carbohydrate ingestion during team games exercise

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Carbohydrate Ingestion during TeamGames ExerciseCurrent Knowledge and Areas for Future Investigation

Shaun M. Phillips, John Sproule and Anthony P. Turner

Institute of Sport, Physical Education and Health Studies, University of Edinburgh, Edinburgh, UK

Contents

Abstract. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5591. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5602. Methodology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5613. Carbohydrate Supplementation Immediately before and during Prolonged Intermittent Exercise . 562

3.1 Early Laboratory Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5623.2 Team Game-Specific Laboratory and Field Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5643.3 Mental Function and Skill Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5683.4 Physiological and Metabolic Responses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5723.5 Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 573

4. Mechanisms of Enhancement with Carbohydrate Supplementation during Prolonged Intermittent,

High-Intensity Exercise . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5734.1 Intermittent Exercise Capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5734.2 Sprint Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5744.3 Mental Function and Skill Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 575

5. Modulators of Carbohydrate Efficacy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5755.1 Fluid Volume and Solution Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 576

5.1.1 Fluid Volume . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5765.1.2 Carbohydrate Concentration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5765.1.3 Carbohydrate Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5765.1.4 Solution Osmolality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5775.1.5 Recommendations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 577

5.2 Fluid and Carbohydrate Ingestion Pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 577

5.3 Glycaemic Index of Pre-Exercise Meals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5785.4 Fluid Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5785.5 Carbohydrate Mouthwash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5795.6 Ambient Temperature. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5795.7 Populations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 579

6. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 580

Abstract There is a growing body of research on the influence of ingesting carbo-hydrate-electrolyte solutions immediately prior to and during prolonged in-termittent, high-intensity exercise (team games exercise) designed to replicate

field-based team games. This review presents the current body of knowledgein this area, and identifies avenues of further research. Almost all early worksupported the ingestion of carbohydrate-electrolyte solutions during prolonged

REVIEW ARTICLESports Med 2011; 41 (7): 559-585

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intermittent exercise, but was subject to methodological limitations. A keyconcern was the use of exercise protocols characterized by prolonged periodsat the same exercise intensity, the lack of maximal- or high-intensity workcomponents and long periods of seated recovery, which failed to replicate theactivity pattern or physiological demand of team games exercise. The adventof protocols specifically designed to replicate the demands of field-based teamgames enabled a more externally valid assessment of the influence of carbo-hydrate ingestion during this form of exercise. Once again, the researchoverwhelmingly supports carbohydrate ingestion immediately prior to andduring team games exercise for improving time to exhaustion during inter-mittent running.

While the external validity of exhaustive exercise at fixed prescribed in-tensities as an assessment of exercise capacity during team games may appear

questionable, these assessments should perhaps not be viewed as exhaustiveexercise tests per se, but as indicators of the ability to maintain high-intensityexercise, which is a recognized marker of performance and fatigue duringfield-based team games. Possible mechanisms of exercise capacity enhance-ment include sparing of muscle glycogen, glycogen resynthesis during low-intensity exercise periods and attenuated effort perception during exercise.Most research fails to show improvements in sprint performance during teamgames exercise with carbohydrate ingestion, perhaps due to the lack of in-fluence of carbohydrate on sprint performance when endogenous muscleglycogen concentration remains above a critical threshold of~200 mmol/kgdry weight. Despite the increasing number of publications in this area, few

studies have attempted to drive the research base forward by investigatingpotential modulators of carbohydrate efficacy during team games exercise,preventing the formulation of optimal carbohydrate intake guidelines. Po-tential modulators may be different from those during prolonged steady-stateexercise due to the constantly changing exercise intensity and frequency,duration and intensity of rest intervals, potential for team games exercise toslow the rate of gastric emptying and the restricted access to carbohydrate-electrolyte solutions during many team games.

This review highlights fluid volume, carbohydrate concentration, carbo-hydrate composition and solution osmolality; the glycaemic index of pre-exercise meals; fluid and carbohydrate ingestion patterns; fluid temperature;carbohydrate mouthwashes; carbohydrate supplementation in different ambienttemperatures; and investigation of all of these areas in different subject popula-tions as important avenues for future research to enable a more comprehensiveunderstanding of carbohydrate ingestion during team games exercise.

1. Introduction

The ergogenic effects of ingesting carbohydrate-electrolyte solutions prior to and during prolonged(45 min) moderate to high-intensity (>75% max-

imal oxygen uptake [.

VO2max])[1] steady-state ex-ercise (sub-maximal exercise requiring a constantpower output and a stable heart rate [HR] and

oxygen uptake [.

VO2])[2] have been known for sev-

eral decades.[1,3] During steady-state cycling, exog-enous carbohydrate ingestion appears to maintaineuglycaemia and high carbohydrate oxidationrates, and during steady-state running it has been

demonstrated to reduce net muscle glycogen break-down in type I muscle fibres.[1] Carbohydrate in-gestion can improve both exercise performance,

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defined as distance covered in a set time or thetime to complete a set distance/amount of work,[4]

and exercise capacity, defined as time to exhaus-tion at a fixed exercise intensity.[5]

The mean whole-game exercise intensity dur-ing adult field-based team games (soccer, rugbyand field hockey) has been estimated at 7080%.VO2max, similar to prolonged steady-state ex-ercise,[1,6] and appears sufficient to promote sig-nificant muscle glycogen depletion,[7] althoughthis is not a consistent finding.[8] Muscle glycogenavailability during prolonged intermittent, high-

intensity exercise (hereafter referred to as teamgames exercise) can influence work output, dis-tance covered and sprinting frequency, partic-ularly in the later stages of exercise.[7,9] Therefore,ingesting carbohydrate-electrolyte solutions dur-ing field-based team games may prove beneficialby attenuating performance decrements that canoccur towards the end of a game. In their earlierreview on fluid and carbohydrate replacementduring intermittent exercise, Shi and Gisolfi[10]

provided recommendations for the optimal carbo-

hydrate concentration, composition and osmo-lality of a carbohydrate-electrolyte solution foruse before and during team games exercise. Sincethis review, a large number of publications havespecifically addressed the ingestion of carbohydrate-electrolyte solutions immediately prior to andduring team games exercise, and an updated synth-esis of current knowledge in this field is required.

The aim of this review is to present the currentstate of knowledge on carbohydrate ingestionimmediately prior to and during laboratory and

field exercise typical of field-based team games.Suggestions are provided for further researchthat would increase knowledge in this area inboth breadth and depth.

2. Methodology

To locate articles focusing on the effect ofcarbohydrate supplementation on team gamesexercise performance and capacity, searches inMEDLINE (PubMed) were performed using the

terms carbohydrate prolonged intermittent ex-ercise, carbohydrate intermittent exercise, car-bohydrate team games, carbohydrate endurance

exercise, carbohydrate exercise capacity andcarbohydrate sprint performance. For the influ-ence of carbohydrate supplementation on mentalfunction and skill performance, the followingMEDLINE (PubMed) searches were performed:carbohydrate skill team games, carbohydrateshooting passing performance, carbohydrateskill performance, carbohydrate mental functionteam games, carbohydrate cognitive functionexercise, carbohydrate effort perception ex-ercise. The related citations service in PubMedwas explored for each highlighted abstract to

locate additional relevant articles. The referencelist of each article was also hand searched forother appropriate studies. These searches yieldeda total of 36 articles for the influence of carbo-hydrate on team games exercise performance andcapacity, and 25 articles for mental function andskill performance. Searches were not date limited,as the total research output in this area is man-ageable without using this limitation and the au-thors wanted to retrieve the earliest papers in thefield. Only studies related to soccer, rugby and

field hockey were incorporated, leading to theexclusion of 27 articles. Studies using additionalsupplementations (i.e. carbohydrate with caffeine,carbohydrate with protein) that did not includea direct comparison between a carbohydrate-electrolyte solution and a placebo solution wereexcluded. Discussion of these articles would haveshifted the focus of the review, which is solelyon the effect of carbohydrate supplementation.Using this exclusion criterion, four articles wereremoved. This review focuses on the acute effects

of carbohydrate supplementation in team games;therefore, studies that supplemented the firstbolus of carbohydrate >1 hour prior to the startof exercise were discounted. As a result, threearticles were removed. Articles investigating theinfluence of carbohydrate on immune functionduring team games exercise were incorporatedinto the discussion of the physiological and met-abolic responses to team games exercise withcarbohydrate supplementation, but the influenceon immune function was not discussed. A suffi-

ciently in-depth review of this literature is outsidethe aims of this article. Based on these criteria, atotal of 21 articles were included in the discussion

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of team games exercise performance and capacity,and 11 in the discussion of mental function andskill performance.

