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Immunology, Nutrition, and Exercise, Part I
Dr. Jason Barker, ND
With the ever-increasing popularity of endurance-type sporting events (marathons,triathlons, and “ultra" versions of these and other combined sports), the study ofimmunology in the athlete continues to grow as a burgeoning field. More practically,preventative and natural medicine-oriented physicians continue to provide a role inserving this sector of the population as research continues to explore the role ofnutraceutical and botanical medicines and their role in athletic performance andimmunologic function. The ever-increasing number of such grueling competitions thatchallenge the limits of human endurance continues to push the boundaries of currentdietary recommendations for these athletes. The selection of diet is dependent on factorssuch as the type and duration of the event, recovery time, and total energy expended.
Another interesting new challenge involves exploring the effects of endurance exerciseon the athlete’s immune system. Well-established in the literature, moderate, near dailyexercise leads to positive changes in the immune system that correlates with less frequentupper respiratory tract infections. Therefore, a commonly held belief is that moderate(near-daily physical activity of 30 minutes or more) exercise benefits the immune system.An example of this was detailed in a study during which a group of people that walkedbriskly on a near-daily basis were compared to another group of completely sedentaryindividuals; the walkers experienced a greater than 50% decrease in sick days over aperiod of 3-4 months; these changes were noted without observed immune functionchanges.1 It is hypothesized that the improvement in host protection andimmunosurveillance that is concordant with moderate exercise is related to an additiveeffect from the acute positive immunologic changes that occur with individual exercisesessions. Additionally, the benefits of moderate exercise may be applied to numerousdisease processes as both a preventative and treatment therapy. Over time the mostnotable benefit of a regular, moderate exercise program equates to less sick days with thecommon cold or other upper respiratory tract infections.2 This is not to negate the otherfar-reaching effects of exercise on several disease processes; epidemiologic studiesdemonstrate a beneficial effect from exercise at decreasing risk from heart disease,stroke, hypertension, diabetes mellitus (type 2), osteoporosis, falls and fractures in theelderly, dementia, anxiety, and depression. In addition, this research shows a link, albeitweaker, between sedentary lifestyles and higher risks of colon, breast, and lung cancers.3Exercise is without a doubt perhaps the single greatest contributor to one’s state of health;nearly every person can benefit from some form of physical activity.
On the other hand, intensive, prolonged exercise causes several changes in immunefunction that are a reflection of the physiologic stress and suppression endured in suchcircumstances; training of this type leads to an increased predisposition to upperrespiratory tract infections (URTIs) and excessive physical exertion that extends beyondroughly 90 minutes is correlated with adverse immune system changes in several areasincluding the skin, upper respiratory tract mucosa, blood, lung tissue, and muscle.2
The risk of exercise-related infection seems to follow a J-shaped curve, with nonexercisersexperiencing more infections in comparison to regular, moderate exerciserswhile those that exercise regularly at a strenuous level have the highest risk. This risk ofinfection continues to grow when factors such as travel (exposure to new pathogens),inadequate sleep, weight loss, excessive mental stress and inadequate nutrition are addedto an aggressive exercise regimen. Following strenuous exercise, athletes enter a periodof time in which they experience impaired immune resistance in which they are moresusceptible to URTIs. However a definitive causal relationship has not yet been clearlydefined.4 According to Neiman, anecdotal, survey, and epidemiologic data all correlatewith an increased risk of URTI among endurance athletes both during and following(perhaps up to 1-2 weeks) periods of intensive training.5 The period of time in which theathlete is more susceptible to infection is referred to as the open window; the timeappears to vary from as little as three hours to seventy-two hours.6
In a review of the literature focusing on the effects of chronic exercise training on humanimmune function, it was revealed that although immune cell numbers remain at normallevels during the training episodes, other evidence shows slight impairment in immuneparameters such as neutrophil function, serum and mucosal immunoglobulin levels,plasma glutamine concentration, and cytotoxicity of natural killer cells.7Whether an athlete is clinically immunosuppressed or not, the possibility exists that thecombined effects of these small changes in immune function may contribute to acompromised resistance to minor illnesses thereby having a detrimental effect onperformance and by preventing the athlete from competing at their maximal level.
