpace

The COVID-19 pandemic continues to create profound social and economic challenges that pervade seemingly every aspect of our day-to-day lives. Not only are basic social interactions significantly constrained and fundamental economic decisions potentially complexified, but also the manner in which citizens and athletes move, recreate, and train are similarly, meaningfully impacted.

In that regard, the following overview seeks to 1) elucidate the traditional J-Curve and Open Window constructs that address the foundational relationship between physical activity / training and immunological health, 2) emphasize select, current evidence that addresses essential issues and challenges specific to the still-evolving understanding of the aforementioned relationship between physical activity / training and immunological health, and 3) to offer a series of reasoned recommendations for productive, safe movement, physical activity, exercise, and / or training.

PART I: THE J-CURVE MODEL & OPEN WINDOW HYPOTHESIS

The J-Curve model is a model that has traditionally extrapolated across multiple scholarly disciplines. The model has depicted, for example, the hypothetical short-term and longer-term impact of currency depreciation on a country’s trade deficit.

With respect to exercise and immunological health, the J-Curve model profiles the relationship between exercise workload and upper respiratory tract infection (URTI) risk. The general, evidence-based premise of the J-curve model is that consistent daily or near-daily moderate-intensity physical activity imparts an approximate forty-to fifty percent (40 – 50%) risk reduction for the development of an URTI. By contrast, the model further indicates that chronic, vigorously intense exercise workloads associate with an approximate two-fold to six-fold (2-fold to 6-fold) elevation in risk for development of an URTI.

More generally, the J-Curve model strongly suggests that consistent, moderate-intensity physical activity imparts a protective impact on infection and illness incidence and thus supports consistent, moderate-intensity physical activity as a fundamental constituent of healthful, daily living.

Notably, notwithstanding recent International Olympic Committee (IOC) support for the J-Curve model (Schwellnus et al., 2016), the committee duly cautioned that the model may not apply to elite athletes executing relatively high training loads. Indeed, the committee noted that such training loads within the context of highly-trained, elite athletes have not been compellingly linked to increased risk of illness.

The IOC caveat regarding elite athletes catalyzes a recognition and investigation of the Open Window Hypothesis. This hypothesis purports that immune function is inevitably compromised for some period subsequent to vigorous, intense exercise / training and that there is a consequent “open window,” or increased probability of infection(s), during the period following such exertions. While acknowledging the potential intuitive appeal of this construct and recognizing that there is a breadth of experimental literature that arguably supports such a hypothesis, very recent investigations have challenged this traditional hypothesis and support a reconsideration of the relationship between vigorous, intense exercise / training and immunocompetency.

PART II: THE EVIDENCE; WHAT DOES THE SCIENTIFIC LITERATURE CONVEY?

Debunking the Myth of Exercise-Induced Immune Suppression: Redefining the Impact of Exercise on Immunological Health across the Lifespan (2018)

Authors John Campbell and James Turner undertake a reasoned reinvestigation of the traditional perception that an acute bout of vigorous exercise transiently suppresses immune function. The authors argue, for example, that supposedly supportive evidence is minimally reliable, that classic markers of post-vigorous exercise / training immune function do not actually signal transient immune system suppression, and that the post-vigorous exercise / training immune response is actually a purposeful redistribution of immune system agents to peripheral tissues that reflect an amplified immune response and thus improved, post-vigorous exercise / training immunosurveillance.

The Compelling Link between Physical Activity and the Body’s Defense System (2019)

Imminent exercise immunologist David Nieman and co-author Laurel Wentz share a broad-ranging review that corroborates the Campbell and Turner (2018) contention that an acute bout of moderate-to-vigorous exercise / training is correctly portrayed to catalyze a transient redistribution of multiple, immune cell subtypes that support heightened, post-exercise / post-training immunosurveillance and thus enhanced immunological health.

Moreover, Neiman and Wentz synthesize multiple decades of accumulated evidence in order to emphasize that, for instance, chronic high-volume training workloads and uniquely demanding competitive pursuits (marathons and ultra-marathons, for example,) infuse a combination of physiologic, metabolic, and psychological stresses that appear to cause transient immune system depression and associated immune dysfunction.

In that regard, the authors note that the IOC continues to focus on a constellation of load management factors (training workload, competition workload, nutrition, sleep, hygiene, and psychological response, for example,) as a component of an ongoing commitment to optimize athlete health and safety.

Finally, this overarching review duly espouses that both traditional and novel, evidence-based nutritional strategies such as adequate, consistent carbohydrate intake during prolonged, intense exercise and consumption of biologically-active polyphenols are very specific to supporting and sustaining immunological health.

Can Exercise Affect Immune Function to Increase Susceptibility to Infection? (2020)

First author Richard Simpson and colleagues construct an engaging debate article that allows for a direct exchange of distinct perspectives from previously-referenced investigators such as David Neiman, John Campbell, James Turner, David Pyne, and other well-recognized, accomplished exercise immunology scholars.

Two points of general agreement include that 1) consistent bouts of moderate-to-vigorous exercise / training are beneficial for the normal functioning of the immune system and very likely contribute to a diminished risk for respiratory infection / illness and 2) susceptibility to infection and illness inevitably is a multifactorial phenomenon that necessarily includes explicit recognition of not only the specific nature of chronic physical activity / training but also allied factors such as sleep, stress, and nutrition.