3. Carbohydrate SupplementationImmediately before and duringProlonged Intermittent Exercise

The following sections discuss early researchthat supplemented carbohydrate during prolongedintermittent exercise atypical of team games ac-tivity, followed by the more recent body of work

that attempted to utilize team games-specific pro-tocols and practices. The influence of carbohydratesupplementation on mental function and skill per-formance, and on physiological and metabolic re-sponses during team games exercise is also discussed.

3.1 Early Laboratory Work

All studies in this section were placebo con-trolled and are summarized in table I. This initialbody of work demonstrated that (i) consuming

carbohydrate-electrolyte solutions during prolongedintermittent exercise can significantly improveexercise performance and capacity; (ii) consum-ing carbohydrate-electrolyte solutions may sig-nificantly attenuate muscle glycogen utilizationduring prolonged intermittent exercise; (iii) solidcarbohydrate is not significantly different froma carbohydrate-electrolyte solution in improvingintermittent exercise capacity; and (iv) the effi-cacy of carbohydrate-electrolyte solutions duringprolonged intermittent exercise may be influenced

by the intensities at which exercise is performed.However, prevalent methodological issues mustbe discussed prior to interpreting these conclusions.

Murray et al.[11] and Coggan and Coyle[12] wereamong the first to study the effects of carbohy-drate supplementation during prolonged inter-mittent exercise. It is unclear why Murray et al.[11]

conducted their study in a high ambient tem-perature. A thermoneutral trial should have beenincluded for comparison due to the possibility ofincreased glycogen breakdown in high ambient

temperatures.[16-18] Although both protocols wereintermittent, neither was consistent with the activ-ity pattern or physiological demand of intermit-

tent exercise in the field due to the nature of therecovery provided, the lack of a maximal- or high-intensity component, the structured and pro-longed duration of the workloads and the use of acycle ergometer. However, at this early stage ofstudy, the authors may have been more concernedwith establishing a baseline of data using control-led research designs rather than maximizing ex-ternal validity.

Research by Murray et al.[13] and Yaspelkiset al.,[14] while again supporting carbohydrate sup-plementation, is subject to similar methodological

issues. The regimented and specifically timed ex-ercise intensities, with no maximal work and longperiods of seated recovery, did not accurately re-flect the physiological demand of team games.Additionally, exercise performance and capacitywas assessed using steady-state rather than in-termittent exercise. Yaspelkis et al.[14] did notprovide body mass (BM)-standardized volumesof the carbohydrate or placebo solutions, mean-ing subjects of lower BM received a larger rela-tive carbohydrate intake. Furthermore, muscle

biopsy data were not collected during the solidcarbohydrate trial, preventing full data inter-pretation and hindering the ability to understandthe mechanisms behind the improvement in ex-ercise capacity.

The lack of improvement in intermittent ex-ercise capacity with carbohydrate supplementa-tion shown by Nassis et al.[15] is in contrast withthe literature discussed to this point. As the au-thors stated, the protocol probably made largedemands on muscle glycogen stores; therefore, it

would be expected that carbohydrate ingestionwould have improved exercise capacity. How-ever, while the volume of fluid ingested during exer-cise was similar to most related studies (2 mL/kgBM), the lower pre-exercise bolus (3 mL/kg BM)facilitated a lower overall carbohydrate intakeduring the protocol than most related work. Thetotal amount of carbohydrate ingested during thetrial (~36 g/hour) was above the minimum intakeof 16g/hour that is required for performance en-hancement,[1] but was notably lower than the re-

commended intake for maximizing carbohydratedelivery (6070 g/hour).[19] Furthermore, the lowervolume of fluid entering the stomach may have

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resulted in a suboptimal rate of gastric emptying(GE), possibly further attenuating the delivery ofcarbohydrate to the intestine. Therefore, carbo-hydrate may not have been systemically presentin sufficient amounts to alter metabolism. This issupported by no significant between-trials dif-ference in blood glucose concentration (with theexception of one timepoint), blood lactate con-centration or respiratory exchange ratio (RER).However, due to the variable intensities of theprotocol, RER may not have been a valid methodof assessing metabolism. Buffering of H+ ions

produced during the high-intensity periods of theprotocol leads to greater production of CO2 re-quiring removal at the lungs, thereby over-inflatingRER.[15] It is also possible that the exercise inten-sity in the final part of the protocol (90%

.VO2max)

was too intense, possibly causing fatigue to occuras a result of factors other than glycogen avail-ability, such as phosphocreatine depletion.[20,21]

If so, this negates the goal of the study and mayhelp to explain the result being somewhat out ofstep with other research in the area.

3.2 Team Game-Specific Laboratoryand Field Work

All studies discussed in this section are sum-marized in table II. Leatt and Jacobs[22] attempt-ed to expand the research base by investigating,for the first time, the effect of carbohydrate in-gestion on muscle glycogen depletion during anexhibition soccer match. Unfortunately, in anindependent study design comprising two groups,

only five subjects per group were used, plac-ing the rigour of any statistical analyses underquestion. The authors attempted to control thebetween-groups physical demand of the game byusing players from the same positions on thefield. However, significant variations in exerciseintensity and distance covered and, hence, muscleglycogen utilization, could have occurred be-tween groups due to factors including team tac-tics, the activity profile of the opposing team [37]

and the score in the game. This could have influ-

enced the reported efficacy of the carbohydrate-electrolyte solution. However, Leatt and Jacobs[22]

attempted to control the influence of team tactics

and activity profile by analysing an intra-squadmatch. A time-motion analysis of each player wouldhave been useful to confirm the physical demandexperienced. Solutions were administered in asingle-blind fashion, suggesting the potential forexperimenter bias. However, the investigators hadno direct contact with subjects during the match.All subjects consumed 0.5 L of the carbohydrate(containing 35 g carbohydrate) or placebo solu-tion rather than a volume matched to individualBM. The authors stated that post-match bloodsamples and muscle biopsies were taken within

20 minutes and 45 minutes of the match ending,respectively. If these tests were administered atdifferent times between subjects, the reliability ofthe results could have been affected due to inter-subject differences in lactate dynamics[38] and theonset of rapid glycogen resynthesis, particularlyin the carbohydrate group.[39,40] While this maybe speculative, it would have been beneficial tostandardize these measurements. It may also havebeen prudent to collect some performance mea-sures during the match to investigate whether gly-

cogen sparing in the carbohydrate trial facilitatedany improvement or maintenance of perfor-mance compared with placebo.

In a defining study, Nicholas et al.[23] demon-strated, for the first time, a 33% improvement inintermittent exercise capacity when a carbohydrate-electrolyte solution was consumed immediatelyprior to and during the Loughborough IntermittentShuttle Test (LIST), a protocol specifically de-signed to replicate the physiological demand ofsoccer.[41] Carbohydrate supplementation did not

significantly improve sprint performance duringthe protocol. Solutions were prescribed relativeto BM and in a double-blind, counterbalancedfashion, ensuring equal fluid and carbohydrate(0.90 g/kg BM) intake across all subjects. Thesestrengths are in direct comparison to the issueshighlighted in section 3.1.

Most subsequent research investigating car-bohydrate supplementation during team gamesexercise employed the LIST protocol or a slightmodification of it. Almost without exception, this

research demonstrates that carbohydrate sup-plementation improves intermittent exercise ca-pacity[24,26,27,30,32,33] or promotes physiological

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Table II. Summary of team games-specific laboratory and field studies on the effects of carbohydrate (CHO) supplementation immediately bef

on the intermittent exercise performance and capacity of adultsa

Study No. of subjects

and training level

Protocol Supplementation Significant findings Lim

Leatt and

Jacobs[22]10 highly trained

soccer players

90 min outdoor friendly soccer

match, 10 min interval

Treatment (n =5) and PLA

(n = 5) group

7% glucose polymer solution

0.5L ~10 min before match

and at half-time

~39% reduction in muscle

glycogen use with CHO

ingestion

Lo

Si

So

Va

m

No

Nicholas et al.[23] 9 trained games

players

Standard LIST

Double-blind design

6.9% CHO-E solution

5 mL/kg BM prior to exercise

2 mL/kg BM every 15 min

during exercise

33% longer time

to exhaustion

Sprint performance

unchanged

No

Davis et al.[24] 10 active Standard LIST

Double-blind design

20% CHO solution

20% CHO+BCAA solution

5 mL/kg BM 1h and 10min

before exercise


during exercise (CHO only)

Significant increase in time

to exhaustion (52% CHO,

42% CHO+BCAA)

No difference between

treatments

Sp

Nicholas et al.[25] 6 trained games

players

Extended LIST (part A only,

90 min duration)

6.9% CHO-E solution

5 mL/kg BM prior to exercise2 mL/kg BM every 15 min

during exercise

Sprint performance

unchanged22% reduction in muscle

glycogen use

Ex

Blno

Davis et al.[26] 8 active Standard LIST

Double-blind design

6% CHO-E solution

5 mL/kg BM 10 min before

exercise


during exercise

32% longer time to

exhaustion

Sp

Welsh et al.[27] 10 (5 F) trained

games players

Modified LIST:

4 modified part A, with a

20 min recovery between the

second and third set

Modified part A:

3 20 m walking

2 vertical jumps at 80%

maximum height

1 20 m sprint

3 20 m run at 120%.

