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El11cacy of Nutritional Ergogenic Aids in Hot Environments

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B.R. Ely, S.N. Cheuvront

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Many athletes seeking a competitive edge rely on nutritional ergogenic aids to improve performance. Carbohydrate (CHO) and caffeine (CAF) supplementation appear efficacious at enhancing endurancc exercise performance when studied under ideal circumstances, but the unique challcnges imposed by environmental strcssors such as heat may minimize or negate these effects. Simil3f to findings in temperate or cool environments, CHO intake during endurance exercise in hot environments produces a consistent performance benefit. But in contrast to the benefits observed in moderate environments. CAF affords no apparent perfonnanee advantage in the heat. These findings raise interesting questions about nutritional ergogenic mechanisms of action and otTer direction fo r future research.

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calfcine. carbohydrate, endurance performance, heal

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CURRENT TOPICS IN NlITRACEUTICAL RESEARCH Vol. 8, No. I, pp. 1-6.20 10 ISSN 1540-7535 prim, Copyright © 2010 by New Century Healm Publishen, LLC

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EFFICACY OF NUTRITIONAL ERGOGENIC AIDS IN HOT ENVIRONMENTS

Brcn R. 8y and Samuel N. Cheuvront

U.S. Army Research Institute of Environmental Medicine. Natick, MA

[Received November 19, 2008; Accepted January 5, 2009)

ABSTRACf; Many athkus suiting a comp~titiv~ ~dg~ rt/y on nutritional ugog~nic aids to improv~ paformanu. CArbohydrate (CHO) and cafftint (CAE) JUpplcnmlation ap~ar eJ!icm:ious al ~lIhancing mduranc~ exercis~ JK7:formanu whm studi~d unJ~r ideal drr:umstanus, but th~ uniqu~ challmg~s impoud by ~nvironm~mal stressors such as h~al may minimize or negau thtse effoCIS. Similar to findings in temperat~ or cool mvironments, CHG intake during mdurance exercise in hOI e,wironmenlS produces a consistmt pufomlance bnufit. But in contrast to the bnujirs oburved in modaau t1wironmeflts, CAE affords no apparmt pnfonnance advamage in the heat. These jindingr raise interesting questions about tlutritiOflal ergogenic mechanisms of action and offer Jirtction for future rnearclJ.

KEYWORDS: Caffeine, Carbohydr:lte, Endurance Performance,

H"" Corresponding Author. Dr. Samuel N. Cheuvront, U.S. Army Research Insritute of EnvironmCnt;,t1 Medicine, Thcrmal & Mount;,tin Medicine Division, K;,tnus St, Building 42, Natick, MA 01 760, USA; Phone: 508-233-5607; Fax: 508-233-5298; E--mail: S:unud.n.cheuvront@us.army.mil

lNTRO DUCfION Thc aerobic energy system used in endurance exercise requires

nutritional suppon: for optimal athlctic pcrformance.1t has been well documcntl:d mat athletes require more food energy, especially from ca rbohydratcs (C HO), to fuel high training volumes (American College of Sports Medicine, 2009). Beyond a alorically adequatc diet rich in CHO, cndurance athletes may also rely upon nutritional supplements in an effon to optimize nutrition and potcnrially gain :l competitive edge. While the

market abounds with myriad ergogenic nutritional spans aids, few haye consistcntly demonstr:Hcd performancc benefits for endurance exercise. C H O and caffeine (CAF) haye both been widely studied and appear to reliably enhance endurance performance under ordinary sporting circumstances (Hawley et

aI., 1997; Coyle, 2004; Dohcny and Smith, 2004; Hargreaves et aI., 2004).

Most nutritional ergogenic aids arc: studied under ncar-ideal conditions, but exposurc to environmcntal stressors such as heat can rcsult in profound decrements in aerobic performance (Galloway and Maughan, 1997; Elyetal., 2007; Ely et aI., 2010). Relatively few studies have explicitly addressed whether C HO or CAF can abate or surmount the perform:mce challenges imposed by the heat, but athletic competitions rcgularly occur in warm weather venucs. The purpose of this paper is to succinctly review me limitations to endurance exercise: imposed by hot environments. explorc potcntial mechanisms by which CHO and CAF might offset performance decrements, and examine mc perfonnance evidcnce.

