Biochemical Aspects of Over Training in Endurance.5

Biochemical Aspects of Overtraining inEndurance SportsA Review

Cyril Petibois,1,2,3 Georges Cazorla,1,3 Jacques-Rémi Poortmans4 and Gérard Déléris1,2

1 University Victor Segalen Bordeaux 2, Bordeaux, France2 Bio-Organic Chemistry Group, Bordeaux, France3 Faculty of Sport Sciences and Physical Education, Talence, France4 Free University of Brussels, Brussels, Belgium

Contents Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8671. Metabolic Aspects of Overtraining . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 868

1.1 The Skeletal Muscle Structure Hypothesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8681.2 The Carbohydrate Hypothesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8701.3 The Branched-Chain Amino Acid Hypothesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8701.4 The Glutamine Hypothesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8711.5 The Polyunsaturated Fatty Acid Hypothesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8721.6 The Leptin Hypothesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 872

2. The Protein Metabolism Hypothesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8733. Validity of Clinical Analyses to Diagnose Overtraining . . . . . . . . . . . . . . . . . . . . . . . . . 874

3.1 Summary of Biochemical Markers of Overtraining . . . . . . . . . . . . . . . . . . . . . . . . . 8743.2 Perspectives for a Clinical Diagnosis of Overtraining . . . . . . . . . . . . . . . . . . . . . . . . 875

4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 875

Abstract Top-level performances in endurance sports require several years of hard train-ing loads. A major objective of this endurance training is to reach the most ele-vated metabolic adaptations the athlete will be able to support. As a consequence,overtraining is a recurrent problem that highly-trained athletes may experienceduring their career. Many studies have revealed that overtraining could be high-lighted by various biochemical markers but a principal discrepancy in the diag-nosis of overtraining stems from the fact that none of these markers may beconsidered as universal. In endurance sports, the metabolic aspects of trainingfatigue appear to be the most relevant parameters that may characterise overtrain-ing when recovery is not sufficient, or when dietary habits do not allow an optimalreplenishment of substrate stores. From the skeletal muscle functions to the over-all energetic substrate availability during exercise, six metabolic schemes havebeen studied in relation to overtraining, each one related to a central parameter,i.e. carbohydrates, branched-chain amino acids, glutamine, polyunsaturated fattyacids, leptin, and proteins.

REVIEW ARTICLE Sports Med 2002; 32 (13): 867-8780112-1642/02/0013-0867/$25.00/0

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We summarise the current knowledge on these metabolic hypotheses regard-ing the occurrence of overtraining in endurance sports.

Fatigue has been defined as an inability to main-tain a given exercise intensity[1] and may be con-sidered as an alarm signal from the organism indi-cating a stress situation diminishing its initialcapacities. On the other hand, fatigue inductionthrough training exercises is the first rule of thetraining adaptation process to improve athlete ca-pacities by stimulating organism functions. Thebalance between stress and recovery factors de-fines the quality of the training programme. How-ever, the stress limits and minimal recovery peri-ods an athlete must use to optimise his/her trainingprogramme are not known and must be indi-vidualised. To date, to optimise training, it isvery tempting to reduce the recovery periods andincrease the training loads as long as fatigue seemsbearable. Overstepping this maximal ability totrain may result in a fatigue accumulation, and pos-sibly overtraining. One consequence is a greatersusceptibility to various pathologies, such as asthe-nia,[2] upper respiratory tract infections (URTIs)[3]

and viral or bacterial infections,[4] which may beattributed to an impairment of the body defences.[5]

Another consequence is that recovery from over-training may take several weeks or months withoutphysical activity before returning to trainingagain.[6]

It has been estimated that 70% of high-level en-durance athletes have experienced (or will experi-ence) overtraining during their career.[7] However,the exact aetiology of overtraining is not fully un-derstood[8] and there is no universal tool to predictits occurrence before it is clinically diagnosed. Theonly available diagnostic tool is a recurrent perfor-mance decrement while maintaining or increasingtraining load.[9] This late diagnosis may result inthe loss of several months in the training pro-gramme due to a long-lasting recovery period.Therefore, diagnostic tools are needed to recognisenew cases of overtraining and, if possible, to pre-vent its occurrence. For endurance sports, there isa growing body of evidence that overtraining stems

from dysfunctions in carbohydrate, lipid, and/orprotein metabolism.