VO2max2 vertical jumps at 80%

maximum height

18% and 6% CHO-E solution



(6% only)

5 mL/kg BM at half-time

(18% only)

37% longer time to

exhaustion

Significantly faster sprint

performance during final

15min

Similar physiological

function between trials

No

mo

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Table II. Contd


and training level


3 20 m jogging at 55%.

VO2maxDouble-blind design

Motor skill, jumping, cognitive

and emotion tests undertaken

before, during, and after

protocol

Morris et al.[18] 9 active Modified LIST in 30C heat:

5 part A, followed by 60 sec

run/60 sec rest until exhaustion

6.5% CHO-E solution

6.5mL/kg BM prior to

exercise

4.5mL/kg BM every 15 min

during exercise

No difference in sprint

performance or time to

exhaustion



Su

ex

An

dis

Ve

the

Bl

sta

Winnick et al.[28] 20 (10 F) active Modified LIST:

4 15 min modified part A,

5 min interval after set 1 and

3, 20 min interval after set 2

Modified part A, see Welsh

et al.[27]

Double-blind design

Motor skill, jumping, force

sensation, cognitive and

emotion tests undertaken

before, during and after

protocol

6% CHO-E solution


and at beginning of 20 min

interval

3 mL/kg BM beginning of

each 5min interval, 10 min

into 20 min interval, and

immediately after fourth set

Significantly faster sprint

performance during final

15min



No

mo

Ali et al.[29] 16 trained games

players


90 min duration) following

glycogen-depleting exercise

Shooting and passing tests

undertaken before and afterexercise

6.4% CHO-E solution



during exercise

Significantly faster mean

sprint performance during

protocol

Ex

Bl

sta

Patterson and

Gray[30]7 trained games

players

Standard LIST

Double-blind design

CHO gel

0.89 mL/kg BM prior to

exercise

0.35 mL/kg BM every 15 min

during exercise

45% longer time to

exhaustion



CH

so



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Table II. Contd


and training level


Clarke et al.[31] 12 trained games

players

Soccer-specific motorized

treadmill protocol (2 45min

with 15 min recovery)

6.9% CHO-E solution


and during recovery (trial 1)

Same total volume as trial 1

at 15 min intervals (trial 2)


function and metabolic

response between trials

Significant attenuation in

gut fullness in trial 2

No

Davison et al.[32] 10 untrained Modified LIST:Part A for 60 min followed by

incremental run to exhaustion

Double-blind design

6% CHO-E solution8 mL/kg BM 15 min before

exercise

8% longer time toexhaustion

CH

Foskett et al.[33] 6 active games

players

Modified LIST:

Part A for 90 min, and then

continuously to exhaustion

Double-blind design

6.4% CHO-E solution



during exercise

21% longer time to

exhaustion

Sprint performance

unchanged



Lo

Abbey and

Rankin[34]10 trained games

players

5 15 min intermittent exercise:

2 55 m jogging at 55%.

VO2max2 55 m running at 120%.

VO2max2 55 m walking

4 55 m sprinting

Agility and shooting tests

performed during exercise

6% CHO-E solution

8.8mL/kg BM 30 min prior to

exercise and at half-time

No difference in time to

exhaustion


performance

CH

en

CH

lim

Bl

sta

Ali and Williams[35] 17 trained games

players


90 min duration) following

glycogen-depleting exercise

Passing test performed before,

every 15 min during and after

exercise

6.4% CHO-E solution



during exercise


performance



Ex

Bl

sta

Roberts et al.[36] 8 trained games

players

BURST test 9% CHO-E solution

1 h before exercise and

21, 46, and 77 min during

exercise

Volume ingested: 1.2 g/kg

BM/h


performance



Pr

da

Bl

sta

a All studies were PLA controlled.

BCAA =branched-chain amino acids; BM=body mass; BURST=Bath University Rugby Shuttle Test; CHO-E= carbohydrate-electrolyte

Intermittent Shuttle Test; PLA=placebo;.

VO2max=maximal oxygen uptake.



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and metabolic alterations that infer greater per-formance and/or capacity.[25,31] Improvements inintermittent exercise capacity with carbohydrateingestion during part B of the non-modified LISTrange between 32% and 52%, with effect sizesranging from d = 0.442.69.[23,24,26,30] The validityof this performance measure should be consid-ered, as team games athletes are rarely required tocontinue running to exhaustion during trainingor competition. However, the intermittent run toexhaustion should perhaps not be viewed as anexhaustive exercise test per se, but rather as an as-

sessment of the ability to maintain high-intensityexercise, which is a recognized marker of perfor-mance and fatigue during field-based team games.[37]

Despite this, the fixed workloads of most teamgames protocols (e.g. part A of the LIST proto-col) do not permit the subject to alter their workrate; therefore, the influence of carbohydrate onself-governed work rate during team games ex-ercise cannot be quantified. Future protocols, suchas that proposed by Ali et al.[42] should addressthis. The influence of carbohydrate supplementa-

tion on sprint performance during team games ex-ercise is contentious, with only three studies showingany form of improvement[27-29] (see section 4.2).

Abbey and Rankin[34] found no effect of car-bohydrate supplementation on exercise perfor-mance or capacity during a team games protocol.However, the different protocol and tests of sprintperformance and exercise capacity from thosediscussed above, along with less frequent carbo-hydrate ingestion, may help to explain this. Morriset al.[18] found no performance or capacity bene-

fits with carbohydrate ingestion during a slightlymodified LIST in 30C heat. Lack of perfor-mance enhancement was attributed to carbohy-drate availability not being a limiting factor in theunacclimatized subjects. As the authors must haverecognized this prior to the study, it raises thequestion of why they failed to account for it by,for example, acclimatizing the subjects. The rateof rise in rectal temperature was greatest in thecarbohydrate and placebo trials compared withthe flavoured water trial. The authors suggested

this was indicative of greater thermal strain dueto impaired fluid delivery with ingestion of thecarbohydrate-electrolyte solution. However, this

is confused when it is noted that mean rectaltemperature at the end of the protocol was notsignificantly different between the three trials.Furthermore, impaired fluid delivery with carbo-hydrate ingestion is dependent on multiple factorsthat were not measured in this study (section 5.1),and this does not explain the similar rate of rise inrectal temperature in the placebo trial. An ordereffect was reported for the total distance run (19%increase in trial 3 compared with trial 1), despite arandomized and counterbalanced approach totrial ordering. This may reflect a learning and/or,

possibly, an acclimatization effect across the threetrials. Finally, only four of the nine subjects com-pleted the full protocol in the flavoured water trial,three in the placebo trial and only one in the car-bohydrate trial. This invalidates any statisticaltests carried out on the data. As a result of theseissues, the findings of this study should be inter-preted with extreme caution.

3.3 Mental Function and Skill Performance

All studies in this section are summarized intable III. Carbohydrate intake during team gamesexercise has been associated with significantlybetter maintenance of whole-body motor skillsand mood state,[27,28] and reduced perception ofexertion,[29] fatigue[27] and force production[28] inthe latter stages of exercise. Carbohydrate intakedoes not appear to influence cognitive functionduring team games exercise.[27,28] Roberts et al.[36]

found no influence of carbohydrate on the samemotor skills test used by Welsh et al.[27] and Winnick

et al.[28] and attributed this to the different pro-tocol used in their study. The lack of influence ofcarbohydrate on agility in the study of Abbey andRankin[34] may have been due to carbohydratenot being a limiting factor in the exercise protocol.

