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Science paper
   PLEASE SCROLL DOWN FOR ARTICLE ! #$ &'(#)*+ ,&$ -.,/*.&-+- 0123/2 45 6+0'7&'1 4899 :))+$$ -+(&#*$2 :))+$$ ;+(&#*$2 6'++ :))+$$  <70*#$ +' !&1*.' = 6'&/)#$  >/?.'@& A(- B+C#$(+'+- #/ D/C*&/- &/- E&*+$ B+C#$(+'+- F7@0+'2 98G4HIJ B+C#$(+'+- .??#)+2 K.'(#@+' L.7$+M 5GNJ9 K.'(#@+' O('++(M A./-./ E9! 5PLM QR D7'.S+&/%P.7'/&*%.?%OS.'(%O)#+/)+ <70*#)&(#./ -+(&#*$M #/)*7-#/C #/$('7)(#./$ ?.' &7( .'$ &/- $70$)'#S(#./ #/?.'@&(#./2 ((S2TT,,,U#/?.'@&,.'*-U).@T$@SST(#(*+V)./(+/(W(G9JIH45IJ :/(#.X#-&/(%$7SS*+@+/(&(#./%&/-%+/-7'&/)+%('&#/#/C2%E#/%.'%*.$$Y K#)& Z'.$$ & [ 3*#\+' ]&7@ & [ L&/$ L.SS+*+' &&  >/$(#(7(+ ?.' :/&(.@1M Q/#\+'$#(1 .? ]+'/M ]+'/M O,#(^+'*&/-3/*#/+ S70*#)&(#./ -&(+2 9G P&/7&'1 4899  .%)#(+%( #$%:'(#)*+  Z'.$$M K#)& M ]&7@M 3*#\+' &/- L.SS+*+'M L&/$_4899` a:/(#.X#-&/( $7SS*+@+/(&(#./ &/- +/-7'&/)+('&#/#/C2 E#/ .' *.$$YaM D7'.S+&/ P.7'/&* .? OS.'( O)#+/)+M 992 9M 4G b 54  .%*#/c%(.%( #$%:'(#)*+2%;3>2% 98U98d8T9GJe95H9885eHH8dd QBA2% ((S2TT-XU-.#U.'CT98U98d8T9GJe95H9885eHH8dd Full terms and conditions of use: http://www.informaworld.com/terms-and-conditions-of-access.pdfThis article may be used for research, teaching and private study purposes. Any substantial orsystematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply ordistribution in any form to anyone is expressly forbidden.The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug dosesshould be independently verified with primary sources. The publisher shall not be liable for any loss,actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directlyor indirectly in connection with or arising out of the use of this material.  ORIGINAL ARTICLE Antioxidant supplementation and endurance training: Win or loss? MICAH GROSS, OLIVER BAUM, & HANS HOPPELER  Institute for Anatomy, University of Bern, Bern, Switzerland  Abstract Typically, free radicals are thought of as perpetrators of cell damage, ageing, even cancer, whereas antioxidants areseen as the defence against these threats. Accordingly, antioxidants are among the most common sports supplementsused by amateur and professional athletes. However, the sensibility of this practice has recently been challenged in thescientific literature. This article briefly summarizes both positive and negative physiological effects of free radicals andantioxidants, culminating with emphasis on the signalling roles played by free radicals during training adaptations and theability of superfluous antioxidants to weaken these desired signals, as revealed in several recent publications. The aim of this article is not to explicitly condemn antioxidant supplementation by athletes, but to underscore complexity of thesituation and to champion efforts to achieve a deeper understanding of circumstances (e.g. dosage, timing, and setting)that might deem antioxidant supplementation as either largely beneficial or largely detrimental for endurance athletes intraining. Keywords:  Oxidative stress, free radical, vitamin, endurance training  Introduction Over the years, the terms ‘‘free radical and antiox-idant’’ have become common lingo. Typically, freeradicals are thought of as perpetrators of celldamage, ageing, even cancer, while antioxidantsare seen as the defence against these threats. Alongwith awareness about the harmful effects of freeradicals, consciousness regarding the importanceof dietary antioxidants has also increased. As aresult, many health-conscious people have turnedto nutritional supplements. Worldwide, antioxidantsupplements have become a multi-billion dollarindustry.A particular interest in antioxidant supplementshas arisen among athletes and highly active people.Indeed, antioxidants are among the most commonsports supplements used by amateur and profes-sional athletes (Braun et al., 2009; Froiland,Koszewski, Hingst, & Kopecky, 2004; Knez, Jenkins, & Coombes, 2007; Krumbach, Ellis, &Driskell, 1999; Margaritis & Rousseau, 2008; Sobal& Marquart, 1994). Such individuals are generallyquite health-conscious, and free