1. Metabolic Aspects of Overtraining

In endurance sports, a heavy training load con-sisting of repetitive long-duration exercises at spe-cific intensities is necessary to enhance given me-tabolic pathways for energetic supply to skeletalmuscles. The major metabolic adaptations to en-durance training load have been situated within theskeletal muscle cell,[10] liver,[11] and kidney.[12] In-deed, these locations are potentially implied in me-tabolic aspects of the overtraining process. An-other consequence is that the study of overtrainingrequires taking into account parameters from dif-ferent biological tissues and/or to combine differ-ent analytical approaches, i.e. biochemical, physi-ological, endocrine, neuronal, myologic; each onebeing potentially involved in the understanding ofthe metabolic aspects of overtraining. In the pasttwo decades, many studies have been conducted toinvestigate given biochemical parameters impliedin the occurrence of overtraining or to diagnose it.There is a growing body of evidence that only alimited number of metabolic schemes may high-light overtraining occurrence. To date, six metabo-lic schemes seem to provide pertinent informationabout overtraining occurrence. In this review, wepropose to summarise the current knowledge onthese metabolic schemes, each one centred arounda key parameter, i.e. carbohydrates,[13] branched-chain amino acids (BCAA),[14] glutamine,[15] poly-unsaturated fatty acids (PUFAs),[16] leptin,[17] andproteins.[6] As a starting point in this review, thehypothesis that mechanical and/or chemical stresson myocytes may favour or induce overtraining ispresented.

1.1 The Skeletal Muscle Structure Hypothesis

During intense and/or eccentric exercises, my-ocyte alterations may be induced, either by me-chanical (disruption of cellular architectural pro-

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teins) or metabolic stress (chemical aggressions onsubcellular contents). During oxido-reduction pro-cesses, highly reactive oxygen species (ROS) arecontinuously produced since 1 to 3% of oxygen isincompletely reduced.[18] Successive reactionsmay occur from ROS, producing superoxide an-ions (O2–) that may induce peroxidation of thephospholipids located within skeletal muscle cellmembranes.[19] Hydroperoxide (H2O2) may alsobe produced and this generates hydroxyl radicals(OH) in the presence of Fe2+, which are highly re-active and attack other families of biomolecules,namely proteins, DNA and lipids. Lipidic radicalsmay also be produced from hydroxyl radicals bysubtracting hydrogen to PUFAs, which leads to theformation of lipoperoxyl (LOO) and alkoxyl (LO)radicals, and aldehydes (malondialdehyde) as by-products. These have been found to alter skeletalmuscle cell membrane functions.[20] Peroxidationof the lipids located in cellular membranes havebeen observed during intense exercise by measur-ing plasma malondialdehyde concentration.[21] Ithas also been postulated that superoxide anion pro-duction (O2–) might induce oxidation of the cate-cholamines implied in the mobilisation of sub-strates used by skeletal muscles during enduranceexercise.[22]

During exercise, oxygen consumption may in-crease up to 40-fold. As a consequence, ROS pro-duction is also greatly increased.[21] A defencesystem exists, which includes enzymatic andnon-enzymatic (vitamin) actions to reduce ROSaggressions within skeletal muscle cells. This de-fence system may be enhanced by endurance train-ing. An imbalance between ROS actions and anti-oxidative defence capacities of skeletal musclecells has been suggested to be a potential factorfacilitating overtraining occurrence.[23] However,it has never been demonstrated that a long-termperoxidation of skeletal muscle cell biomolecules(phospholipids and contractile proteins) could suf-ficiently alter the cellular functions to induce over-training.[19,20,24] Nevertheless, it is well known thatROS actions increase membranes’ permeability,releasing several cellular biomolecules that may