Findings on the influence of carbohydrate onmental function during exercise may be influencedby the assessment procedure used, with Backhouseet al.[46] suggesting the Profile of Mood States testmay not be sensitive enough to detect treatmenteffects on psychological responses to exercise. Using

the Felt Arousal Scale, a subjective measure ofperceived arousal, the authors demonstrated a sig-nificantly better maintenance of perceived arousal

568 Phillips et al.



11/27

Table III. Summary of team game-specific laboratory and field studies on the effects of carbohydrate (CHO) supplementation immediately bef

on mental function and skill performance in adultsa


and training

level

Protocol Supplementation Significant findings

Zeederberg et al.[43] 22 trained

games players

90 min outdoor competitive soccer

game

Tackling, heading, dribbling, shooting,

passing and ball control performance

recorded throughout game

6.9% CHO-E solution

5 mL/kg BM 15 min prior to

match and at half-time

No significant effect

tackling, heading,

dribbling, shooting,

passing or ball contr

ability

Northcott et al.[44] 10 active

games players

90 min circuit designed to replicate

soccer, 15 min interval

Passing and shooting tests undertaken

every 15 min during protocol

8% CHO-E solution

8 mL/kg BM 15 min prior to


Significantly better

maintenance of

passing and shootin

performance in last

15 min of exercise

Ostojic and Mazic[45] 22 trained

games players

90 min outdoor soccer match, 15 min

interval. Treatment (n =11) and PLA

(n =11) group

Dribbling, precision, coordination and

power tests undertaken after the match

7% CHO-E solution

5 mL/kg BM immediately

prior to match


during match

Significant

improvement in

dribbling performanc

and precision scores

No difference in

coordination or powe

Welsh et al.[27] 10 (5 F) trained

games players

Modified LIST:

4 modified part A, with a 20 min

recovery between the second and

third set

Modified part A:

3 20 m walking

2 vertical jumps at 80% maximum

height

1 20 m sprint

3 20 m run at 120%.VO2max

2 vertical jumps at 80%

maximumheight

3 20 m jogging at 55%.VO2max

Double-blind design

Motor skill (hopscotch), jumping,

cognitive (SCWT) and mood (POMS)

tests undertaken before, during and

after protocol

18% and 6% CHO-E solution



(6% only)

5 mL/kg BM at half-time

(18% only)


maintenance of moto

skill in last 15 min

Significantly lower

sensation of fatigue

exhaustion

No difference in

cognitive function



SportsMed2011;41(7)


12/27

Table III. Contd


and training

level

Protocol Supplementation Significant findings

Winnick et al.[28] 20 (10 F) active

games players

Modified LIST:

4 15 min modified part A, 5min

interval after set 1 and 3, 20min interval

after set 2

Modified part A, see Welsh et al.[27]

Double-blind designMotor skill (hopscotch), jumping, force

sensation (perception of force at wrist

extensors), cognitive (SCWT) and

mood (external POMS) tests

undertaken before, during and after

protocol

6% CHO-E solution


and at beginning of 20 min

interval

3 mL/kg BM beginning of

each 5 min interval, 10 mininto 20 min interval and

immediately after fourth set


motor skills during fi

30min

Significantly improve

mood during final

15minSignificantly reduced

force sensation

No influence on

cognitive function

Ali et al.[29] 16 trained

games players

Extended LIST (part A only, 90 min

duration) following glycogen-depleting

exercise

LSST and LSPT undertaken before and

after exercise

6.4% CHO-E solution



during exercise

Significant reduction

RPE during final 15 m

of exercise


maintenance of

shooting performanc

No difference in

passing performance

Backhouse et al.[46] 17 trained

games players


duration)

Measures of pleasure-displeasure

(scale) and perceived arousal (felt

arousal sale) recorded throughout

exercise

6.4% CHO-E solution



during exercise

Significantly greater

perceived activation

last 30 min

Trend for attenuation

RPE in last 30 min o

exercise

Abbey and Rankin[34] 10 trained

games players

5 15 min intermittent exercise:

2 55 m jogging at 55%.VO2max

2 55 m running at 120%.VO2max

2 55 m walking

4 55 m sprinting

Agility and shooting tests performedduring exercise

6% CHO-E solution

8.8 mL/kg BM 30 min prior to


No significant

difference in agility

No significant

difference in passing

performance

Ali and Williams[35] 17 trained

games players


duration) following glycogen-depleting

exercise

LSPT performed before, every 15 min

during and after exercise

6.4% CHO-E solution



during exercise

No difference in

passing performance



SportsMed2011;41(7)


13/27

during the final 30 minutes of the LIST with car-bohydrate ingestion, along with a non-significantattenuation in the rating of perceived exertion(RPE). Exercise performance and capacity werenot assessed, making it impossible to observe wheth-er increased arousal influenced these measures.

Zeederberg et al.[43] found no effect of a car-bohydrate-electrolyte solution on aspects of skillperformance in two teams during two outdoorsoccer matches. The ability to successfully com-plete these actions was determined according toset criteria defined by the authors. For example,

passing performance was governed by the criter-ion a player kicks the ball to a team-mate with-out interception by the opposition or over thesideline for a defensive clearance. This does notaccount for the possibility that the player mis-kicked the ball (e.g. in attempting a shot on goaland the ball happened to reach a team-mate). Italso does not quantify the quality of the pass, whichmay have been successful due to poor positioningof the opposition players rather than passing ac-curacy. Hypoglycaemia may inhibit performance

of skills requiring sensory-visual information,small and precise postural changes and tacticalthinking and inter-player cooperation,[29,43] pro-viding a rationale for carbohydrate ingestion toimprove skill performance. However, the absenceof post-match hypoglycaemia in either trial in theZeederberg et al.[43] study suggests carbohydrateavailability was not an issue, possibly negating therequirement for carbohydrate ingestion. The con-flicting results reported by Ostojic and Mazic[45]

(table III) may be due to differences in the tests

administered or the degree of test familiarizationthe subjects were given. Additionally, Ostojic andMazic[45] conducted their tests after a soccer match,and therefore presented no evidence that carbohy-drate ingestion modulated skill aspects during soc-cer. As both studies were conducted in the field, theextraneous factors that can affect field-based soccerperformance (section 3.2) could have also influencedthe measures of skill in both studies.[29]

Northcott et al.[44] found a significantly bettermaintenance of passing and shooting performance

with carbohydrate ingestion. However, no infor-mation was provided on the validity or reliabilityof the shooting and passing tests, or the exerciseTa

bleIII.Contd

Study

No.ofsubjects

andtraining

level

Protocol

Supplementation

Significantfindings

Limitations

Currelletal.[47]

11trained

gamesplayers

10

6minexercise:

10s

ecwalk,10secjog,10seccruise,

10s

ecjog,10seccruise,15secwalk,

5se

csprint,15secjog,5secsprint

Exe

rcisepatternrepeatedfourtimes

per6minexerciseblock

Testsofagility,dribbling,kickingand

headingperformedduringexercise

7.5

%

CHO-Esolution

6

mL/kgBM30minpriorto

exercise

4

mL/kgBMathalf-time

1

mL/kgBMevery12min

duringexercise

Significant

improvementin

dribbling,agilityand

shootingp

erformance

Nosignific

ant

difference

inheading

performan

ce

Blindingproceduresused

werenotstated

Robertsetal.[36]

8trained

gamesplayers

BUR

STtest

Motorskill(hopscotch)testperformed

befo

re,duringandafterexercise

9%

CHO-Esolution1h

beforeexerciseand21,46

and77minduringexercise

Volumeingested:1.2g/kg

BM/h

Nodifferentinmotor

skillsthrou

ghout

protocol

Protocoldesignbasedon

activityprofiledataofR

ugby

Unionforwardsonly

Blindingproceduresused

werenotstated

a

AllstudieswerePLAcontrolled.

BM=body

mass;BURST=BathUniversityRugbyS

huttleTest;CHO-E=carbohydrateelectrolyte;F=females;LIST=LoughboroughIntermittentShuttleTest;LSPT=Loughborough

SoccerPa

ssingTest;LSST=LoughboroughSocce

rShootingTest;PLA=placebo;POMS=

ProfileofMoodStates;RPE=ratingofp

erceivedexertion;SCWT=StroopColourWord

Test;. VO2max=maximaloxygenuptake.




14/27

protocol. Distance covered increased significantlyduring the first and second 45-minute periods ofthe protocol in the carbohydrate trial. This mayhave been independent of the solution consumed,possibly representing a protocol reliability issue.

The recent development and validation ofspecific laboratory tests of soccer shooting accu-racy and passing performance[48] has enabled amore objective quantification of the influence ofcarbohydrate supplementation on these variables.Carbohydrate ingestion before and during teamgames exercise has been demonstrated to signif-

icantly improve or maintain shooting accuracyin glycogen depleted[29] and non-glycogen de-pleted[47] subjects, with no significant influenceon passing performance.[29,34,35] However, the ob-servation that the performance of a dribbling testis significantly better maintained during the last30 minutes of the LIST when a non-carbohydratefluid is consumed compared with no fluid inges-tion,[49] suggests that the relative influence offluid and carbohydrate intake on skill perfor-mance in team games should be quantified. This

will determine whether one is more importantthan the other with regard to skill performance,and whether an additive effect is evident whenfluid and carbohydrate are co-ingested.

