High-Intensity Training & Immunity

By Dan Gwartney, MD

Don’t Let Your Ego Write Checks Your Body Can’t Cash

In the 1986 box office smash “Top Gun,” Tom Cruise played the character of a gutsy, daredevil combat pilot who excelled by pushing the envelope of his F-14 and himself. After the heroic rescue of another plane’s crew whose pilot had frozen after a Soviet jet got a missile lock on their F-14, Maverick (Tom Cruise) and his R.I.O., Goose (brilliantly played by Anthony Edwards), were summarily chewed out and complimented by their squadron commander who used the phrase oft-repeated by coaches, commanders and instructors: “Son, your ego is writing checks your body can’t cash.”

Nearly every athlete with the heart to compete has been warned of pushing herself too far— save certain of those born with the genetics and raw talent to surpass the less fortunate without exerting themselves. As the vast majority has to be disciplined and dedicated to maintain progress, it is understandable why naturally gifted athletes are scorned when they fail due to a poor work ethic or destructive lifestyle.

So, what happens to the hard-driving athlete who constantly gives 110 percent to set a personal record or prepare for a competition? Ideally, the result is progress with unprecedented growth and performance; too often though, burnout or overtraining occur. Progression turns to regression, frustration builds and many competitors push themselves even harder in a futile attempt to reverse the loss of muscle, strength or athletic prowess.

As frustrating as the effects of overtraining are on performance, the situation can easily worsen, as many have experienced. Overtraining is associated with the feelings of fatigue, depression, weakness, aches and frailty caused by overtaxing the body’s ability to recover physically. Yet, it is not just the neuromuscular system and endocrine organs that are affected by the stress. Another vital component of the human physiology is also greatly affected by excess or unaccustomed exercise that impacts one’s health as well as his progress as an athlete: the immune system.

Exercise is associated with health, as it should be, when practiced in moderation. However, high-intensity or high-volume exercise is interpreted by the body as a condition of severe stress, especially during periods of poor nutrition or hypocaloric dieting. Remember, the concept of recreational exercise is a modern phenomenon. The human body has evolved/adapted to essential activities associated with fulfilling life’s basic needs (food, clothing, shelter and breeding). If a primitive person were running, she was either hunting, being hunted or in combat. Prolonged running/fighting meant that conditions were dangerous and stressful. In such situations, injuries such as cuts, bites, scratches and wounds were more important to defend against or repair (from an immune system point of view) and the body was primed to divert resources from lower priority threats like viral infections (common colds, cold sores) which are handled by a separate subset of the immune system. If the conditions continued, the affected people would begin to succumb to colds, the flu, stomach infections, etc. Soon, the weakest would die, allowing the fittest a better chance of surviving, as the smaller population would require fewer resources as a group (less food during times of famine, fewer huts/caves/homes during periods of overcrowding, less water during drought, etc).

Modern medicine and domestic/international aid have altered the balance of nature, leading to the modern-day challenges of equating national resources with the demand for universal health care and entitlement programs. Of course, these are issues well outside the scope of this article.

The immune system deals with various threats to human health. Some are obvious, like an infected cut or the flu; others are less obvious. Researchers are actively developing immune-based therapies to fight many forms of cancer by stimulating the antibody response against tumor cells; other researchers are seeking to suppress immune reactions that attack normal, healthy cells in conditions like rheumatoid arthritis and certain kidney disorders.

For the common person, the immune system is taken for granted until it fails. Every year, sniffles and sneezes fill the school halls as students gather together in enclosed rooms, spreading every pathogen with their classmates and teachers. New parents who have not been sick in years will catch every runny nose bug once their kids enter day care. Prisoners are at high risk for the spread of many infectious diseases due to the confined quarters they share; tuberculosis is particularly worrisome.

Even for the athlete, recreational or competitive, there is no safe haven. Consider every fall and winter, when people come in to the gym blowing their nose, looking like death warmed over, hoping they can burn off the cold on the treadmill…maybe they are dedicated and don’t want to miss a workout regardless of how they feel. Great for them, but for the rest of the gym, breathing in spraylets of virus-laden vapor released with every cough and sneeze, that ill member is subject zero for the latest contagion. In many cases, the increase in illnesses is more a reflection of increased exposure (enclosed gyms, large crowds, shared equipment, deeper breathing, etc.) rather than a failure in immune protection.

Fortunately, the immune system does provide a great deal of protection, when coupled with hand washing if the sick person has the manners to cover his mouth when hacking out a lung. Yet, what happens if the immune system is compromised for some reason? People see a greater incidence of infection, particularly upper respiratory infections.