be measured within blood, namely creatine phos-phokinase (CPK), myoglobin, skeletal troponine I(sTi) and 3-methylhistidine. Blood release of 3-methylhistidine results from contractile proteindegradation and its concentration may remain ele-vated up to 72 hours after an exhaustive enduranceexercise.[25] The CPK enzyme activity depends onthe cytosolic biochemical equilibrium for the cou-pling between metabolic (myoglobin) and contrac-tile (sTi) proteins. It has been proposed to measureCPK blood concentrations during and after intenseendurance exercises to study athletes’ recovery ca-pacities.[26,27] However, since membrane perme-ability usually remains elevated 48 to 96 hours af-ter exercise, the CPK release within the bloodstream will necessarily follow the membrane re-structuring kinetic.[9,28] Moreover, this enzymediffuses within the blood stream whether skeletalmuscle cell damage is reversed in a few hours ornot, and its presence is constant in most cells ofthe organism for adenosine triphosphate (ATP)resynthetic processes. Indeed, whatever the impor-tance of the skeletal muscle cell damage, bloodCPK may be found to be elevated after intense ex-ercises.[29,30] As a consequence, its utilisation forhighlighting overtraining occurrence during heavytraining periods would be very difficult to appre-ciate. Myoglobin is a free cytosolic protein respon-sible of O2 transport into mitochondria. It has beenshown that this protein diffuses easily within theblood stream while cellular membrane permeabil-ity increases.[31] Oxidative-type fibres are the mostexposed to ROS peroxidations, leading to abun-dant release of myoglobin out of the skeletal mus-cle cell area. The study of its blood concentrationkinetic following intense endurance exercises ap-pears to be useful in estimating the chemical stresslevel in skeletal muscle cells and to give informa-tion about the fibre which is the most damaged.[31]

However, it has never been demonstrated that themuscular proteins found in high concentrationswithin blood could be sensitive parameters for dis-criminating reversible training fatigue from over-training.[32] Peroxidation processes do not appearto induce overtraining since skeletal muscle cell

Biochemistry of Overtraining 869


destructuring is one of the most important sourcesof feelings of stiffness for athletes, as well as ec-centric exercises that disrupt cellular compo-nents.[19] Muscular pains appear as alarm signalsand inhibit athletes’ capacities to realise other in-tense exercises.[33] Indeed, it seems unlikely thatovertraining might appear as a consequence of suc-cessive alterations in the skeletal muscle sys-tem,[34] but rather that skeletal muscle cell damagemay participate in the overtraining process.

1.2 The Carbohydrate Hypothesis

During endurance exercise, fatigue may inducea slight transient hypoglycaemia, which is due tohepatic and/or muscular glycogen store depletion,and/or to a failure in glycogenolytic metabolicflux. Following several intense and long-lastingendurance training sessions, glycogen depletionmay become chronic if carbohydrate ingestion isinappropriate,[35] leading to its slower and delayedrepletion.[36] It has been found that exercisehypoglycaemia could have higher severity in over-trained athletes[8,13] and that the lactacidaemiaincrease could be lower,[37-39] suggesting a poorparticipation of glycolysis to skeletal musclemetabolism. As a consequence of this poor glycol-ysis, purine nucleotide metabolism would be pro-longed over the hydrolysis of ATP and adenosinediphosphate (ADP), which may produce higheramounts of inosine monophosphate (IMP) andNH4+.[11] This process is known to release by-products such as hypoxanthine and xanthine oxi-dase which are toxic if found in high concentra-tions within the muscle cells. However, althoughovertrained athletes present higher glycogen de-pletion in response to long-term endurance exer-cises, glycogen store replenishment between exer-cises is generally found to be optimum.[39,40]

Indeed, rather than being responsible for the occur-rence of overtraining in endurance-trained ath-letes, these repeated glycogen depletions might in-duce subtle changes within the metabolic pathwaysthat contribute to the skeletal muscle energy sup-ply.[13,41] It has been suggested that long-term gly-cogen depletion would lead to an increased BCAA

oxidation, which is much more likely to be respon-sible for a central fatigue process.[13]

1.3 The Branched-Chain Amino Acid Hypothesis

During endurance exercise, BCAA (leucine,isoleucine, valine) may be widely captured by skel-etal muscles (but not the liver), to be oxidised forATP resynthesis.[42] Along with BCAA, plasmafree fatty acids (FFA) may also be oxidised inhigher amounts by skeletal muscles as glycogenstores are depleted.[43,44] FFA are not water solubleand thus necessitate albumin binding to be trans-ported into the blood stream. However, there iscompetition between tryptophan and FFA bindingto albumin for blood transport. Indeed, an increasein FFA transport to skeletal muscles induces ahigher utilisation of albumin transport capacities.In turn, this albumin utilisation leads to the bloodrelease of free tryptophan.[45] BCAA and aromaticamino acids utilise the same carrier in the haemato-encephalic barrier. Thus, a consequence of the in-crease in free tryptophan concentration (while thatof BCAA decreases), is the facilitated free trypto-phan entry into the cortical area.[14] Cerebral tryp-tophan is then converted to serotonin, in specificcortical areas. This serotonin may have severalfunctions: (i) sleep induction; (ii) motoneuron ex-citability and inhibition of post-synaptic reflexes(notably during exercise);[43] and (iii) endocrinefunctions inhibiting the release of hypothalamichormones, which may impair various endocrineregulations within the body.[46] Such phenomenahave been observed in overtrained athletes.[14]

