Exercise, muscle mass, pain and the immune system
by Martin Krause: uploaded ~2001, updated Dec 2004, April 2005

- have you ever looked upon muscle as an important source of protein for the immune system?

- have you wondered whether exercise could enhance the immune response?

- have you considered the role of pain and inflammation during exercise and the immune response? 

Abstract

As we age, our adaptive immune system develope's specific responses as a result of antigen exposure.  The ability to develop these responses is in part dependent upon protein-telomere shortening of eukaryotic carrier sites.  These carrier sites represent protein endings which are splice off through mRNA.  Subsequently, as we age, the protein sequences shorten.  There is a finite limit to this shortening-splicing process. Therefore, the greater the protein mass in our youth, the greater the ability to respond to novel antigen exposure throughout life.  Unfortunately, the aging process (beyond the 3rd decade) also reduces protein quality and quantity in the greatest store of protein in our body i.e. muscle.  Furthermore, the innate immune system is also compromised by reduced muscle mass as the amount of natural killer (NK) cells are directly proportional to muscle mass in the aged population. This is particularly the case in people who commit to muscle atrophy through sedentary lifestyles, as well as those who deal poorly with psychogenic stressors which release catabolic hormones.  Therefore the importance of regular exercise cannot be underestimated.  A variety of exercise including cardiovascular, progressive resistance training and mind-body awareness (e.g. Yoga, Chi Gong, etc) is recommended. 

There is direct relationship with muscle mass and immune function. The better your muscle mass the better the reservoir of proteins for the immune system to use at times of need. However, when in vigorous training and/or over-training the protein is being used for muscle recuperation and repair and therefore little is left over for the immune system. Moreover, animal experiments have demonstrated impaired proliferative responses of mononuclear or spleen cells to mitogens (Sacerdote P et al 1994 Brain Behav Immun, 8, 251-260) and IgG antibody responses to novel antigen exposure (Laudenslager ML et al 1988 Brain Behav Immun, 2, 92-101 cited in Pain Clinical Updates XIII, No1) in the presence of intermittent pain.  Plausibly, immune suppression is more evident when you are doing potentially muscle damaging stuff such as downhill running, plyometrics , etc. I find it beneficial before doing such exercise to do an eccentric stretching regime as well as afterwards doing concentric X's such as bike riding.  Additionally, mind-body awareness and relaxation through yoga can be particularly useful.  These factors may then aid in reducing pain and inflammation associated with eccentric exercise.  Also keep in mind that if you've hit the flu season, plenty of freshly squeezed oranges are great and better than the bottled stuff.  Drink it within minutes of squeezing, otherwise the enzymes (due to lack of added preservatives and exposure to oxygen) start to break down the goodness.  Naturally, all other essential dietary vitamins and minerals are also important.  Emphasis has been placed on the omega 3 and 6 FFA's such as fish oil and evening primrose oil (EPO), anti-oxidants such as Vit E (slightly controversial recently), magnesium and Vitamin B's.  However, there are a plethora of other minerals which help catalyze metabolic reactions (e.g. Zinc, Molybdenum, etc).  Therefore, athletes should seek advice from a qualified sports dietician. 

Progressive resistance training (PRT) can enhance muscle function by creating hypertrophy of muscle tissue.  40-60 minutes of PRT, 3 times per week, using a gym based weights programme can enhance your ability to fight infection by increasing the protein reservoir residing in your muscle mass.  Ideally, 6-8 exercises are chosen which incorporate the largest muscle groups e.g. quadriceps, calfs, gluteals, pectorals, deltoids and trapezius.  Since cachexia is a cascade of cytokine-immune responses, then eccentric exercises should also be chosen, as this may 'fine tune' the regulation of the cytokinine-immune response from the microtrauma induced by exercise.  Additionally, such exercises can provide better functional stability and balance for ADL.  Concentric exercise such as cycling can enhance muscle bulk.  Ergogenic aids such as protein-carbohydrate supplementation, as well as creatine phosphate - HMB preparations taken during and/or within 20 minutes after cessation of exercise aids the restorative-regenerative training effect.  Creatine phosphate is also thought to improve insulin sensitivity. Similarly, moderate exercise every 72 hours is thought to reduce the susceptibility to metabolic syndrome (i.e. hypertension, hyper-cholesterol, hyper-lipidemia, artherosclerosis, cardiovascular disease, & insulin resistant diabetes) .  Since magnesium acts as an antithesis to calcium, it's supplementation may also aid in the prevention of injury, by improving the rate of muscle relaxation thereby reducing the chance of stiffening up and/or cramping during exercise as well enhancing the rate of recovery.  