PERFORMANCE LIMITI NG fACTORS IN HOT ENVIRONMENTS

A large maximal oxygen uptake (V0lm.) is a prerequisite for success in sports where aerobic endur.ulce is comesu.'d. Although high fitness and positive heat acclimation statUS will dampen the negative effects of heat stress (Pandoif, 1979), acute exposure to heat Stress reduces VO.z-as a consequence of dispbced peripher.al blood flow and a reduction in maximal cardiac output (Rowdl ct

aI., 1 %9; Nybo et aI., 2001; Gonzalez-Alonso and Callxt, 2003). Sustainable, submaximal exercise intensities are merd'ore reduced si nce the same workload rO:luires a larger fraaional utili7.3.tion of oxygen (Arngrimsson ct aI., 2003). The subStantial body heat storage that frequently occurs with exercise in mc heat cn also impair motor-neural function (Nybo and Nielsen, 2oo1a) and cerebral blood flow (Nybo and Nidsen, 2oolb), and accderatc CHO subs[fatc depletion (Fcbbraio, 2000; Jcntjens et aI., 2002) at a time when oxidation rates ofingested CHO arcsimulrancously reduced Oemjcns Ct aI., 2002). Sweat losses can be vcry high during excrcise-heat stress as evaporative ~eating is me principal

means of h~t loss wh~n air tcmperatures are h.igh. (> 35°C). Bccausesweat losses frequently outmatch fluid intake, dchydration ensues and can accentl41tC performance-limiting flctol'S (Gon7.a1C1.­Alonso et aI., 1997; Gonz.ait.'1.-Alonso et aI., 2000; Nybo Ct aI., 2001 ; Cheuvront et al., 2003). The same work task in the heat is

2 Nutritional Ergoglmit Aids

also often perceived as more difficult than in tempcr.J.te conditions (Nybo and Nielsen, 200 I c).

Ahhough performanct-limiting factors occur in concert, rather than in isolation, one or more factors may predominate. For example, the duration of exercise is inversely proportional ro its intensity, so the potential for heat to accelerate substrate depletion will become more important as exercise duration increases. The rate of heat storage, sweating rate/efficiency, and overall balance between heat gain and heat loss will also depend on the exercise intensity x environment (air temperature x humidity) interaction. It is clear, however, that even when controlling for substrate depletion, excessive heat storage, and hydration, performance is reduced by 15-20% for exercise of only 15-min duration when air temperature is increased from 20-40GC ( Ely er aI., 2010). The magnitude of the heat stress effea can be calibrared against a similar performance decrement (for the same exercise durarion) observed when ascending from sea level to 3,000m alticude (M:l7.zeo and Fulco, 2006). It therefore appears that environmental heat stress impairs aerobic exercise performance reliably, but variably, depending on circumstance.

H OW M IGHT CHO OR CAF IMPROVE P ERFORMANCE?

CHO is the most efficient fuel source (i.e., more ATP is produced per unit of oxygtn consumed) during exercise and ample CHO stores are necessary for sustaining high levels of performance during endurance exercise. The human body has a limited glycogen stornge capaciry, and CHO intake during endurance events (i .e., 42km marathon, Iron man triathlon) is considered critical for maintaining perfonnance. The ergogenicity ofCHO supplememation for endurance exercise has been widely studied. Recommendations for athletes looking to optimize performance include a high-CHO pre-exercise snack and 30-GOflhrofCHO forevenrs lasting longer than one hour (American CollegeofSpons Medicine, 2009). Pre-exerciseCHO loading delays fatigue (Hawley et al., 1997) and consumption ofCHO­electrolyte spons beverages during activi~ provides fuel to

muscles and delays hypoglycemia, thereby maintaining central nervous system (CNS) function (Nybo, 2003). CHO may also reduce the perception of effort (Nybo, 2003) or possibly improve motivation and central drive via putative CHO receptors in the mouth (Carter et al.. 2004).

The efficacy ofCAF has been demonstratl-d consistently across a range of intakes from 3 - 13 mg/kg, hut with no apparent dose-response relationship (Doherty and Smith, 2004). Several potential ergogenic benefits ofCAF have bt'en investigated. The original mechanism believed responsible for CAF's ergogenic effects with endurance exercise centered about increased adrenergic mobilization of free fatty acids and the consequent conservation of glycogen Storcs. However, the fr.J.gility of rhis explanadon is now clear and it is no longer a tenable primary hypothesis (Gr.J.ham, 2001). In vitro, CAF induces skdetal muscle contraction without membrane depolarization by interacting with calcium channels of the sarcoplasmic reticulum.

Howe"er, the dose of caffeine required to elicit this response is beyond [he range associated with toxiciry in humans (Davis et aI., 2003; Kalmar and Cafarelli, 2004). The present leading mechanistic explanation for CAF's performance-enhancing potenrial is adenosine receptor anragonism. CAr acts as a CNS stimulant by blocking adenosine binding sites (Davis et ai., 2003; Kalmar and Cafarelli, 2004). During endurance exercise, this may delay fatigue by increasing dopamine binding. As 'central fatigue' could be related to hyperthermia (Nybo and Nielsen, 2001a) through a neurotransmitter phenomenon, this might afford a porential performance benefit during conditions of environmental heat stress (WatSOn et al., 2005). The well­established analgesic effect of CAF to reduce the perception of pain (Mod et aI., 2003) and effort (Doherty and Smith, 2005) would also be advantageous.