Therefore, a decrease in the free tryptophan toBCAA blood concentration ratio (free trypto-phan/BCAA ratio) has been proposed as a diagnos-tic tool for detecting overtraining in endurance ath-letes.[47]

However, ingestion of BCAA during or afterendurance exercise has not been shown to restorethe free tryptophan/BCAA ratio and did not signif-icantly change the loss of performance level due tometabolic fatigue induced by glycogen store deple-tion.[44,48] In fact, ingestion of BCAA increased

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amino acid carbon skeleton drainage through thetricarboxylic acid cycle to form acetyl-CoA. Thiscycle leads to ATP production from amino acidcarbon skeletons, but ammonium ions will alsobe produced in large amounts, rapidly becomingtoxic for the muscle cell. In skeletal muscle cells,BCAA represent the first amino group donor to 2-oxoglutarate for glutamate formation. Glutamateleads to glutamine synthesis via glutamine syn-thase. This is the main biological ammonium vec-tor, a compound highly toxic when free.[42] Thus,the ingestion of BCAA did not appear useful indecreasing the high free tryptophan/BCAA ratioobserved in overtrained athletes because of the po-tential induction of other metabolic stress.[48] Fur-thermore, the relationship between the higher de-pletion of glycogen stores observed in overtrainedathletes and the central fatigue potentially inducedby the free tryptophan/BCAA ratio increase hasnot been described. It has not been demonstratedwhether the free tryptophan/BCAA ratio remainssignificantly elevated between intense endurancetraining sessions, while glycogen store replenish-ment is optimum.[49] Therefore, the continuous se-rotonin secretion that may appear during heavytraining load does not seem to be a sufficient cen-tral fatigue inductor to lead to overtraining in en-durance athletes. Regarding other metabolic as-pects of overtraining, the link between BCAAoxidation during training exercises and serotoninsecretion is one of the factors that may increase thesusceptibility of athletes when experiencing theovertraining syndrome, i.e. along with other cen-tral and/or peripheral fatigue inductors.

1.4 The Glutamine Hypothesis

Glutamine is one of the most abundant aminoacids within the human body. It is metabolised byimmune cells, such as lymphocytes and macro-phages.[50] Their proliferation depends on gluta-minolysis,[51] suggesting that a decrease in bloodglutamine concentration may at least be partiallyresponsible for immune function deficiency or im-pairment. A portion of the glutamine found withinthe body enters the gut to be metabolised. Skeletal

muscle appears as the most important glutamineproducer, which is further released within theblood stream. Indeed, during an intense enduranceexercise, the blood glutamine concentration con-stitutes a metabolic link between active skeletalmuscles and immune system capacities of reac-tion.[52] Under catabolic stresses, such as infec-tions, surgical interventions, trauma, burns and ac-idosis, glutamine homeostasis is placed understress. Glutamine stores, notably within skeletalmuscles, may be widely depleted. With respect toglutamine metabolism, exercise may also be con-sidered as a catabolic situation.[15]

Intense endurance exercises induce a biphasicresponse in glutamine blood concentration.Firstly, it augments during exercise and, secondly,it falls significantly during rest periods, for severalhours before again reaching the basal concentra-tion. Indeed, it may be argued that insufficient restperiods between intense training sessions maylimit glutamine release from skeletal muscles and,therefore, the immune system may become muchmore stressed.[52] Gut function may also be unset-tled by this lower glutamine disposition, leading tohigher risks of bacterial and viral translocationswithin organisms. Important glutamine depletionshave been reported in overtrained athletes experi-encing URTI.[4] A significant relationship betweenURTI frequency and the prolonged decrease inglutamine concentrations has also been reported inovertrained endurance athletes.[53,54] Indeed, thepersistence of this situation would contribute to theoccurrence of overtraining, these infections play-ing the role of immune stressors while a metabolicstress fatigue is already experienced by the athlete.However, decreases in glutamine concentrationshave not been systematically observed in over-trained athletes and URTIs may appear with thesame frequency in well-trained and overtrainedathletes.[55] Moreover, immunosuppression hasbeen observed in overtrained athletes, but withoutdecreases in glutamine concentrations and in theabsence of URTIs.[2,3,56]