3.4 Physiological and Metabolic Responses

Ingestion of carbohydrate-electrolyte solu-tions does not appear to directly influence.VO2, HR, core temperature (Tcore), plasma vol-ume (PV) or fluid loss during team games

exercise.[11,13-15,22-25,27-29,31,35,36,43,45,46,50]Some au-thors have reported a significantly lower HRthroughout exercise with carbohydrate inges-tion,[24,26] attributed to a trend for better main-tenance of PV. However, other work has reportednon-significantly greater PV losses with carbo-hydrate supplementation without a significantalteration in HR response.[23] Yaspelkis et al.[14]

reported a significantly higher HR at exhaustionwith carbohydrate supplementation, which mayreflect an increased ability to continue exercise due

to carbohydrate-mediated central and/or periph-eral alterations (section 4.1). The significantlyhigher

.VO2 with carbohydrate supplementation

reported by Ali et al.[29] and Coggan and Coyle[12]

could relate to an augmented work rate (section 4.2).Ostojic and Mazic[45] found a significantly lowerBM loss after a soccer match, attributed to largersweat and urine losses in the placebo trial. How-ever, sweat rate and urine loss were not measuredin the study. Furthermore, the limitations associ-ated with using BM loss as a measure of hydra-tion status should be considered.[51] Extraneousfactors associated with conducting the study inthe field, such as possible differences in exerciseintensity both within and between teams, as well

as differences in the timing of BM measurementbetween players before, during and after the match,may also have contributed to the different BM los-ses, independent of carbohydrate intake.

Carbohydrate ingestion alters the metabolicresponse to team games exercise, with a significantincrease in blood glucose concentration foundeither periodically,[11,12,14,15,23,26,29,31,33,35,36,46,50,52]

or throughout exercise.[13,24,45] Studies that havenot recorded increased blood glucose concentra-tion may have been hampered by infrequent blood

sampling opportunities[22,43]

or a small samplesize.[25] Significant increases in blood insulinconcentration may also occur with carbohydratesupplementation,[12,14,31,33] but this is not con-sistently observed.

Significantly greater carbohydrate oxidation ratesare recorded with carbohydrate ingestion,[12,14,29,31,35]

along with a strong trend for attenuated bloodfree fatty acid (FFA) levels[12,14,24,26,31,33,35] andfat oxidation rates,[31,35] although this is not con-sistent.[23,25,29,36,45] Nassis et al.[15] found no in-

crease in carbohydrate oxidation rates withcarbohydrate intake, but this may be due to pro-tocol issues (section 3.1). RER appears to be sig-nificantly higher during prolonged intermittentexercise when carbohydrate is ingested.[12-14] Aliet al.[29] did not find a between-trials difference inRER during the LIST, despite a higher rate ofcarbohydrate oxidation in the carbohydrate trial.This highlights the issues associated with usingRER to quantify metabolic responses to inter-mittent exercise (section 3.1).

The blood lactate response to prolonged inter-mittent exercise is largely unaffected by carbohy-drate ingestion,[11-13,23,24,26,27,29,33,35,36,45] except

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at exhaustion, where it has been reported to besignificantly higher.[14,15] This may reflect theability to continue exercising to a higher in-tensity, as previously discussed in this section andsection 4.1. However, if this is the case, bloodlactate concentration is not a reliable marker ofthis phenomenon, as numerous studies have de-scribed enhanced intermittent exercise capacitywithout a significant increase in blood lactateconcentration. It is also worth noting that bloodlactate concentration only reflects activities un-dertaken a few minutes prior to sampling, and the

balance between lactate movement into and outof the blood.[53,54]

3.5 Summary

Early research was almost unanimous in support-ing the consumption of carbohydrate-electrolytesolutions during prolonged intermittent exercisefor maintaining and/or improving exercise per-formance and capacity. However, the studies pre-sented significant methodological concerns that

limit their applicability to actual team games. Akey concern is the failure to use protocols thataccurately replicate the physiological demands ofteam games.

Contemporary research constructed method-ologies and protocols more representative of theactivities and physiological demands of teamgames and was almost unequivocal in its supportfor the efficacy of carbohydrate supplementationin improving intermittent exercise capacity. Mostresearch shows no benefit of carbohydrate sup-

plementation on sprint performance. The minorityof research showing no influence of carbohydratesupplementation on intermittent exercise capa-city displays methodological issues that could sig-nificantly impact the findings. Therefore, this workshould be interpreted with caution.

Carbohydrate supplementation may elicit al-terations in effort perception and mood state,which could facilitate improvements in exerciseperformance or capacity late in the exercise bout.The presence and extent of any such influence of

carbohydrate will likely depend on factors in-cluding pre-exercise muscle glycogen status, theintensity and duration of the exercise bout and

the amount and timing of carbohydrate inges-tion. More work is required using appropriateevaluative tools to confirm the presence of suchan effect, as well as its influence on exercise per-formance and/or capacity. Carbohydrate sup-plementation may facilitate a better maintenanceof shooting accuracy during team games, withnegligible support for improvements in passing,dribbling, tackling or heading. Again, these stud-ies may be influenced by such factors as pre-exercise glycogen concentration; the existing skilllevel of subjects; the validity and reliability of and

ability to compare between the various skill testsemployed; the extent of test familiarization; andthe type, intensity and duration of exercise. Fur-ther work using consistent, well controlled pro-tocols and a uniform battery of standardized testswill enable greater understanding of the influenceof carbohydrate on skill performance.

Carbohydrate ingestion does not directly alterthe physiological response to prolonged inter-mittent exercise. Any alterations that may occurare likely due to carbohydrate-mediated augmen-

tations in work rate. The general metabolic re-sponse to prolonged intermittent exercise withcarbohydrate supplementation is an increase inblood glucose concentration and significantlygreater carbohydrate oxidation rates, along withattenuated blood FFA levels and fat oxidationrates.

4. Mechanisms of Enhancement withCarbohydrate Supplementation duringProlonged Intermittent, High-Intensity

Exercise4.1 Intermittent Exercise Capacity

It appears that carbohydrate supplementationextends intermittent exercise capacity via reducedmuscle glycogen utilization in the first ~75 min-utes of exercise.[23,24,26] Nicholas et al.[25] seemedto confirm this by showing a combined 22% re-duction in type I and II muscle fibre glycogenutilization with carbohydrate ingestion during90 minutes of the LIST. This was attributed to

factors including exogenous carbohydrate oxida-tion sparing endogenous stores, greater activityof the pyruvate dehydrogenase complex due to




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hyperinsulinaemia and lower blood lactate con-centration and glycogen resynthesis in type II fi-bres due to elevated blood glucose and insulinlevels. Other studies support the hypotheses ofcarbohydrate-mediated muscle glycogen sparingand/or glycogen resynthesis during team games ex-ercise due primarily to observations of increasedblood glucose and/or blood insulin concentra-tions during exercise.[14,18,23,24,26] However, onlyYaspelkis et al.[14] measured muscle glycogenconcentration, finding a 25% greater concentra-tion at the end of exercise in type I muscle fibres

in the carbohydrate trial. This suggests sparing ofmuscle glycogen rather than its synthesis duringexercise, which is suggested to occur in type IImuscle fibres.[25] Supporting evidence for greaterpyruvate dehydrogenase activity with carbohy-drate supplementation is lacking. However, workinto the mechanisms of carbohydrate efficacyshould continue when it is considered that onlya small amount of exogenous carbohydrate ap-pears to be oxidized, or made available for oxi-dation, in the first hour of exercise regardless

of whether carbohydrate exerts an ergogenic ef-fect[55] or not.[56]

The potential influence of carbohydrate onperceptual responses to exercise may enable en-hanced intermittent exercise capacity (see section3.3).[29,46,57] While this hypothesis requires morework, as the relationship between carbohydrateingestion, RPE and performance during teamgames exercise has not been clearly established, itdoes appear that carbohydrate may modify theperception of effort during team games.