Anecdotally and epidemiologically, this happens in many groups of athletes, particularly as they increase the frequency, duration and/or intensity of training. Sadly, most coaches still adhere to the “Death March” mentality in preparing athletes for competition, often wearing them down so that they are no longer at peak functioning capacity on the days of competition. Wrestlers begin the season dropping weight, often over 15 percent of bodyweight in a short period of time through a combination of extreme dieting and excessive training; distance runners increase the weekly volume of their training runs in the months leading up to a marathon or ultra-endurance event; football players go through grueling two-a-day or even three-a-day practices to get ready for the home opener— all these examples are situations which the body recognizes as stress. As noted earlier, stress decreases the ability of the immune system to prevent or fight off viral infections; long-term, it may even affect health in more serious ways (e.g., increased risk of certain cancers).

There has been an explosion of research looking at how high-intensity exercise affects the immune system and whether it actually increases a person’s susceptibility to a viral infection. Though there is not a consensus (agreement), it is generally accepted that people at the extremes, exercise-wise, have a less robust immune system than people who exercise moderately. In other words, sedentary couch potatoes and those who are engaged in high-intensity exercise over a long period are more likely to get sick than people who exercise without killing themselves every workout. Many studies have shown that athletes in a variety of sports are more prone to upper respiratory illnesses.

Most of the research studied the effects of a single-, high-intensity or long-duration training session. The findings are reasonably consistent in stating that the immune system is impaired after training sessions lasting longer than 1 1/2 hours at moderate to high intensity (55 percent-75 percent VO2 max). This post-exercise immune dysfunction lasts for 3-24 hours and represents a window of opportunity for all the nasty bugs wanting to invade your healthy body.

The risk of a relative immune deficiency is greater if no calories are consumed during the training session, as is often the case— particularly in sports like wrestling and boxing. If the hardcore training persists for greater than one week, the effects on the immune system may last longer, effectively lowering the athlete’s resistance to colds, flu, etc. Interestingly, people who regularly include ~2 hours/day of moderate exercise have a lower risk of many infections.

An excellent review on the topic written by Michael Gleeson of Loughborough University discusses the basis for the immune response to high-intensity exercise and suggests some approaches to minimize the effect if necessary. Interestingly, Gleeson also discusses the possible long-term advantages to the immune-lowering effect of exercise that are not intuitively evident.

The change in immune function is multifactorial, meaning there are many different elements involved. Exercise, especially high-intensity or high-volume training, forces the body to pump out stress-related hormones to maintain blood sugar levels, chews up conditional fuel sources like BCAAs and glutamine, the blood becomes more acidic, body temperature rises and contracting muscles release a legion of cytokine messengers which affect the entire body.

Active muscle can increase metabolic demand 40 times the normal and that increase in demand has to be fed by burning calories. High-intensity exercise burns a higher percentage of carbohydrates for calories than low to moderate intensity and local stores of glycogen (a storage form of carbohydrate in the liver and muscle) only lasts for a brief time. The remainder of the sugar must come from the circulation or by converting certain amino acids into sugar, a process called gluconeogenesis. Many hormones are increased during immune-impairing exercise to meet the demand for sugar; coincidentally, some of these hormones also suppress the immune response (e.g., epinephrine, growth hormone, cortisol and prolactin).

As mentioned above, the muscle and liver will scavenge certain amino acids to create more sugar. Glutamine levels in the blood and inside muscle drop during immune-impairing exercise, which is relevant, as certain cells in the immune system use glutamine as a fuel source. Shuttling glutamine to active muscle or the liver for sugar production robs the lymphocytes of metabolic fuel, a point which many supplement companies have promoted though there are conflicting results among the studies as to whether supplementing the diet with glutamine improves immune function in athletes. As glutamine is relatively inexpensive and is readily found in high concentration in many protein powders, a dietary deficiency in this amino acid should be easy to avoid. However, the point remains that during prolonged exercise or high-intensity work, glutamine availability may be limited and affect immune status.

Though many of the hormones noted above relate to increasing sugar stores and are stimulated to a large degree by a drop in blood sugar, cortisol is unique. Cortisol is also a potent anti-inflammatory hormone, often used to treat herniated discs, organ rejection, allergies and other conditions related to an overly vigorous immune reaction. Often called the “stress hormone” and referred to as being catabolic, since high levels lead to muscle wasting, cortisol has an interesting relationship with working muscle. Like every other tissue in the body, muscle communicates with other tissues through a biochemical messenger system. This includes a class of chemical hormones called cytokines. The major player released by active muscle is IL-6, which has also been noted to be involved in the release of fat from fat cells and activates certain genes in muscle. IL-6 acts on two or more levels to increase the concentration of cortisol (increasing ACTH release from the pituitary gland and acting directly on the adrenals). Additionally, IL-6 induces the release of two other cytokines that have anti-inflammatory roles and suppresses the release of a potent activator of inflammation (TNF-α).