Sarcopenia is characterized by the reduction in muscle mass and may contribute to immunosenescence, as it is thought that muscle provides an important reservoir of heat shock proteins (HSP) and glutamine as well as act as a site for the action of insulin. HSPs are the cellular link, which activate T lymphocyte proliferation. Exercise can activate HSPs and may provide the ‘danger signal' for T lymphocyte re-activation after a period of quiescence. Muscle glutamine appears to be important for inter-organ transport as well being a precursor for anti-oxidant activity. Dose specific exercise is thought to improve immune function through an improved ratio of TNF-a: IL-6, which may promote the anabolic effects of IGF-1 and insulin thereby possibly improving muscle mass, leading to enhanced mechanical loading tolerance. Additionally, older people with preserved muscle mass have the highest number of NK cells. The weight of evidence suggests that the preservation of muscle mass and/or reversal of sarcopenia through exercise could be a useful anabolic method to provide a protein reservoir for later use when the older person is exposed to infection, inflammation and/or severe trauma.  Click on diagram for PDF file

 

Introduction
Sarcopenia is characterized by reduced muscle mass [1]. This is significant as skeletal muscle contains 50-75% of all proteins in the human body and represents a store of energy and nitrogen which becomes a vital supply of fuel for the immune system, as well as a substrate for wound healing during malnutrition, injury and disease [2]. Loss of muscle mass may result in immunosenescence, which is characterized by impaired cellular immune function concomitant with increased inflammatory activity [3]. Little is known about the effects of exercise on the senescent immune system [3]. Moreover, exercise has been suggested as a prototype for studying the effects of stress factors on the cellular immune system, because several other physical stressors, such as burns, surgery, acute myocardial infarction and hyperthermia induce similar immune responses [3].

Muscle accounts for 90% of the cross sectional area in active young men, but only 30% of that area in frail older women [4]. The prevalence of clinically significant sarcopenia is estimated to range from 8.8% in 'young old' women to 17.5% in 'old old' men [1]. This review will entertain the notion that the loss of muscle mass due to sarcopenia attenuates the inflammatory-immune response due to the lack of protein stores (glutamine [5], heat shock protein [HSP] [6]). Cytokines such as Interleukin-6 (IL-6) could enhance the immune response through inhibition of Tumor Necrosis Factor ? (TNF-?) production and insulin resistance [7]. TNF-? is considered to enhance protein degradation through the activation of HSP [8], whereas Insulin [9, 10] and Insulin like Growth Factor (IGF-1) [11] are thought to enhance protein synthesis. Additionally, it will be hypothesized that dose specific exercise training can ameliorate some of the effects of sarcopenia as well as enhance immune function by balancing protein synthesis with proteolysis.

T lymphocyte immune responses

Cytokines are a group of low molecular weight regulatory proteins secreted by white blood cells and a variety of other cells which generally function as intercellular messenger molecules [12]. Proliferation of T lymphocytes is essential for the first step in an adaptive immune response [3]. Essentially there are 2 types of immune cells, the 'memory cells' and 'virgin cells'. With age, the percentage of memory cells increases with a corresponding decrease in virgin cells [3]. Consequently, a reduction in naïve cell responses, and a resulting shift to memory cell proliferative response has been shown in the older person [13]. Fortunately, with advancing age a progressive increase in natural killer (NK) cell number appears to compensate for a decreased number and function of T lymphocytes [14]. More importantly, these findings suggest that cells from older individuals do not suffer from a quantitative decline in cytokine production on a per cell basis [13]. Indeed, centenarians and nonagenarians who presented with the highest number of NK cells and best preserved cytolytic function, also had preserved integrity of muscle mass [14]. Therefore, this suggests that the maintenance of muscle cell mass could preserve the immune response to environmental stressors.