The significantly lower HR reported by someauthors[24,26] during team games exercise whencarbohydrate is ingested (section 3.4) infers re-duced stress on the cardiovascular system and anability to exercise at a higher intensity for a givenHR, and may possibly contribute to improvedintermittent exercise capacity. However, the com-mon observation that carbohydrate exerts no in-fluence on PV or HR during team games exercisesuggests that altered HR response is not a plau-sible or consistent ergogenic mechanism of car-

bohydrate supplementation. Furthermore, Aliet al.[29] found a trend for a higher HR with car-bohydrate ingestion during the LIST; however,

this may have been due to the faster sprint timesreported in the carbohydrate trial (section 4.2).

4.2 Sprint Performance

Improved sprint performance during teamgames exercise following ingestion of a carbohy-drate-electrolyte solution has been attributed tomaintenance of blood glucose levels,[27,29] whichmay enable greater muscle and cerebral metabo-lism,[29] thereby maintaining central nervous system(CNS) function and allowing better maintenance of

power output or muscle glycogen sparing.[28]

Thesehypotheses are debatable, as blood glucose con-centration did not reach hypoglycaemic levels in thecarbohydrate or placebo trial in the studies of Ali etal.[29] or Welsh et al.,[27] and muscle glycogen levelswere not measured by Winnick et al.[28] It shouldbe stated that the subjects in the Ali et al.[29] studybegan exercise with depleted glycogen stores. Thismay explain the improved sprint performancewith carbohydrate supplementation in this study,as short-duration, maximal-intensity exercise can

be attenuated if muscle glycogen levels fall belowa critical threshold (~200 mmol/kg dry weight).[58,59]

Therefore, ingestion of carbohydrate may haveprovided a sufficient supply of glucose to the mus-cle to enable greater sprint performance in theglycogen-depleted state compared with placebo.However, the extent of glycogen depletion wasnot quantified; therefore, this hypothesis is spec-ulative. Furthermore, Foskett et al.[33] and Ali andWilliams[35] reported a significant attenuation ofsprint performance during the LIST protocol in

the carbohydrate and placebo trials when sub-jects began exercise in a glycogen-depleted state.However, the extent of glycogen depletion wasnot reported. It also does not explain the improvedsprint performance documented by Welsh et al.[27]

or Winnick et al.,[28] as subjects in these studieswere not glycogen depleted prior to exercise.

When glycogen availability is not compro-mised, phosphocreatine concentration and its rateof resynthesis rather than carbohydrate avail-ability is more related to short-duration sprint

performance,[60] perhaps helping to explain the lackof effect of carbohydrate on sprint performancein most studies. However, it should be considered

574 Phillips et al.



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that, while phosphocreatine availability is thedetermining factor when short sprints are inter-spersed with adequate passive recovery, duringteam games, subjects are required to jog, run andwalk between each sprint. In this situation, phos-phocreatine resynthesis may not be completeenough to contribute fully to each sprint, partic-ularly in the later stages of the protocol. If thiswere the case, other substrates, notably carbohy-drate and fat, would become more prevalent fuelsduring the sprints.[61] Therefore, carbohydratesupplementation may be important for main-

taining sprint performance during the later stagesof team games exercise. This may be particularlypertinent when pre-exercise muscle glycogenstores are not optimal,[29] but may also help toexplain the findings of Welsh et al.[27] and Winnicket al.[28] who found a significant improvement insprint performance in the late stages of exerciseonly. It may also help to explain the non-significantbetween-trials difference in sprint performanceobserved in most studies. However, this requiresfurther investigation.

4.3 Mental Function and Skill Performance

Studies confirming improved mood, force out-put and effort perception with carbohydrate sup-plementation during team games exercise haveimplicated carbohydrate-mediated alterations inbrain chemistry, particularly attenuated serotoninproduction,[62,63] as a potential mechanism.[27,28,46]

However, none of the studies collected data thatcould directly confirm this, instead inferring in-

creased brain glucose uptake based on signif-icantly elevated blood glucose concentrations inthe carbohydrate trial.[64] Cerebral glucose uptakebegins to decline when blood glucose concentrationfalls below ~3.6mmol/L,[65] which did not happenin the placebo trial in the studies of Backhouseet al.[46] or Welsh et al.[27] and, in the Winnicket al.[28] study, blood glucose levels were not mea-sured. It is therefore difficult to accept this expla-nation. Furthermore, the concept of CNS fatigueremains unclear and difficult to experimentally

isolate and confirm, particularly from a mecha-nistic perspective.[66] It is also extremely difficultto differentiate central from peripheral effects

when carbohydrate is ingested during exercise.[67]

Work needs to be conducted that is sensitive en-ough to resolve the nature of the influence of car-bohydrate on mental function during team gamesexercise, yet using tests that are externally valid toteam games performance.

The significantly improved, or better main-tained, performance of certain skills reported bysome authors has also been largely attributed tocarbohydrate-mediated alterations in CNS func-tion that enable better motor control and henceskill performance.[27-29,47] However, the issues

with this are discussed above. Ali et al.[29]

suggestedan augmentation of neuromuscular function withcarbohydrate supplementation that may also en-able greater motor control, but this was not sup-ported with data. Maintenance of blood glucoseconcentration, sparing of muscle glycogen andtherefore, possibly, attenuation of muscle fatigueand, perhaps, better performance of the anaero-bic component of the skill test have also beenpostulated.[27-29,44,45] However, no muscle glyco-gen measurements were taken,[27,29] and some

studies did not measure blood glucose concentra-tion.[28,44,45] Furthermore, hypoglycaemia did notoccur in any of the other studies,[27,29] and Ali andWilliams[35] failed to show a significant improve-ment in passing performance with carbohydratesupplementation despite very similar between-trial blood glucose responses to their 2007 study.However, the possible effects of low blood glu-cose concentration on skill performance have notbeen elucidated.[29] Further work must attempt toquantify the mechanisms responsible for improve-

ments in skill performance during team gamesexercise when carbohydrate is ingested.

5. Modulators of Carbohydrate Efficacy

Research supporting the use of carbohydrate-electrolyte solutions during team games exercisegenerally focuses on supplementation of an ap-proximate 6% carbohydrate-electrolyte solutionof similar composition. The current research out-put does not provide a sufficient thesis on factors

that modulate the efficacy of carbohydrate sup-plementation during team games exercise. Potentialmodulators may be different from those during




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prolonged steady-state exercise due to the con-stantly changing exercise intensity and frequency,duration and intensity of rest intervals, the po-tential for team games exercise to slow the rateof GE[68] and restricted access to carbohydrate-electrolyte solutions during many team games.Work must be undertaken to further understand-ing in this area, and ultimately lead to the formu-lation of clear guidelines for the optimal ingestionof carbohydrate during team games exercise. Someof these important modulators are discussed insections 5.15.7.

5.1 Fluid Volume and Solution Composition

If carbohydrate-electrolyte solutions are con-sumed during exercise, then fluid and carbohy-drate intake are interdependent and should notbe considered in isolation. Therefore, the follow-ing discussion on fluid volume, carbohydrate con-centration, carbohydrate composition and solutionosmolality is presented as one topic.

5.1.1 Fluid Volume

Mild dehydration increases Tcore, RPE andBM loss, and impairs skill performance duringteam games exercise.[49,69,70] Team games athletesshould maintain adequate hydration status inorder to maximize performance. This can beachieved by replacing the same amount of fluidthat is lost during exercise and is a recommendedpractice for team games athletes.[10,71-75] Failureto ingest an appropriate volume of fluid duringexercise may prevent the athlete from maximizing

their performance even when ingesting carbohy-drate. More specific fluid ingestion recommend-ations are difficult due to the numerous factorsthat can influence fluid requirements, such as BM,exercise intensity, individual sweat rates and en-vironmental conditions. Section 5.2 further dis-cusses fluid intake strategies for team games.

5.1.2 Carbohydrate Concentration

Only three studies have employed differentcarbohydrate concentrations during prolonged

intermittent exercise.[11,13,27] Unfortunately, theuse of different carbohydrate compositions,[11]

relatively small increases in carbohydrate inges-

tion between solutions[13] and different carbohy-drate concentrations within the same trial,[27] limitthe usefulness of the results. Ingesting too littlecarbohydrate may not meet energy requirementsduring exercise (section 3.1). However, consumingtoo much carbohydrate can attenuate GE rate,cause gastrointestinal distress and impair intestinalfluid absorption (section 5.1.4).[68,76,77] A 57%carbohydrate-electrolyte solution is currently re-commended for team games,[10] along with the re-commendation of Jeukendrup and Jentjens[19] foran optimal carbohydrate intake of~1.01.1 g/min.

However, neither of these recommendations havebeen thoroughly tested using externally valid teamgames protocols.