1.5 The Polyunsaturated Fatty Acid Hypothesis

Immunosuppression seems to be recurrent inovertrained athletes. An alternative metabolicscheme has been proposed, starting with inhibitionof lymphocyte proliferation due to PUFAs. It hasbeen suggested that PUFAs may cause this inhibi-tion with a higher chronicity than saturated fattyacids.[16] Exercise metabolic stress increases theblood concentration of fatty acids, notably duringand after intense endurance exercises since glyco-gen stores are depleted. Lymph nodes are associ-ated with adipose tissue. Thus, during fatty acidmobilisation from adipocytes, lymph nodule cellsmay be exposed to high PUFA concentrations.Depending on the fatty acids contained in adiposetriglycerides, their high mobilisation during endur-ance exercise would lead to a lymphocyte prolifer-ation inhibition in lymph nodules, along with anincreased PUFA flux. An elevated sensitivity ofadipocytes to lipolytic hormones, and a triglycer-ide composition in fatty acids shifting towardsPUFAs may also be implied in the immunosup-pression observed in overtrained athletes.[57] How-ever, this immunosuppression has not been dem-onstrated in overtrained athletes, which shouldtake into account the turnover and differentiationof fatty acids synthesised for triglyceride stores re-plenishment between intense endurance trainingsessions.

1.6 The Leptin Hypothesis

Leptin, the product of the ob-gene, is specific-ally released by adipocytes and reflects the bodyfat content. In addition to its metabolic functions,i.e. a putative satiety signal in humans, it seems toaffect the feedback mechanisms of the hypotha-lamic-pituitary-gonadal-axis. Leptin secretion iscomplexly regulated in humans. Insulin has beenshown to stimulate leptin secretion, whereas invitro data suggest that catecholamines and FFAsinhibit leptin secretion. It has been shown thatshort-term exhaustive exercise has no immediateor delayed effect on circulating leptin concentra-

tion.[58] On the other hand, several studies showedthat endurance exercise sessions decrease theplasma leptin concentration after 48 hours, in as-sociation with a preceding decrease in insulin.[59]

Nevertheless, its long-term effects on metabolicadaptations to training remain controversial. Glob-ally, serum leptin levels decrease in highly-trainedendurance athletes in comparison with nonsportingindividuals. Serum leptin levels in top-level ath-letes parallel the changes in body fat content andare not an independent predictor of endurancetraining level. It seems that plasma leptin is notsensitive to an increase in training volume fortrained individuals. In fact, training level inducedby resistance and/or endurance exercises did notinfluence leptin production when considering vari-ations in body composition.[60]

Leptin, as well as inhibin B, colecalciferol (vi-tamin D3), and possibly activin and resistin, arenow considered as indicators of tissue overload inhighly-trained endurance athletes. It has recentlybeen suggested that metabolic functions of thesehormones may become potent biochemical mark-ers of overtraining occurrence in endurancesports.[17] Correlations have been found betweenvariations in these neuroendocrine axis parametersand performance in fatigued athletes after 3 weeksof excessive training. The underlying mechanismsmight help us to understand how overloaded pe-ripheral organs and tissues ‘tell’ the brain of theirfatigue. However, there are no studies on leptinlevels regarding overtraining occurrence.[60] Datafrom recent studies strongly suggest plasma leptinis not sensitive to an increase in training volumeand that this hormone may not be indicative ofchanges in fat mass with an increase in trainingvolume in female athletes. These data suggest thatleptin may not be useful in monitoring relativetraining stress in athletes.[61,62] Furthermore, theauthors found no evidence of alterations in leptinlevels in patients with chronic fatigue synd-rome,[63] a fatigue syndrome thought to be closeto overtraining in its biological effects on the hu-man body.[34,64] Therefore, to date, the monitoringof changes in leptin levels along training duration

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and intensity does not appear to be a useful tool indiagnosing or preventing overtraining. Neverthe-less, in conjunction with other stress hormones,leptin may interfere in overloaded tissues in induc-ing a preventive resistance to further metabolicstress. This mechanism might be responsible for adown-regulation of the carbohydrate–lipid meta-bolism during exercise, facilitating the occurrenceof overtraining.[17]

2. The Protein Metabolism Hypothesis

As a general rule, overtraining has not been as-sociated with important variations in blood proteincontent.[5,53,65] However, an intense enduranceexercise strongly augments metabolic processeswithin skeletal muscles, liver, and kidney, whichmay be associated with tissue inflammation.[66]