Muscle as a source of protein for immune function

Since muscle represents approximately 40% of body weight, it is thought to be an important reservoir of proteins [8], which may be called upon by the immune system in response to injury [10]. Even a 10% loss in lean body mass (LBM) corresponds with impaired immune function [15] and a loss of approximately 30% of the body proteins can result in death [2]. During severe trauma, such as burns, the need for essential amino acids drives the catabolic loss of protein from skeletal muscle, which can be as high as 1% per day of illness [16]. Accelerated muscle proteolysis is the primary cause of this loss of lean body mass characteristic of many diseases [17]. Some peptides generated by the breakdown of cell proteins are transported to the cell surface where they are presented to cytotoxic lymphocytes, which destroy cells presenting foreign (eg viral) peptides [17]. This may partially explain why successful aging has been associated with the preservation of muscle mass [14] as this would endow these individuals to draw on their store of protein for inflammatory-immune responses.

Increased cytokine activity has been associated with aging and muscle weakness [18], possibly resulting in inactivity. This age associated progressive dysregulation of immune response [19], is seen in older women with high IL-6 serum levels who have a higher risk of developing physical disability and experience steeper declines in walking ability than those with lower levels [18]. However, the statistical interaction of IL-6 concentration with disability was non-linear and the progression of disability and IL-6 concentration over time was not investigated [18]. Therefore, it is difficult to conclude whether elevated levels of IL-6 are the cause or the effect of skeletal muscle weakness, as the progressive withdrawal of a number of anabolic stimuli, also causes skeletal muscle weakness [18]. Furthermore, high levels of TNF-? and IL-6 have been shown to be associated with classical risk factors such as smoking, physical inactivity and body mass index (BMI) [20]. It is difficult to conclude whether the high levels of cytokines lead to inactivity or whether they are the result of reduced skeletal muscle loading capacity from inactivity. However a cycle of inactivity and chronic inflammatory-immune response is plausible.

Heat shock protein (HSP)

Numerous attempts to link exercise to meaningful alterations in immune function have been largely unconvincing [21]. It has been argued that it is local as opposed to the global immune system activation by HSPs, which is at the core of immune effects of exercise [21]. It is thought that HSPs on a cells surface activate T lymphocyte proliferation when accompanied by a co-stimulatory 'danger signal' [21]. These T lymphocytes can switch between the active and quiescent memory states, to be later re-activated in the presence of an antigen and co-stimulatory 'danger signal' [21]. Decreases in HSP72, a heat shock protein specifically related to skeletal muscle [22], is considered to promote apoptosis [23] possibly due to the lack of this secondary 'danger signal' [21]. Not surprisingly, apoptosis has been linked with sarcopenia [24]. HSP are involved in protein folding and sorting, in the assembly of protein complexes as well as binding of denatured proteins and are primarily induced in response to stress [6]. Impaired recovery from acute complications and the reduced renewal of damaged and toxic proteins are potential undesired consequences of low-protein turnover [25]. Indeed, critically ill patients have been shown to have low protein synthesis in skeletal muscle correlating with metabolic status and clinical indices of the severity of the disease [26]. Conversely, specific adaptation in muscle associated with enhanced proteolysis can occur [17], whereby HSPs provide a link between immune response to infection and autoimmunity during fever [27]. In fact older people have been shown to have a reduced incidence of fever in response to injury [28]. Although, exercise induced HSP activity has been described [6, 22] it is uncertain what level of exercise can provide the 'danger signal' required to enhance immune function [21].

Exercise may provide a hormonal stimulus to regulate HSP proteolysis. Recently, rodent investigations have demonstrated that Insulin-Like Growth Factor 1 (IGF-1) inhibits both lysosomal and ubiqitin-proteasome dependent stress protein breakdown in skeletal muscle [11] thus suggesting a hormonal regulating mechanism. Increased IGF-1 concentrations have been demonstrated near the Z-bands in the elderly after resistance training regime [29]. In particular, eccentric exercises have been associated with damage to these Z-bands [30]. Therefore, IGF-1 production in muscle may be responsible for the regulation of protein synthesis after HSP induced proteolysis as a result of exercise induced trauma.

Muscle glutamine and inflammatory-immune response

Skeletal muscle may represent an important source of anti-oxidants. Alterations of respiration in mitochondria of muscle cells has been associated with aging and sarcopenia [10, 31-34]. Interestingly, the muscle protein glutamine is a substrate for glutathione, which acts as an endogenous scavenger with an ability to counteract oxidative injury from oxygen free radicals [16]. This may be particularly important, as the level of oxidative stress imposed on the aging muscle is influenced by two fundamental biological processes: the increased generation of reactive oxygen species (ROS) and age-associated changes in antioxidant defense [33, 35]. Therefore, since glutamine accounts for nearly two thirds of the free intracellular amino acid pool and is abundant in skeletal muscle [16] it is likely to be an important source of anti-oxidants.