radical productionis known to be greater during exercise (e.g. Davies,Quintanilha, Brooks, & Packer, 1982; Kanter, 1998;Koren, Sauber, Sentjurc, & Schara, 1983; Sastreet al., 1992). A quick survey of the labels on mostenergy bars and recovery drinks would seem to eraseany doubt that athletes need vast amounts of supplementary antioxidants. However, some havebegun to question whether this is truly necessary(Margaritis & Rousseau, 2008), or at all sensible(e.g. Padilla & Mickleborough, 2007).Indeed, there is a growing body of evidence thatthe appearance of free radicals fulfils importantphysiological functions in cells, and that a balancebetween antioxidants and free radicals is necessaryfor desired physiological adaptations (e.g. Gomez-Cabrera, Domenech, & Vina, 2008b; Ji, 2007, 2008; Ji, Gomez-Cabrera, & Vina, 2006). Thus, it becomesnecessary to evaluate the prudence of antioxidantsupplementation, particularly among athletes intraining. In this article, we wish to explore thepossibility that superfluous antioxidant supplemen-tation by endurance athletes is erroneous, and toprovoke a more critical appraisal of the situation,which should lead to more detailed research. Correspondence: M. Gross, Institute for Anatomy, University of Bern, Baltzerstrasse 2, Bern 9, CH-3000, Switzerland. E-mail:micah.gross@ana.unibe.ch European Journal of Sport Science , January 2011; 11(1): 27    32 ISSN 1746-1391 print/ISSN 1536-7290 online # 2011 European College of Sport ScienceDOI: 10.1080/17461391003699088  D o w nl o ad ed A t :23 :3823 F eb r u a r y2011  Origin of free radicals The term free radical refers to reactive oxygen(ROS) and nitrogen species (RNS), which are highlyreactive because of an unpaired valence electron.In animal muscle fibres, five main radicals have abiological impact. The first, superoxide (O 2   ), isformed in mitochondria and in the cytosol. A smallamount of molecular oxygen passing through theelectron transport chain in mitochondria is prema-turely released as O 2    (Chance, Sies, & Boveris,1979). Superoxide can also be formed in theextracellular space by NADPH oxidase or by theenzyme xanthine oxidase (XO) during the conver-sion of xanthine to uric acid. Xanthine oxidase isfound mostly in microvascular endothelial cells, butis also present in leucocytes, which may infiltratemuscle fibres following strenuous exercise (Hellsten,Frandsen, Orthenblad, Sjodin, & Richter, 1997).The second, hydrogen peroxide (H 2 O 2 ), can bereleased during both steps of the hypoxanthine  0 xanthine 0 uric acid conversion by XO, or it can beformed from O 2   by superoxide dismutase (SOD)isoforms in mitochondria, cytosol, and the extra-cellular space (Boveris & Cadenas, 2000; St-Pierre,Buckingham, Roebuck, & Brand, 2002; Valko,Izakovic, Mazur, Rhodes, & Telser, 2004). Third,the hydroxyl radical (  OH) is formed when O 2   orH 2 O 2  reacts with metal ions such as iron or copper(Valko et al., 2004). The fourth radical, nitric oxide(NO  ), is formed from L-arginine by nitric oxidesynthase (NOS), mainly the neuronal isoform(nNOS) in skeletal muscle, but also endothelialNOS (eNOS) (Stamler, Sun, & Hess, 2008; Valkoet al., 2007). Lastly, the peroxyl radical, peroxynitrite (ONOO  ), is formed in the cytosol whenO 2    reacts with NO   (Pryor & Squadrito, 1995).Because their srcins are closely linked, increasedactivation of the electron transport chain and NOSduring exercise leads to elevated production of eachof these five radicals.Substrate depletion, leading to a fall in glutathionereductase activity, and hyperthermia, which pro-motes mitochondrial uncoupling and loss of respira-tory control, may also contribute to free radicalproduction during exercise. Furthermore, transienthypoxia during anaerobic exercise leading to acido-sis, as well as reperfusion of hypoxic muscle, mayincrease oxidative stress (Kanter, 1998). Finally,mechanical stress of exercise can itself increase freeradical formation (Symons, 1988). Negative effects of free radicals Because of their high reactivity, reactive oxygenspecies