5.1.3 Carbohydrate Composition

Carbohydrate oxidation rate depends on mul-tiple factors, one of which is the composition ofingested carbohydrate.[19] This suggests that differ-ent carbohydrate compositions may have differentefficacies during exercise. Ingestion of multipletransportable carbohydrates, typically glucose

and fructose in a ratio of~2 : 1, appears beneficialduring prolonged steady-state exercise for increas-ing GE rate,[78] intestinal carbohydrate and waterabsorption (section 5.1.4)[78,79] and exogenouscarbohydrate oxidation rates,[79-82] although thelatter is not universally found.[83] In the only studyto manipulate carbohydrate composition duringprolonged intermittent exercise,[11] it was notpossible to discern between effects due to changesin carbohydrate concentration and composition(section 5.1.2). Therefore, the effect of alterations

in carbohydrate composition during team gamesexercise should receive close attention in futurework.

Recently, the first study investigating the effectof a carbohydrate gel during team games exercisereported a 45% improvement in intermittent ex-ercise capacity compared with a placebo solu-tion,[30] analogous to the effect of carbohydratesolutions (section 3.2). This is supported by evi-dence of a similar time-course of carbohydrateoxidation and peak carbohydrate oxidation rate

between carbohydrate gels and drinks of the samecomposition.[84] This represents a step forward forthe research base by investigating carbohydrate de-

576 Phillips et al.



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livery in essentially a different medium. Althoughinitial findings are positive, more research isrequired.

5.1.4 Solution Osmolality

Following ingestion of isocaloric carbohydratesolutions of differing composition and osmolal-ity, less than 5% of the variance in GE rate is dueto differences in osmolality.[85] Similar findingshave been replicated numerous times at rest andduring exercise.[86-90] Solution osmolality oftenincreases in proportion to caloric content, indi-

cating that the inhibition of GE originally attri-buted to osmolality[91,92] may have been confusedwith the influence of increased caloric density.[93]

Significant negative correlations between carbo-hydrate content and GE rate with ingestion ofiso-osmotic carbohydrate solutions, and posi-tive correlations between solution caloric contentand the half-time of GE, have been reported.[94,95]

Calbet and MacLean[95] confirmed that caloriccontent explained 92% of the variance in GE rate.This, along with the observation of a similar GE

rate when solutions with the same carbohydrateconcentrations but significantly different osmo-lalities are consumed,[94,96] suggests that carbo-hydrate content and caloric density are moreimportant than solution osmolality in modulat-ing GE rate.

Rapid fluid and carbohydrate delivery tothe systemic circulation is crucial for exerciseperformance. The osmolality of a carbohydrate-electrolyte solution appears inversely related tothe rate of water absorption in the small intes-

tine,[97-101] with conflicting findings[86,96,102-105]attributed to the activity and number of intestinalsolute transporters, alterations in osmolality overthe length of the small intestine, and solution com-position.[10,106,107] Increasing the carbohydrateconcentration of a carbohydrate-electrolyte so-lution can increase osmolality, and therefore at-tenuate the rate of intestinal water absorption,[108]

when carbohydrate concentration reaches ~8%.[103]

This should be considered when manipulating theconcentration of carbohydrate-electrolyte solu-

tions (section 5.1.2), as increasing carbohydrateconcentration may allow increased absorption ofcarbohydrate, but could attenuate GE rate and

intestinal water absorption, and result in sub-optimal hydration status.

Carbohydrate type can also influence solutionosmolality and, therefore, intestinal water absorp-tion[10] when carbohydrate concentration is>6%.[103]

Incorporating multiple transportable carbohy-drates into a solution can offset the effect of highosmolality on intestinal water absorption[109] byactivating a greater number of intestinal solutetransport mechanisms. This could enable a highvolume of carbohydrate delivery while maintain-ing adequate intestinal water absorption. For a

more detailed discussion on this topic, the readeris referred to the review of Shi and Passe.[110]

5.1.5 Recommendations

Future work must study the effects of alteringfluid volume, carbohydrate concentration, com-position and solution osmolality, independentlyand in an integrated fashion. This will enable dis-covery of the optimal composition of a carbohy-drate-electrolyte solution for maximizing intestinalfluid and carbohydrate absorption during team

games exercise.

5.2 Fluid and Carbohydrate Ingestion Pattern

Fluid may take ~4060 minutes from the timeof ingestion to be transported around the sys-temic circulation and become physiologicallyuseful.[111,112] This, coupled with the potentialattenuation of GE due to the intensity of teamgames exercise[68,76] and the addition of carbohy-drate to a solution,[90,113] and the insufficient op-

portunities to ingest fluid at regular intervalsduring team games,[31] casts doubt on the efficacyof consuming consistent amounts of fluid andcarbohydrate throughout team games exercise.Coyle[111] suggests that it may be beneficial todrink larger volumes early in exercise, ingest fluidthroughout exercise to ensure gastric volume ishigh after 40 minutes, and then ingest little fluidthereafter to minimize gastric volume towards theend of exercise, and thereby minimize the volumeof fluid present that cannot aid, and may inhibit,

performance by adding weight and perhaps causinggastrointestinal discomfort. It would be interest-ing to compare the standard intake regimen




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employed in most team games research (see table II)with one that provides greater volumes of fluid inthe early stages of exercise and then progressivelyless as exercise continues.

Clarke et al.[31] investigated the effect of con-suming a carbohydrate-electrolyte solution in ateam games-specific fashion (a large bolus priorto and at 45 minutes during exercise) comparedwith more frequent ingestion during a team gamesexercise protocol. Exercise performance and ca-pacity were not assessed but the overall metabolicresponse to exercise quantified by measurement

of blood glucose, insulin, non-esterified fattyacids, glycerol and adrenaline concentrations was similar between trials. This suggests that in-gestion of carbohydrate-electrolyte solutionsbefore a game and at half-time is a practical al-ternative for fluid and carbohydrate provision.[31]

However, this is not supported by the study ofAbbey and Rankin.[34] More work is required inthis area.

5.3 Glycaemic Index of Pre-Exercise Meals

This review will not discuss the glycaemic indexin detail, and the interested reader is referred tothe recent review by OReilly et al.[114] Manipu-lating the glycaemic index of a meal consumedseveral hours before team games exercise doesnot significantly affect sprint performance or in-termittent exercise capacity,[115,116] despite in-creased fat oxidation rates with a low-glycaemicindex meal.[116] Lack of effect may be due to therequirement for high-intensity efforts through-

out team games protocols, which would be de-pendent on phosphocreatine and carbohydratemetabolism.[60,61]

Ingesting a carbohydrate-electrolyte solutionbefore and during steady-state endurance exercisenegates the proposed benefits of a pre-exercise low-glycaemic index meal[117,118] by minimizing poten-tial differences in metabolic response or substrateoxidation between low- and high-glycaemic indexmeals.[117,118] Chryssanthopoulos and Williams[119]

reported a significant improvement in steady-

state running capacity when ingestion of a pre-exercise carbohydrate meal was combined withcarbohydrate ingestion during exercise. How-

ever, a low- to high-glycaemic index meal com-parison was not made. No research has inves-tigated the interaction between pre-exercise mealsof differing glycaemic index and ingestion of acarbohydrate-electrolyte solution before and dur-ing team games exercise. This should be carriedout in order to quantify the optimal pre- and dur-ing exercise nutritional strategy for team gamesathletes.[114]

5.4 Fluid Temperature

Provision of cold fluid (4

5

C) encouragesgreater fluid ingestion during exercise in mild andhigh ambient temperatures,[120,121] and may alsoenable significantly greater steady-state endurancecycling performance[122] and capacity[121,123] inthe heat compared with ingestion of warm fluid(1638C). Cold fluid may act as a heat sink, at-tenuating the rise in body heat storage and, pos-sibly, Tcore.

[123] However, endurance capacity hasbeen improved with cold fluid ingestion withoutsignificant changes in Tcore.

[121,122] Cold fluid

intake may significantly reduce skin tempera-ture[122,124] and attenuate skin blood flow andsweat rate[125] during exercise in temperate andhot environments. This may represent a redis-tribution of cardiac output from the skin to theexercising muscles and may enable improved en-durance performance/capacity.[122] However, thisrequires further investigation as skin tempera-ture, blood flow and exercise performance/capa-city have not yet been measured in the same study.The influence of cold fluid ingestion on steady-

state cycling capacity in moderate environmentalconditions appears negligible.[123,126]

The studies discussed above were conductedusing similar exercise protocols (steady-state re-cumbent or upright cycling for ~50120 minutesat 5066%

.VO2max). No work has used prolonged

running as a modality; furthermore, no pro-longed intermittent cycling or running protocolshave been employed. Variable intensity cyclingin high ambient temperatures may significantlyincrease heat storage, the rate of rise in Tcore, whole-

body sweat rate and dehydration, and signif-icantly reduce forearm blood flow compared withsteady-state cycling.[127] This, along with the

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current recommendation for a fluid temperatureof 1521C[73] and the acknowledgement thatpreferred fluid temperature varies greatly betweenindividuals,[73] provides a rationale for investigatingthe effects of fluid temperature during team gamesexercise. This should be conducted using fluid withand without carbohydrate, to observe whetheralterations in the temperature of a carbohydrate-electrolyte solution provide an additional effectover and above that of carbohydrate or fluid alone.