It is thought that TNF may mediate protein degradation in cachexia through a ubiquitin-proteasome pathway [8]. This skeletal muscle proteolysis seen in response to severe injury has been speculated to be for the provision of precursors for glutamine synthesis [16]. Glutamine is also thought to be a significant inter-organ nitrogen and carbon transporter as well as being important in glycogen metabolism and therefore can presumably affect mitochondrial oxidative responses [16]. Additionally, in the fasting state, an increase in glucocorticoids and reduction in insulin results in muscle proteolysis [17] presumably through an inter-organ mechanism. During acidosis, some amino acids from muscle protein are converted to glutamine, which is used by the kidneys in acid excretion and energy metabolism [17] (figure 1). Therefore, muscle may represent an important anti-oxidative organ during an inflammatory-immune response. Hence, in the presence of sarcopenia, reduced ability to counteract damage by oxidative radicals may be expected due to reduced glutamine stores.

Implications for exercise training and immune function

Investigators have recently demonstrated that muscle contractions induce the production and release of IL-6 but not TNF-? into the circulation [7]. Contrary to the conclusions of Ferrucci et al (2002) [18], other authors suggested that muscle-derived IL-6 contributes to mediate the beneficial metabolic effects of exercise and may inhibit TNF-? production and insulin resistance [7]. Administration of insulin has been shown to promote protein synthesis [9]. Furthermore, resistance exercise training has been shown to attenuate the catabolic effect of TNF by suppressing skeletal muscle TNF-? expression [36]. However, a 12 week high intensity progressive resistance training program did not affect immune function in healthy older people or subjects with systemic inflammation [37]. Yet, eccentric exercise is associated with an increase in pro-inflammatory cytokines [3, 30]; whereas concentric exercise has been associated with inflammation from oxidative stress [32] and hydrogen ion accumulation due to hypoperfusion [5]. Furthermore, a recent review concluded that older people have a preserved ability to recruit T lymphocytes and NK cells in response to exercise [3]. However, the cells recruited had a replicative history suggesting they were memory cells. Additionally, a lack of investigations into circulating levels of pro-inflammatory cytokines during eccentric exercise in older versus young populations was lamented by these authors [3]. Nevertheless, it would appear that resistance training could be beneficial for IL-6 production as well as TNF-? suppression post exercise.

Although investigators have demonstrated enhanced muscle size with resistance training in older people [38], there is little evidence to suggest that this change in size is sufficient to improve immune function [3, 39]. Yet, resistance training has been shown to significantly increase lean body mass and strength in HIV associated muscle wasting [40], therefore indicating a role for exercise in the presence of protease inhibitors and retroviral activity. Additionally, improved lean body mass and reduced fat has been shown to enhance IGF-1 levels during endurance training [41]. Consensus indicates that 'moderate exercise' may enhance immune function and may reduce the incidence of infection while long term exhaustive exercise results in immuno-suppression and an increased susceptibility to infections [12, 42-45]. This is consistent with Ji [33] who suggest that the major benefit of non-exhaustive exercise is to induce a mild oxidative stress that stimulates the expression of antioxidant enzymes, as well as the induction of IGF-1 [11] seen in resistance training [29,46]. Additionally, resistance training accompanied by nutritional supplementation has been shown to result in significant muscle hypertrophy [47]. Biomechanical principles dictate that for the same force, the strain in a skeletal muscle is reduced proportional to the skeletal muscle's cross sectional area [48]. Therefore, if contractile skeletal muscle mass is maintained or enhanced, then it is plausible that a greater spectrum of 'moderate exercise' can be entertained.