and reactive nitrogen species are able todeform other biologically important molecules, thuscausing damage to cell structure and obstructingcell function. Superoxide, H 2 O 2 , and  OH are able toacquire the protons adjacent to double bondsin unsaturated fatty acids, such as those in cellmembranes.Thisbeginsachain reactionofdeforma-tion to these fatty acids forming lipid peroxides. Thisprocess, called ‘‘lipid peroxidation’’, results in poorlyfunctioning (i.e. leaky) cell membranes (Kellogg &Fridovich, 1975; Lai & Piette, 1977; Valko et al.,2004). Similarly,   OH, NO  , and ONOO  canoxidize nucleotides causing damage to DNA, whichcan lead to tumours (Valko, Rhodes, Moncol,Izakovic, & Mazur, 2006). Nitric oxide is able tobind to the cysteine groups on proteins, called S  -nitrosylation, changing the proteins’ tertiarystructure and altering their function (Stamleret al., 2008), and ONOO  is able to irreversiblydenature proteins in a similar manner, renderingthem non-functional (Koppenol, Moreno, Pryor,Ischiropoulos, & Beckman, 1992). Nitric oxide hasalso been suggested to have a direct inhibitory effecton contractility in muscle fibres (Kobzik, Reid,Bredt, & Stamler, 1994). Important biological functions of free radicals Although free radicals have traditionally been con-sidered purely a threat to cells, such one-sidedthinking is beginning to be challenged. There isincreasing evidence to suggest that free radicals playan important role in modulating redox-sensitivesignalling pathways on the way to muscular adapta-tions (Jackson, 2009). Results from several recentstudies on animals as well as some involving athletespresent the framework for a functional role of reactive oxygen species, including O 2   , H 2 O 2 , andNO  , as important cell signals.Activation of the MAP kinase signalling pathwayafter aerobic endurance training enhances mitochon-drial genesis and capillarization (angiogenesis), mus-cle and heart hypertrophy, and glucose transportability (Gibala, 2009; Ji, 2007). Adaptations totraining may be dependent on changes to cellularthiol    disulphide ratios, or redox potentials, causedby free radicals (Jackson, 2009) or the transientappearance of O 2   (Gomez-Cabrera et al., 2008b; Ji et al., 2006), as these appear to stimulate theupregulation of certain important transcription fac-tors within this pathway.Meanwhile, H 2 O 2  formed from O 2    by SOD3in the extracellular space acts as a vasodilator, whichcan acutely optimize blood flow. Nitric oxide pro-duced in endothelial cells by eNOS also induces vasodilation in feeding and resistance arteries(Clifford & Hellsten, 2004), leading to an increase inblood-flow velocity (Kayar et al., 1992). The result-ing increase in shear stress in the microvasculature of 28  M. Gross et al.  D o w nl o ad ed A t :23 :3823 F eb r u a r y2011  muscle fibres is an important stimulus for angiogen-esis in muscle (Baum et al., 2004). Endogenousoxidant-defence is also upregulated by negativefeedback from reactive oxygen species, especiallyO 2   (Gomez-Cabrera et al., 2008b; Ji et al., 2006).Free radicals may also have acute positive effects.In low concentrations, they help maintain muscleforce production (Jackson, 2009; Powers & Jackson,2008). Furthermore, during the oxidative burst of phagocytosis, macrophages release O 2   , H 2 O 2 , andNO  as part of the clearing out of damaged or deadcell material, which helps speed the repair process(Valko et al., 2006). Effects of antioxidants Antioxidant substances can be placed into twocategories: endogenous and exogenous. In bothcases, antioxidants scavenge radicals and convertthem into unreactive substances, thus minimizing achain reaction of radical-induced transformations tocell structures.In human skeletal muscle fibres, several endogen-ous enzymes and substrates work together to sca-venge free radicals. Superoxide dismutase isoforms1 (cytosolic), 2 (mitochondrial), and 3 (extracellular)reduce O 2   to H 2 O 2 . In the cytosol, H 2 O 2  can there-after be converted to water by glutathione