This inflammation induces a short-term responseof hepatic proteins, i.e. fibrinogen, haptoglobin, C-reactive protein, α1-acid glycoprotein, and α1-antitrypsin, through their antiproteolytic func-tions.[67] Strenuous endurance training may causethree levels of inflammation: (i) firstly, this maybe observed through a modest increase in α1-antitrypsin concentration, along with heavy train-ing loads, but without change in ferritin concentra-tion; (ii) the subsequent level indicates a severeaffection, characterised by higher increases in α1-antitrypsin and ferritin concentrations; (iii) thelater appears during particularly hard and heavyendurance training loads, which may cause impor-tant iron losses followed by long-term decreases inhaptoglobin blood contents, and a rise in ferritinand α1-antitrypsin concentrations. These eventsmay be observed up to 24 to 48 hours after train-ing.[68]

Continuation of this inflammatory situationmay cause a substantial depletion in functionaliron body stores. On the other hand, such depletionmay occur after anaemia due to a mechanicallyinduced haemolysis (trauma, repeated shocks,haematoma) and/or induced chemically by ROSaction. In addition to the metabolic stress inducedby exercise, peroxidation processes alter the cellu-lar membrane functions of erythrocytes, facilitat-

ing their dehydration. A possible consequence is adisruption of erythrocyte ionic homeostasis thatmay limit their entry into the micro-circulation.This mechanism slightly augments hypoxia withinactive skeletal muscles, which may increase ROSaction on the membrane phospholipids of eryth-rocytes; in turn, this may lead to erythrocyte de-struction and, potentially, to exercise anaemia.[69]

However, it has been shown that erythrocyte de-struction during endurance exercise is not im-portant enough to become deleterious for athletes.To the contrary, this slight exercise anaemia mightbe an interesting way for de novo synthesis oferythrocytes, leading to the blood release of youngand potentially more efficient erythrocytes. Nev-ertheless, successive haematuria during endur-ance exercises may cause a rapid and significantdecrease in blood haptoglobin, haemoglobin,haemopexin, and ferritin concentrations.[70] Long-term and repeated depletions of these proteinstores along intensive endurance training mayweaken muscle and liver defences against inflam-matory processes. Skeletal muscle inflammationmay also be associated with the catabolism of con-tractile proteins and with myofibril degeneration,in addition to the normal exercise protein turn-over.[71] Nevertheless, exercise anaemia has notbeen clearly associated with the occurrence ofovertraining since overtrained athletes do not pres-ent with important and long-term depletions inhaptoglobin, haemoglobin, haemopexin, and ferri-tin concentrations.[30,65] Finally, these processesdo not appear to cause overtraining in enduranceathletes, but rather increase the metabolic stressand/or muscular and hepatic tissue inflammationthat may lead to a chronic fatigue accumula-tion.[57,69]

Another index of the protein metabolism statusis the ratio between free testosterone and cortisolconcentrations (T/C ratio). This ratio has been pro-posed as a marker of the anabolic–catabolic statusof the athlete, i.e. a global appreciation of proteinturnover.[6,29,72] A drop in this ratio of 30% belowthe basal values of the athlete, i.e. before exerciseor training, and/or values <0.35 × 10−3 may be in-



dicative of a too intensive training load[23] and mayreveal overtraining in sprint and strength sports.[72]

However, several studies on high-intensity resis-tance exercise overtraining failed to obtain signif-icant changes in the T/C ratio, notably when over-training occurred after several weeks of intensifiedtraining load.[5,6,9,53] Furthermore, athletes’ abilityto train in endurance sports is not primarily de-pendent on protein metabolism, but rather on en-ergy metabolism, i.e. that which involves mainlycarbohydrates and lipids. Therefore, a biologicalindex such as the T/C ratio may not be sufficientlydiscriminatory to diagnose overtraining if it is notused in conjunction with biological markers of theenergy status of the athlete.