EFFICACY OF CHO IN THE HEAT CHO supplemenration in the heat produces a consistent

performance benefit during long-duration activities (Table I). The effect appears most pronounced (9-24% improvement) in protocols using a fixed-intensi~ time to exhaustion rrrr) as a measure of performance (Carter et:al. 2003; Carter et ai. 2005; Nassif et al. 2008), where the provision of CHO affords an extended exercise duration. However, eltects aTe also seen in time-trial performance measures [TT] when preceded by a fixed­intensity pre-load (Davis et al. 1988a; Davis et al. 1988b; Millard-Stafford et aI., 1992; Below et aI. 1995; Millard-Stafford et al., 1997). When benefitli ofCHO afe observed in the heat, the magnitude of the effect appears similar to what has been reported for Cl-lO supplemcnt:ltion in temperate environment.~. However, the number of direct experimental comparisons between hot and cooler environments is limited (Febbr.J.io et aI., 1996).

'IT's simulate the strategy and bioenergetics of genuine competitive events, thus the smaller (3-1 0%) improvements seen in lTs m:ay be more applicable: to standard athletic competitions (e.g., 15km or 42km running events). Mill:ard-Stafford et al. ( 1992) showed asubsrantial improvemem in the final5km of a 40km run in warm conditions with a 7% CHO-elecrrolyte beverage when compared to an artificially-sweetened placebo . Nassif et al. (2008) found a similar benefit with a 60%V01 .....

lTE test when volunrccrs were aware th:at they were consuming CHO in bever:tge form, bur found no benefit in a double-blind trial when CHO wasconsumoo in capsule form with water. This interesting difference may be attributed to a psychological benefit ofCHO consumption, or to the activation of oral and phaIyngeal CHO receptOrs mat may additionally influence fatigue: (Carter et al. 2004). Other studies investigating differences in CHO beverage concemf"J.tion (Davis et aL, 1988a; Davis et aL, 1988b; Below et aI., 1995; Febbraio Ct al., 1996; Galloway and Maughan, 2000) or thesweerness ofCHO (Caner el al., 2005) have found minimal differences, although the standard 6-8% CHO hcverages (-30-60gCHO/hr) are used most often for optimal absorption. Importantly, consumption of CHO in beverage form allows diminishing fluid and electrolytes levels to

be replenished simultaneously during exercise In the heat, which additionally improves performance as compared to a concemrated CHO bolus with minimal fluid replacement (Below et al., 1995) and offers a vast improvement over exercise in the heat with no fluid or carbohydrate replacement alone (Galloway and Maughan, 2000).

Nutritional Ergogmic Aids 3

depletion, and dehydration to influence pcrl"onnance in a hot, dry environment (4O"C. 25%rh), Cheuvront et al. (2009) sriU found no

advantage ofCAF over placebo during a IS-min 11' preceded by a 3O-min exercise pre-load at 50% ofV0ln~. Although CAF appears ro alford a small improvement in the measurement of maximal voluntuy

TABLE L Summary of exercise performance with C H O in the heat. 'denores significant improvement; TIT", time to exhaustion; IT "' time trial performance; CE", cycle ergometry. All IT protocols involve a submaximal preload of varying duration except Millard-Stafford et at., 1990

PERFORMANCE TASK ENVIRONMENT CHODOSE % BENEFIT

Davis 1988a CE at 65% max, 3x3-min time trial (TI) 26-28aC, 67-68% rh During exercise 3-6% faster

Davis 1988b CE at 75% max, -30 min time [fia! (TT) 26-28aC, 67-68% rh During exercise 9% faster"

Millard-Stafford 1990 Swim 1.5km, CE 40km, run 10km (Tl1 3()<>C, 67% rh During exercise I % faster

Millard-Stafford 1992 Run 40km; final 5km hard (Tn 25-32aC, 62-82% rh Before + during 10% faster"

Bdow 1995 CE, 50-min submax, iO-min rime trial (TT) 31.2aC, 54% rh During exercise 6% faster"

Febbraio 1996 CE at 70% mal( to fatigue (1TE)

Millard-Stafford 1997 Run 15km; final1.6km hard (1-n

Caner 2003 CE at 60-73% max [0 farigue (11'£)

Carter 2005 CE at 60% max to fatigue (lTE)

Byrne 2005 3l(60-min loaded walks (11'£)

Bailey 2008 CE, submax to fatigue (TfE)

Nassif200S CE 60% max to fatigue CITE)