peroxidase(Gpx), which oxidizes glutathione (GSH), or one of several peroxiredoxins with the help of thioredoxin,or to water and molecular oxygen by catalase (Valkoet al., 2007). The dipeptides carnosine and anserinealso act as antioxidants by scavenging O 2   and   OH(Chan & Decker, 1994).Other non-enzymatic antioxidants, which are notsynthesized in humans, must be obtained exogen-ously, and include the vitamins A ( b -carotene), C(ascorbic acid), and E ( a -tocopherol). These sub-stances are able to scavenge various free radicals byproton donation (Sies & Stahl, 1995). Vitamin Cmay help strengthen immune defence (Kreider et al.,2004; Valko et al., 2006), while vitamin E couldenhance energy balance at high altitude (Simon-Schnass & Pabst, 1988). Furthermore, the twofunction together to protect against lipid peroxida-tion. Beta-carotene is particularly well-suited forscavenging O 2   ,   OH, and peroxyl radicals such asONOO  (Valko et al., 2004).It should be noted that under certain circum-stances, non-enzymatic antioxidants may becomepro-oxidative agents. Beta-carotene, in the presenceof increased partial pressure of oxygen, can beconverted to a peroxyl radical, and vitamin C canform DNA-damaging genotoxins from lipid hydro-peroxides in the presence of transition metal ions(Valko et al., 2004).In some cases, counteracting reactive oxygenspecies via acute antioxidant supplementation canpositively affect athletic performance. For example,pharmacologically boosting the capacity to convertH 2 O 2  to water protects against ROS-induced fatigue(Medved et al., 2004; Reid, 2008). However, recentconsensus reports from the International Society of Sports Nutrition (Kreider et al., 2004) and theAmerican College of Sports Medicine (Rodriguez,Di Marco, & Langley, 2009) do not support thebelief that ordinary antioxidant substances suchas vitamins A, C, and E improve performance ordelay fatigue in adequately nourished athletes.Similarly, supplementary vitamin C or E does nothave a protective effect against muscle damage(Beaton, Allan, Tarnopolsky, Tiidus, & Phillips,2002; Childs, Jacobs, Kaminski, Halliwell, &Leeuwenburgh, 2001; Close et al., 2006; Connolly,Lauzon, Agnew, Dunn, & Reed, 2006; McGinley,Shafat, & Donnelly, 2009). Evidence for negative effects of antioxidantsupplementation during training Recently, questions have been raised about theefficacy of high doses of exogenous antioxidantssuch as vitamins C and E during endurance training,with several publications suggesting that these mayactually be counterproductive (Gomez-Cabreraet al., 2005, 2008a; Ji, 2008; Ristow et al., 2009;Wray, Uberoi, Lawrenson, Bailey, & Richardson,2009). It was supposed that reactive oxygen speciesplay an important signalling role for adaptation of endogenous oxidant defence systems and for mito-chondrial genesis and angiogenesis. When radicalappearance is overly suppressed, these signals maytherefore be weakened or abolished.One response to the elevated oxidative stressassociated with exercise is increased oxidant defenceviaupregulationofpowerfulantioxidantenzymeslikeSOD and Gpx. However, antioxidant supplementa-tion may discourage such adaptations by interferingwith the radical-mediated signal (Gomez-Cabreraet al., 2005, 2008b). The importance of this con-sequence may not be obvious if one assumes asurrogate protective effect from exogenous antiox-idants; however, endogenous mechanisms could bemore important when radical production is particu-larly high. Accordingly, Knez and colleagues (Knezet al., 2007) reported significantly greater oxidativedamage following half and full ironman triathlons inathletes who took antioxidant supplements than inthose who did not.Likewise, the challenges faced by energy produc-tion systems during training stimulate enhancedcapacity through the likes of increases mitochondrialmass and capillarization of muscle fibres, and  Antioxidant supplementation and endurance training   29  D o w nl o ad ed A t :23 :3823 F eb r u a r y2011
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