3. Validity of Clinical Analyses toDiagnose Overtraining

The main difficulty in the diagnosis of over-training is the need for repeated analysis of theblood prior to and after exercise. Moreover, thebiochemical markers of overtraining may varywith respect to the characteristics of the sport prac-tised and the nature of the training load. Many fac-tors may also interfere with the biochemical as-pects of overtraining, i.e. psychological, social, orcultural.[5] Furthermore, until recently, only a fewstudies have taken into account the phenomenon ofexercise-induced changes in plasma volume. Asrecently reviewed,[73] cold exposure, psychologi-cal stress, nutrition, hydration, and the durationand intensity of exercise, may markedly changeexercise haemoconcentration. Thus, the compari-son of data obtained from different studies on over-training remains globally impossible. This meth-odological point will be of major importance in thefuture since biochemical parameters of overtrain-ing appear to be much more highlighted by exer-cise than rest analyses.[8,15,39,41,47,65,66,74] There-fore, the diagnosis of overtraining remains as astate of the art; possible in some cases[8] but unpre-dictable in many others.[34] To date, a list of poten-tial markers of overtraining in endurance sportsmay be proposed (table I). However, there is stillno single biochemical marker to propose as a sig-

nal of overtraining, i.e. to assess the limit betweenreversible training fatigue and overtraining.

3.1 Summary of Biochemical Markers of Overtraining

From the skeletal muscle function to the overallenergy substrate availability during exercise, sixmetabolite pools have been studied in relation toovertraining, each one linked to a central parame-ter, i.e. carbohydrate, BCCA, glutamine, PUFA,leptin, and protein (table I). It is clear that in en-durance sports overtraining may appear after sev-eral months of hard training.[34] Indeed, to diag-nose, or rather to prevent overtraining, clinicalanalyses should be addressed throughout the train-ing programme. Ideally, to ensure the best diagno-sis of overtraining, every metabolite which hasphysiologically been linked to overtraining shouldbe analysed (table I). However, in practice, this isimpossible as it would require testing: (i) at rest,for comparison with the normal physiologicalrange of each metabolite; (ii) after an exercise spe-cific to the sport practice, in order to evaluate theresponses of the athlete to normal training stimuli;and (iii) 24, 48, and 72 hours after this exercise inorder to evaluate recovery capacities of the athleteand this adaptation to the training load. Our knowl-edge now extends, at the cellular level of skeletalmuscle, to overload of oxidative stress and/or me-chanical aggressions being abnormally elevated,and can be monitored by studying the plasma ki-netics of CPK, malondialdehyde, tocopherol (vita-min E), ascorbic acid (vitamin C), retinol (vitaminA), myoglobin, 3-methylhistidine, and sTi. Alter-ations of the energy metabolism may also be high-lighted by studying variations in the concentrationsof glucose, lactate, glutamine, and urea, as well aswith the typology of fatty acids contained withintriglycerides. Nonspecific responses of the im-mune system may be perceived from fluctuationsin the concentrations of immunoglobulin (Ig)A andIgG and cell dynamics, and may involve directlinks to amino acid and protein metabolism. Dys-functions of the hormonal system may be observedthrough concentrations of serotonin, cortisol, tes-

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tosterone, the T/C ratio, and catecholamines. Alsorelated to the anabolism–catabolism balance ofproteins, reactions to different levels of tissueinflammation induced by heavy training may beassessed by plasma kinetics of α1-antitrypsin, α1-acid glycoprotein, and α2-macroglobulin. Apossible exercise anaemia may also increase theimbalance of protein turnover, which may be ob-served through wide variations in haptoglobin,haemopexin, transferrin, and ferritin concen-trations. However, the role of exercise-inducedplasma volume changes in the interpretation ofthese type results can no longer be ignored.[73] Sub-stantial biochemical variations in blood becomenonsignificant when corrected for plasma volumechanges, which have been induced by exercise.

3.2 Perspectives for a Clinical Diagnosis of Overtraining

The main biochemical markers of overtrainingremain unpredictable and do not allow their use ina systematic diagnosis of severely overtrained ath-letes.[5] The nature of training load, as well as train-ing monotony, dietary habits, sleep efficiency, andthe psychological status of the endurance-trainedathlete also appear as important factors in the oc-currence of overtraining.[37] The analysis of all the

specific venous blood parameters described in thisreview remain costly since many necessitate non-routine analytical techniques, and may be subjectto significant plasma volume effects. Therefore,another approach is needed to allow longitudinalhealth monitoring of the athletes engaged in heavyendurance training programmes.[74] While onlylongitudinal studies monitoring the metabolic re-sponse to exercise would be able to discriminatebetween good and poor training adaptations, thetraining loads for endurance, sprint, or resistancesport practices may each induce different metabo-lic, immune, mechanical, neuro-hormonal, andmorpho-skeletal adaptations, which may generatedifferent overtraining processes.[8,65] Therefore,while systematic study of the metabolic aspects ofovertraining is needed for each sport, attempts tohighlight the most relevant biochemical parame-ters that may reveal overtraining for each individ-ual sport have, as yet, not been successful.