5.5 Carbohydrate Mouthwash

Insufficient opportunities exist for regular fluidingestion during field-based team games, and anyopportunities that do arise may be brief and notafford the athlete the time to ingest the optimalvolume of fluid or carbohydrate. Furthermore,evidence of an attenuated GE rate and, possibly,increased gastrointestinal discomfort with inges-tion of carbohydrate-electrolyte solutions duringteam games exercise,[77] along with the recentsuggestion by Edwards and Noakes[128] that the

degree of sweat loss and associated dehydrationcommonly encountered during soccer is not crucialto performance, suggests that a carbohydrate-basedergogenic aid that can be rapidly utilized and hasno tolerance issues may be useful for team gamesplayers.

In recent years, the use of carbohydrate mouth-washes has been shown to enhance running andcycling performance lasting~3060 minutes.[129-132]

Other work has failed to show a benefit of car-bohydrate mouthwashes,[133,134] possibly due to

study differences in solution blinding, the influ-ence of dehydration and endogenous muscle gly-cogen availability. The apparent mechanisms forenhancement with carbohydrate mouthwashesrevolve around modification of central drive andmotivation and/or activation of reward and motorcontrol centres in the brain rather than a meta-bolic cause.[129,131] These alterations may elicita more favourable perception of effort duringexercise.[129,130] For more information on the en-hancement mechanisms of carbohydrate mouth-

washes, see Chambers et al.[131]

All previous studies of carbohydrate mouth-washes used steady-state protocols. The potential

of carbohydrate mouthwashes during team gamesexercise is strong, particularly in allowing easier andmore rapid supplementation than carbohydrate-electrolyte solutions and limiting possible gas-trointestinal distress associated with fluid andcarbohydrate ingestion.[133] Research needs to quan-tify this potential benefit, particularly regardingwhether a carbohydrate mouthwash is sufficientto enhance team games performance in the pre-sence of significant muscle glycogen depletion.

5.6 Ambient Temperature

The effect of carbohydrate supplementationduring prolonged exercise in the heat is equivo-cal. If exercise is terminated due to attainment ofa critical Tcore a concept that, while havingsome empirical support,[135,136] is not universallyaccepted[137-139] carbohydrate is not beneficialto performance.[140] However, if subjects do notterminate exercise due to hyperthermia, ingestionof a 6% sucrose/glucose solution has been shownto improve prolonged cycling performance in theheat.[140] During prolonged exercise in a cool en-vironment, a 7% carbohydrate solution is also ableto improve exercise capacity.[141] However, thesefindings apply to prolonged steady-state exercise.Only two studies have investigated carbohydratesupplementation during prolonged intermittentexercise in the heat.[11,18] The major limitations ofthese studies (see sections 3.1 and 3.2) preventconfident interpretation and application of thefindings. Therefore, there is a large scope for fo-cused and well conducted research into the effectof carbohydrate supplementation during teamgames exercise in different ambient temperatures.

5.7 Populations

No research into carbohydrate supplementa-tion during team games exercise has focused ex-clusively on adult female subjects. Females generallyoxidize less carbohydrate and more fat duringexercise than do males,[142,143] with less muscleglycogen utilization recorded during steady-staterunning[144] but not cycling.[145] It would be in-

teresting to observe whether carbohydrate sup-plementation during team games exercise enabledany performance and/or capacity improvements in




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females and, if so, whether mechanisms behindthese improvements were different from thosebehind improvements in male subjects.

A large number of children and adolescents ac-tively participate in organized team games.[146,147]

However, the research base investigating thephysiological responses of this population to thisform of exercise, as well as investigating fatiguemechanisms and avenues of performance enhance-ment is sparse. This is likely due to the manyproblems faced when conducting research inyoung people such as recruitment and retention,

gaining parental consent, child assent and ethicalapproval to undertake all necessary experimentalprocedures,[148,149] ensuring subjects understandand fulfil all procedural requirements of a studyand adequately controlling for the influence ofbiological maturation, which is often hamperedby ethical and consensual restrictions.[150]

Adolescents appear to exhibit a maturation-dependent exercising metabolic response involvinggreater fat and lower carbohydrate oxidation thanadults;[151] however, the large number of poten-

tially confounding factors involved in the studyof developmental changes in energy metabolismmake a firm consensus extremely difficult.[152,153]

They also appear able to oxidize significantlymore exogenous carbohydrate during moderate-intensity steady-state cycling than adults.[154]

Additionally, a significant improvement in steady-state exercise cycling capacity with carbohydratesupplementation has been observed in 10- to14-year-old males.[155] This provides a rationalefor the study of carbohydrate supplementation

during team games exercise in these subjects.We recently demonstrated, for the first time,

that ingestion of a 6% carbohydrate-electrolytesolution immediately before and during a modifiedLIST protocol significantly improved the inter-mittent exercise capacity of trained 12- to 14-year-old team games players by 24% compared with aplacebo.[156] Neither sprint performance nor phys-iological responses to exercise were affected bycarbohydrate supplementation, except at exhaus-tion, where subjects elicited a significantly higher

peak HR in the carbohydrate trial, but with nosignificant difference in RPE compared with theplacebo trial. This was attributed to carbohy-

drate supplementation enabling participants tocontinue working to a higher intensity via bettermaintenance of muscle metabolism (section 4.1),or the influence of carbohydrate on perceptualresponses to exercise (section 3.3). Further workis required to confirm these mechanisms in thispopulation. These positive findings provide aplatform from which to investigate other factorsassociated with carbohydrate supplementationduring team games exercise in adolescents, suchas those discussed in sections 5.15.6, in order towiden and strengthen the research base in this

area.

6. Conclusions

Most early research investigating carbohy-drate supplementation during prolonged inter-mittent exercise was subject to methodologicallimitations that restricted both its scientific rigourand its applicability to actual sporting activity.The development of team game-specific exerciseprotocols enabled a more focused investigative

approach to this topic. The findings of this reviewinto carbohydrate supplementation immediatelyprior to and during team games exercise are asfollows:1. Carbohydrate supplementation significantlyimproves intermittent exercise capacity in adults.Possible mechanisms include muscle glycogensparing or resynthesis during low-intensity peri-ods and altered effort perception during exercise.More research into the mechanisms of carbohy-drate efficacy is required.

2. Initial findings suggest that carbohydratesupplementation significantly improves intermit-tent exercise capacity in adolescent team gamesplayers. Enhancement mechanisms may be, at leastpartially, centrally mediated. Future work shouldinvestigate this further.3. Carbohydrate supplementation has a negligi-ble effect on sprint performance in adults andadolescent team games players. Carbohydrate ef-ficacy may depend on endogenous muscle glyco-gen availability.

4. Carbohydrate supplementation may elicit al-terations in effort perception and mood state thatcould improve performance in the later stages

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of team games exercise and may enable bettermaintenance of shooting accuracy during teamgames, with negligible support for improvementsin passing, dribbling, tackling or heading. Im-provements with carbohydrate intake are attrib-uted to improved cerebral glucose uptake, greaterCNS function and motor control. More work isrequired in these areas.5. Carbohydrate ingestion does not directly alterphysiological responses to prolonged intermittentexercise, with any alterations likely due to anaugmented work rate via carbohydrate supple-

mentation. Carbohydrate supplementation usuallyincreases blood glucose and insulin concentra-tions either periodically or throughout exercise,increases carbohydrate oxidation rates and RER,and attenuates blood FFA levels and fat oxida-tion rates.6. It has been suggested that a 57% carbohydrate-electrolyte solution containing multiple transport-able carbohydrates and sodium, and with anosmolality of 250370 mOsm/kg may be optimalbefore and during team games exercise. However,

very little subsequent work has attempted toempirically test these recommendations, as wellas other potential modulators of carbohydrateefficacy, during team games exercise.7. Several key areas need to be addressed by futureresearch. These include manipulations in ingestedfluid volume, carbohydrate concentration, carbo-hydrate composition and solution osmolality, bothindependently and in an i

carbohydrate ingestion during team games exercise

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