Highly conditioned elderly humans seem to have a better preserved immune system, although it is not possible to conclude if this is linked to training or other lifestyle-related factors [3]. Specifically, a bi-directional neuro-immune system has been implicated for exercise induced modulation of immune function through the autonomic nervous system [49]. Importantly, the effect of exercise on immune function requires future investigation using dose specific criteria [39, 50] as over-training is characterized by reduced catecholamine levels, decreases in neutrophil function, serum and salivary immunoglobulin concentrations and NK cell numbers and possibly cytotoxic activity in peripheral blood, at least in younger individuals [51]. In particular, the 2-4 hours post exercise have been shown to demonstrate inflammation with concomitant suppression of immune function, especially after high intensity, eccentric exercise in young populations [5, 12]. If these findings can be extrapolated to older people then the periodization of training and careful selection of the type, volume, frequency, duration and intensity of exercise would appear to be important to gain the maximum anabolic and minimum catabolic effect. Presumably, this anabolic effect would reduce the prevalence of sarcopenia through the prevention or reversal of muscle mass loss thereby concomitantly ameliorating immunosenescence.

Conclusion

Sarcopenia is characterized by the reduction in muscle mass and may contribute to immunosenescence as muscle is thought to provide an important reservoir of heat shock proteins and glutamine as well as to represent a site for the action of insulin. HSPs are the cellular link, which activate T lymphocyte proliferation. Exercise can activate HSPs and may provide the 'danger signal' for T lymphocyte re-activation after a period of quiescence. Muscle glutamine appears to be important for inter-organ transport as well being a precursor for anti-oxidant activity. Gathering evidence suggests that dose specific exercise could improve immune function through an improved ratio of TNF-?: IL-6, which promotes the anabolic effects of IGF-1 and insulin thereby improving muscle mass, which can lead to enhanced mechanical loading tolerance. Additionally, older people with preserved muscle mass have the highest number of NK cells. Taken together the evidence suggests that the preservation of muscle mass and/or reversal of sarcopenia through exercise could improve the protein reservoir for a person to draw upon when they are exposed to infection, inflammation and/or severe trauma.

Exercise and pain

Individuals may exhibit reduced physical activity due to pain related 'fear-avoidance' behaviour.  Unfortunately, this results in a viscous cycle of muscle atrophy and predisposition to muscle inflammation and injury.   Cardiovascular training may stimulate the carotid sinus reflex, which via the ventral noradrenegic bundles (VNB) of the Locus Coeruleus to the Solitary Nucleus (Nucleus Tractus Solitarius) can provide descending modulation of pain in the neurons of the spinal cord.

 

The Locus Coeruleus is the origin of most noradrenergic neurons in the CNS.  This pontine nucleus resides bilaterally next to the 4th ventricle.  There are both dorsal and ventral bundles of neurones which have descending, ascending and cerebellar projections. The dorsal noradrenergic bundle (DNB) projects throughout the limbic system and neocotex and therefore has substantial influence on higher level brain activity.  Any stimulus that threatens the biological, psychological or psychosocial integrity of the individual increases the firing of the Locus Coeruleus (LC).  Thus, the LC is responsible for global vigilance for threatening and harmful stimuli. Interestingly the LC remains active during waking and non-REM sleep.  Therefore, REM sleep is probably important for replenishing noradrenergic stores in the LC during suppression of sympathetic tone. 

Does a relationship with chronic fatigue, bone density, thyroid function and the peripheral sympathetic nervous system exist? ? ?

The central sympathetic nervous system

Neurons in the medullary reticular formation project to the hypothalamus via the ventral noradrenergic bundle of the the LC.  The hypothalamic paraventricular nucleus (PVN) coordinates the HPA axis.  Neurons of the PVN receive afferent information from the ventrolateral medulla, dorsal raphe nucleus, nucleus raphe magnus, LC, dorsomedial nucleus and nuclues tractus solitarius.  The PVN sends direct projections to the sympathetic intermediolateral cell column of the spinal cord, which in turn sends projections to the peripheral sympathetic nervous system.  The peripheral sympathetic nervous system innervates the lymphatic glands. Prolonged nociception may cause sustained stress responses.  Clinically this may manifest as fatigue, dysphoria, myalgia, nonrestorative sleep, somatic hypervigilence, reduced apetite and libido, impaired physical functioning and reduced concentration. Interestingly, these are also the signs and symptoms of over training. Well structured training regimes have the capacity to engage the body's capacity to regenerate and repair.

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Rises in Estrogen levels around ovulation have recently been demonstrated to increase the risk of traumatic knee injury in athletes.  Furthermore, researchers hypothesized that oral monophasic contraceptive pill may reduce the incidence of cruciate ligament rupture. See the ABC website Catalyst for transcript, Thursday, 7th April 2005.