The surge in IL-6 release following exercise makes sense and accounts for the reason many people feel “looser” after warming up. Activity is often associated with trauma, especially if one looks back a few generations. It benefits the body to reduce inflammation following a long battle or hunt (or a workout). Also, structural damage to tendons or muscle fiber is allowed to heal rather than becoming inflamed and swollen. Consider the times an area afflicted with tendonitis feels better during a workout, only to swell and ache again the next day.

In addition to feeling better short-term, exercise-related IL-6 release may be responsible for many of the long-term health benefits associated with exercise. Promoting the anti-inflammatory effects of IL-6, though it does reduce the body’s ability to respond to infected tissue, reduces the negative effects of chronic low-grade inflammation— a condition seen in obesity and cardiovascular disease. It has been stated that a slightly higher risk of certain viral infections is a good trade-off for protection from cardiovascular and metabolic disease.

Most observations of the effect of exercise on the immune system relate to a single training session or look at extreme populations, such as ultra-endurance runners or wrestlers. What about the athlete who periodizes his workouts and is sensitive to overtraining? There appears to be many benefits for this person. At rest, the athlete and the couch potato have similar functional immune capacity; for instance, both groups respond equally well to vaccinations. Yet, as was noted earlier, people who include moderate exercise in their daily routine experience a 29 percent decrease in the rate of upper respiratory tract infections (sore throat, colds, runny nose, etc). Overtrained athletes tend to have a higher risk of infections, or the re-emergence of a recent infection (like a cold sore). Fortunately, it is extremely rare for an athlete to become clinically immune deficient due to overtraining. So, though sore throats and colds may be a problem, serious infections are still managed, assuming the athlete is otherwise healthy (e.g., not suffering from AIDS, eating disorders, etc).

Some might wish to reduce exercise-induced immune suppression, though as stated above, in the long run, it seems to offer many benefits to diseases that impact many Americans (heart disease, diabetes, etc.). It may make sense to try to block the exercise-related immune suppression in certain circumstance, for short periods of time. A major competition may be looming; perhaps one has already come down with a cold or has a cold sore; there may be a vacation ahead involving plane travel (nasty environment for spreading colds); or, it could be that the New Year’s resolution crowd is flocking to the gym wearing gray sweats and bearing a collection of rhinoviruses…all of these are reasonable considerations for avoiding an untimely sore throat.

Gleeson suggests some simple steps that are geared around lowering the oxidative stress related to exercise and preventing the stress response to low blood sugar levels. Four weeks of supplementing with vitamins C (500mg/day) and E (400 IU/day) markedly blunted IL-6 release from active muscle, with a corresponding decrease in cortisol. Drinking 30-60 grams of carbohydrates per hour while training protects against the decrease in a specific immune cell involved in antiviral protection. Normally, this class of cells decreases with exercise, but drinking a sugary drink blocked this effect. Many people are already downing antioxidants and swilling carbohydrate-laced drinks in the gym. This practice actually seems more prevalent with the “no-result” crowd that has been doing the same workout without any noticeable change for years. Perhaps the IL-6 suppressive effect of their carb drinks and those antioxidant cocktails accounts for some of their failure to manage their weight or see any apparent improvement? One research group in Copenhagen suggests this might be the case.

In the end, what is learned? Overdoing it not only creates wear and tear on the mind and muscle, but also sets a person up for greater risk of a cold, flu or sore throat. Yet, suppressing the immune function a bit through regular, moderate exercise appears to be well tolerated (with no increase in infections) while providing certain benefits related to long-term health, primarily by reducing inflammation. If a situation or event is scheduled that might cause a person to wish to have his immune function fully primed, cutting back on exercise a bit, adding a few simple anti-oxidants, some glutamine and a sugary drink to the routine may be all that is needed.

Common sense should tell us when we are overdoing it. If progress has halted or reversed, you are tired and irritable all the time and begin to catch every cold that comes around, it is time to step back and let your body recover. Once you have restored your physiologic equilibrium, work back toward your goals, utilizing periodization techniques and good nutrition to avoid overtraining again. Maverick pushed the envelope all the way to the point where he crashed and burned. Don’t be the athlete whose ego writes checks his body can’t cash.

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