4. Conclusion

The exact aetiology of endurance overtrainingis not fully understood but many biochemical pa-rameters appear to allow pertinent information tobe documented about the possible occurrence ofthis chronic fatigue phenomenon. The difficulty is

Table I. Summary of the biochemical plasma parameters potentially implicated in the occurrence of overtraining in endurance sportsa

Centralparameter

Implied organ Plasma variation (rest) Plasma variation (exercise)

Reactive oxygenspecies

Muscle CPK ↑; myoglobin ↑; sTi ↑; 3-MTH ↑; retinol(vitamin A) ↓; ascorbic acid (vitamin C) ↓;tocopherol (vitamin E) ↓

CPK ↑; myoglobin ↑; sTi ↑; 3-MTH ↑; MDA ↑;retinol ↓; ascorbic acid ↓; tocopherol ↓

Carbohydrates Liver, muscle GLN ↓; urea ↑ GLC ↓; lactate ↑; GLN ↓; urea ↑BCAA Body serotonin ↑ BCAA ↓; fTrp ↑; fTrp/BCAA ↑; serotonin ↑Glutamine Muscle, gut GLN ↓; IgA ↑; IgG ↑ GLN ↑; IgA ↑; IgG ↑PUFAs Lymph nodes PUFAs ↑Leptin Adipocytes Leptin ↓; inhibin B ↓; colecalciferol (vitamin D3) ↓ Leptin ↓; inhibin B ↓; colecalciferol ↓Proteins Muscle, liver,

kidneyHaptoglobin ↓; haemoglobin ↑; haemopexin ↓;ferritin ↑; α1-antitrypsin ↑; α1-acid glycoprotein ↑;α2-macroglobulin ↑; T/C ↓

Haptoglobin ↓; haemoglobin ↑; haemopexin ↓;ferritin ↑; α1-antitrypsin ↑; α1-acid glycoprotein↑; α2-macroglobulin ↑; T/C ↓

a Plasma variations of these parameters are presented as increased (↑) or decreased (↓) during overtraining in athletes well tolerated tohard training.

3-MTH = 3-methylhistidine; BCAA = branched-chain amino acids; CPK = creatine phosphokinase; fTrp = free tryptophan; GLC = glucose;GLN = glutamine; Ig = immunoglobulin; MDA = malondialdehyde; PUFAs = polyunsaturated fatty acids; sTi = skeletal troponine I; T/C = freetestosterone/cortisol concentration ratio.



that none of these metabolic parameters may beconsidered individually as a standard, allowing asystematic diagnosis of overtraining, or its preven-tion. This lack of a useful tool in the sport medicinearea stigmatises the complexity of the overtrainingprocess. Attempts have recently been made toredefine the overtraining syndrome as the unex-plained under-performance syndrome.[75] How-ever, regarding all the parameters that have beenlinked to overtraining, it appears that only six me-tabolic schemes have been described in relation toovertraining. A more precise description of each ofthese metabolic schemes is needed before a moreglobal model of the overtraining process can bedeveloped.

No study has precisely shown the shift from thewell-trained state towards the overtrained state inendurance athletes because of methodological dif-ficulties in the longitudinal health monitoring ofathletes. Therefore, future research will need tofind a global explanation of the steps that lead toovertraining. Currently, variations in energy meta-bolism appear highly relevant, notably for alter-ations in carbohydrate and lipid metabolism duringtraining exercises. There is a growing body of ev-idence that impairment of carbohydrate–lipid me-tabolism leads to other biochemical aspects ofovertraining, i.e. tissue inflammation and proteincatabolism, probably in response to metabolicoverload in peripheral organs, skeletal muscles andadipose tissue.

Acknowledgements

The authors have provided no information on sources offunding or on conflicts of interest directly relevant to thecontent of this review.

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Correspondence and offprints: Cyril Petibois, UniversitéVictor Segalen Bordeaux 2, Faculté des Sciences du Sport etde l’Education Physique, Av. Camille Julian, 33607 Pessac,Cedex, France.E-mail: [email protected]

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Biochemical Aspects of Over Training in Endurance.5