Key points of ongoing debate that merit additional experimental insight and future resolution involve 1) whether athletes are more susceptible to illness and infection relative to the general population and 2) whether extended periods of chronic, intense workloads elevate illness and / or infection risk.

In combination, the distinct perspectives provide an excellent synthesis of the current state of exercise immunology research and remind all stakeholders that very challenging research questions remain to be definitively addressed.

Extracellular Superoxide Dismutase, a Molecular Transducer of Health Benefits of Exercise (2020) 

Accomplished University of Virginia researcher Zhen Yan and colleague Hannah Spaulding co-author an extensive literature review that has just been released via the most recent 2020 edition of the peer-reviewed, data-based publication Redox Biology.

Notably, the authors focus on a skeletal muscle enzyme, extracellular superoxide dismutase (EcSOD), a protein that is induced by endurance exercise and serves to mitigate the structural and functional oxidative cellular damage that can be manifest through a variety of pathological conditions.

In particular, the investigators emphasize evidence that the increased expression of skeletal muscle EcSOD supports a subsequent redistribution of that protein to peripheral tissues such as lung tissue. Within lung tissue, available data suggest that the protein does attenuate the previously-referenced structural and functional damage and correspondingly diminishes both the risk of developing and the severity of Acute Respiratory Distress Syndrome (ARDS). Centers for Disease Control and Prevention (CDC) data indicate that twenty to forty-two percent (20 – 42%) of patients hospitalized for COVID-19 will develop ARDS while pre-pandemic research suggests that approximately forty-five percent (45%) of patients who develop ARDS will die.

In summary, this very recent research review posits yet another profoundly positive, protective adaptation specific to continuous, large muscle group activity such as endurance exercise and, most notably, proposes an adaptation that is specifically applicable to the potential complications that may arise from COVID-19 infection.

PART III: RECOMMENDATIONS FOR SAFE PHYSICAL ACTIVITY & TRAINING

While recent experimental literature not only conveys select, consistent themes regarding the relationship between physical activity / training and immunological health but also reminds interested individuals that certain very challenging experimental questions continue to merit further exploration, one can evolve a series of reasoned, evidence-based recommendations for proponents and devotees of physical activity and / or training:

1) Emphasize the critical importance of consistent, daily movement for all populations. Irrespective of whether an individual is currently sedentary, mildly-active, recreationally-active, or highly, formally active (i.e. engaged in structured training), there are myriad health and fitness benefits to movement. Available evidence compellingly suggests that regular, moderate-intensity physical activity and formal training are both appropriate for and beneficial to immunological function.

2) Yet again irrespective of one’s current physical activity status, purposeful movement such as recreational physical activity, moderate-intensity exercise, and / or formal moderate-to-vigorous intensity training should always reflect defining fitness and conditioning principles such as specificity, progression, and overload. Assuming no physician restriction(s) to initiating relatively structured daily or near-daily physical activity, a previously sedentary individual, for example, need not and should not simply attempt to initiate vigorous, daily physical activity. Rather, that hypothetical individual might augment her / his activities of daily living by beginning to incorporate brief, mild, daily or near-daily walking bouts. Recognizing that there is persistent equivocality regarding the potential impact of chronic, vigorous exercise / training on acute immunological health, an elite athlete might be duly cautious in introducing any significant volume and / or intensity progressions into their current training.

3) Current guidelines for social distancing practices should be honored and practiced. While the case for daily or near-daily movement, physical activity, exercise, and / or training is absolutely compelling, honoring the existing guidelines for social distancing is entirely appropriate to minimizing the risk for COVID-19 transmission and infection.

4) Re-emphasize all the practices that support healthful movement, physical activity, exercise, and / or training. Most specifically, emphasize a) regular, consistent sleep b) sufficient, daily overall energy intake c) corresponding, sufficient daily carbohydrate and protein intake and d) adequate daily hydration.

5) Understand that there is not a universal, “one-size-fits-all” activity, exercise, or training prescription. One individual’s current movement / activity / exercise prescription might be five (5) minutes of daily or near-daily walking. By comparison, a highly-trained endurance athlete’s current training prescription could involve daily sessions that collectively encompass multiple hours of appropriately-intense, well-tolerated training stimuli. In either instance, it would be prudent to incorporate allied principles such as conservatism and consistency in order to optimize the probability of short-term and long-term health in addition to continuous fitness enhancement.

6) DO NOT allow COVID-19 to provide a reason or catalyst for neglecting a commitment to physical activity, health, and fitness. Indeed, the enduring pandemic evokes an all-too-vivid reminder of the defining importance of taking responsibility for enhancing and sustaining one’s health and fitness. Movement, physical activity, exercise, and / or training are clearly supportive of both immunological health and general health / fitness. Always be certain to secure a physician’s formal clearance and endorsement prior to formally initiating structured physical activity. Always be cognizant of and honor the principles that underlie prudent physical activity such as specificity and progression. Always be consistent, conservative, and prudent in all movement, activity, exercise and / or training decisions. Do not be mislead by general, non-evidence-based statements that movement, activity, exercise and / or training are inherently compromising to immunological health.

MOVE. BE ACTIVE. EXERCISE. TRAIN.

BE SMART. BE SAFE. BE HEALTHY. BE WELL!