EFFICACY OF CAF IN THE HEAT The abili£y fOr Q1.F to improve endurance perfimnance in a ttmperate

environment has betrn well established (Doheny and Smith, 2(04). Ho~er, when supplemented in hot environmentS, null findi.ng-; prt.v.lll. Cohenet al. (J 996) compan:darangeofCAF intake; duringa half-marathon (2lkm, ~90 minutes) numingcompeticion at WBGT 24-2S<>C and found no effectS. 1be authors sprt-ulatoo that the -4% dehychation incurred during completion of the competitive 21 km run may ruve obscured or overwhelmed the potential benefits of CAE Howt'Ver, similar null remits were also seen in a 45km cycling Tr (~90 minutcs) when fluid ingcstion '.'.laS pemlined (Ferreira et aI., 2005). When controlling the potential fOr excessive h~rthermia, substrate

33<>C, 20-30% rh Before + during <5% longer

27-28aC, 62-76% rh Before + during 5% faster'

35<>C, 30% rh Before + during 18-19% longer-

35aC, 30% rh Before + during 12-16% longer'

35<>C, 55% rh Before + during 9% longer

35aC, 70% rh During exercise 9% longer'

28aC, 79% rh During exercise 11-24% longer'

com:mctions inter.>pers<..--d throughout prolonged, submaximal exercise in the heat (Del Coso et ai., 2(08), rhis h.as yet to translate into a demonstrable exercise perfOrmance advantlgC.

Table 2 summarizes the small effects of CAF (1-4%) on performance il} the heat, which are subsmntially less than the effccrs reported for temperate environmems (-12%) (Doheny and Smith, 2004). Heat stress prr I~reduces performance (Ely et aI., 2010) while increasing performance variability (Tyler and Sunderland, 2008; Cheuvront e( aI., 2009;), thus small effects might be obscured by added measurement noise. But large inconsistencies in individual responses to CAF, apparent wirhin the Table 2 studies, suggest a genuine non-phenomenon.

TABLE 2. Summary of exercise performance with CAF in the heal. ' denotes significant improvemellt vs. placebo; IT = time trial performance; C& cycle ergometry

PERFORMANCE TASK ENVlRONMENT CAFDOS£ % BENEFIT

Cohen 1996 21-km running race (TI) WBGT 24_28aC 5 mg· kg" ' I % faster

9mg.kg"' <J%slower

Ferreira 2005 45-km cycling race (TI) 28-32aC; 71 -78% rh 5 mg.kg· ' 4% faster

(WHGT 24.5-27"C) 9 mg·kg ' I % faster

Del Coso 2008 CE at 630/0 max for 120-min wirh 3GaC 29% rh 6 mg·kg·1 3% increase in power"

4l(4-sec power mea..~urements ern Cheuvront 2009 CE, 30 min ar 50% max, 40aC, 25% rh 9 mg·kg·1 2.5% more work

15-min time trial (TI")

4 Nutn'tional Ergogmie Aids

The combination ofCHO and C'AF may also have a synergistic metabolic effect (Yeo et al., 2005) and might improve endurance performance: in Ihe heal as a consequence: (Cureton et aI., 2007; Dd Coso et ai., 2008). Many nmritiol'! products (energy drinks, gds or chews. even sofi drinks) targeted mward endurance: athletes contain both CAF (25- loomgpcrserving) and C HO (~25 gper serving), which may work in concc:n to dday fatigue (Del Coso et aI., 2008), reduce sensation of effort and possibly improve performance in a warm or hot environment (CuretOn et aI., 2007). Further study of CAF and CHO combinations in a hOI environment may be a fertile area for nlture research.

CONCLUSIONS Environmemal h(:at stfCSS poses a unique challenge ro endurance

performance and may reduce or negate the efficacy of established nutritional ergogenic aids. CHO and O\F are among the few nutritional ergogenic aids with recognized legitimacy in ordinary envIronments. When examined against the larger body of literamre, comparably fewer studies have aamined the potential efficacy of CHO or O\F in hot environments (Tables 1, 2). Although a variety of performance tests, a ercise intensities, and exercise durations have been used over a range of warm to hOI environments, study outcomes show a consistent significant benefit ofCHO (8/12 studies) in the heat but no benefit ofCAF (0/4 studies). These ohscrv:nions raise important questionsconceming the underlying mechanism(s) ofCHO and CAF ergogenicity and also suppon: fufUrt study of the potential ergogenic interaction of CHO and CAr for enhancing endurance exercise performance in hot environments.

ACKNOWLEDGEMENTS The authors would like ro thank Dr. Asker Jeukendrup for his

responses 10 our queries. The opinions or assertions contained herein arc the private vit.'Ws of (he author(s) and are nOt (0 be construed as official or as reflecting the views of rhe Army or rhe Department of Defense.

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