Exercise and Immune Function
In an attempt to better understand the mechanisms by which heavy exercise traininginfluences infection resistance, several recent studies have attempted to identify theseeffects.8 Among these findings are altered numbers of circulating leukocytes and theirsubsets, plasma cytokine concentrations, natural killer cell activity, rate of secretoryimmunoglobulin A secretion, as well as neutrophil and macrophage phagocyte activity.This research also indicates that these perturbations may persist for several hours to daysfollowing the exercise while some athletes have shown decreased resting/post exercisevalues of non-specific immune parameters such as complement, acute phase proteins, andneutrophil activation. Additionally, chronically intensive exercisers displayed progressivedecreases in neutrophil function and certain unspecified subclasses of serum andsecretory immunoglobulins.
Other studies indicate the role of T lymphocytes in exercise and immune function;decreased T-lymphocyte function has been observed following intensive exercise, andthis is suspected to be a part of the causality of increased URTI among certain athletes.9Intense exercise may also suppress mucosa-related immune parameters as well; onerecent study noted a decrease in salivary IgA and IgM concentrations following intenseperiods of exercise and that the degree of immune suppression and rate of recoveryfollowing exercise were associated with exercise intensity and duration.10
Decreased levels of salivary IgA and IgM are associated with an increased risk ofrespiratory illness in athletes; however the mechanisms for this mucosal immune cellsuppression are not known at this time. These data represent only a small fraction of theresearch in this area; it does suggest that highly trained athletes suffer from immunesuppression that is clinically relevant. In addition, it is important to note that thepsychological stress associated with training and competing at an elite level may also be afactor in the effects of intense exercise on the immune system.
While at first glance the increased risk of viral URTIs in endurance athletes seems to betrivial, for the competitive athlete such illnesses can be the deciding factor in whether anathlete can remain competitive or not. Additionally, the prevalence of such infectionsappears to be quite high in this group, both statistically and on a clinical basis. Althoughactual numbers are not available, the prevalence of athletes one may encounter withpersistent URI-type illnesses is quite high and is a popular topic. One often-quotedexample is a study based on participants in a marathon event. Those who trained at 97kilometers or more per week experienced twice the URIs compared to those who trainedat 32 kilometers per week, while the faster runners in the marathon had a 6-fold increasein URI in the week following the race.11 Infections of minor severity can lead to missedworkouts or competition, while actual physical performance decreases performancegenerally. Surprisingly, the exact mechanism of performance degradation has beendebated, with one source weighing decreased motivation against the actual effects of thedisease!12 Performance following a URTI generally rebounds quite vigorously once themajor symptoms have subsided, however some athletes may be saddled additionally bypostviral fatigue syndrome.13
In general, URTIs may be treated by a moderation of training and other medicines.However, once systemic, the severity of illness can become worse; signs of systemic viralinfections include fever, myalgia, cough, vomiting, diarrhea, fatigue andlymphadenopathy. Those with symptoms such as this should temporarily discontinueexercise altogether, until symptoms are gone.
Preserving Immune Function
Many factors are suspect in exercise-related immune depression; researchers continue toexplore nutritional influences that can alter this phenomenon. Of the many possiblecauses for immune dysfunction in athletes, inadequate and or inappropriate nutritionalintake seems to take precedence as evidenced by the literature. In general, insufficientdietary intake of protein and certain micronutrients have long been known to adverselyaffect immune function, regardless of fitness caliber. Fats and carbohydrates play a roleas well, and in some instances, competitive athletes may adopt unusual diets in order toenhance performance thereby predisposing them to immunologic challenges.
Fat intake (and large increase in body fat, although unlikely in endurance athletes), inexcessive amounts is detrimental to immune function, while insufficient fat intake andconcomitant reduction in body mass can impair immune function in an already leanathlete. Likewise, moderate intake of polyunsaturated fatty acids (PUFAs) has manypositive health effects for the athlete. PUFAs (namely omega-3-type) exert beneficialeffects on immune function14 and play a major role in altering inflammatory processes,making them useful for injury recovery as well.
Carbohydrates in particular have been shown to have impressive effects on the immunefunction of athletes engaged in long-term fitness events. Athletes that perform in acarbohydrate-depleted state have greater increases in circulating “stress" hormones andwider negative variations in immune functions while those that consume 30 to 60 gramsof carbohydrate during sustained, intense exercise appears to decrease the rise in stresshormones and limits the degree of immune suppression as well.15 Among the effects ofcarbohydrate ingestion during competition include higher plasma glucose, decreasedcortisol and growth hormone, stabilization of leukocyte counts, lower granulocyte andmonocyte phagocytosis and oxidative burst activity, as well as decreased pro and anti inflammatorycytokine release. Investigators have not concluded, however, thatcarbohydrate ingestion during competition decreases the amount of infections followingintensive exercise.
The majority of athletes demonstrate only minimal deficiencies in essential nutrients (andare therefore far from maintaining optimal nutrient levels). Among the most commonmicronutrients that are deficient in athletes and which have the greatest impact onimmune function are the vitamins A, E, B-6, B-12, folate, essential fatty acids, the aminoacids glutamine, arginine, and L-carnitine, as well as trace elements.16, 17
Psychological Influences
The link between the hypothalamus and immune function allows for neurologicperceptions to modulate the impact of physical activity on physiologic systems, includingimmune cells. Psychological stress and intense physical activity appear to have similareffects; both induce similar neurohormonal responses including secretion of cortisol,growth hormone, epinephrine and norepinephrine. Undoubtedly, athletes participating incompetition experience more intense psychological challenges in comparison to athletesworking at similar intensity in a laboratory setting thereby predisposing them to greaterpotential immune dysfunction. One study demonstrated an increased propensity for thosewho restricted social contacts and/or spent excessive amounts of time in goal-orientedactivities to suffer from a greater number of upper respiratory infections;18 theseparticular lifestyle patterns are similar to those of athletes pursuing intensive athleticcompetitive goals. Stress modulation is an important tool (perhaps as important asnutrition) when considering treatment options for prevention of exercise-inducedimmunosuppression.
Maintaining nutritional health in athletes is a strongly debated and controversial topic.Practitioners must assess the individual’s nutritional needs and at the minimum ensurethat the basic requirements for nutrition are met. Adequate energy intake must be metwith a correctly balanced ratio of macronutrients; this ratio differs from person to personand is also heavily dependent on the fitness activity of choice. An essential point is thatthe application of nutritional therapies to endurance athletes cannot necessarily be appliedusing a one-fits-all, cookbook approach to diet and nutrition. The use of immune-boosting supplements must be applied in much the same way; the field of naturalmedicine is quite full of numerous products that alter immune function in a variety ofways; selection of the correct therapy for the athlete becomes a more complex issue whentaking the entire person, as an individual, into account. The next article in this series willaddress individual nutrients and botanicals and their application to the immune system inexercise-induced immunosuppression.
Article References
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9 Green KJ. Improving understanding of exercise effects on in vitro T-lymphocytefunction--the role of fluorescent cell division tracking. Exerc Immunol Rev. 2002;8:101-15.
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11 Nieman DC, Johanssen LM, Lee JW, Arabatzis K. Infectious episodes in runnersbefore and after the Los Angeles Marathon.J Sports Med Phys Fitness. 1990 Sep;30(3):316-28.
12 Shephard RJ: Physical Activity, Training and the Immune Response. Carmel, IN,Cooper Publications Group, 1997
13 Behan PO, Behan WM, Gow JW, et al: Enteroviruses and postviral fatigue syndrome.CIBA Foundation Symposium 1993;173:146-159
14 Stulnig TM. Immunomodulation by polyunsaturated fatty acids: mechanisms andeffects. Int Arch Allergy Immunol. 2003 Dec;132(4):310-21
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18 Totman R, Kiff J, Reed SE, Craig JW. Predicting experimental colds in volunteersfrom different measures of recent life stress.J Psychosom Res. 1980;24(3-4):155-63.