23 4
Par t 3 O pt im al Nut r it ion f or t h e P h y s i c a l l yA c t i v e f 'e r s o n :M a k i n g I n f o r n r e da n d H e a l t h f u lC h o i ce s
Answer these 10 staternentsabout intensetraining and sportscompetition. Use the scoring key at the
end of the chapterto checkyour results.Repeatthis test after you have read the chapterand compare
your results.
1 . T F O n e s h o u l dfa s t fo r 24 hoursbeforesportscompeti ti onor i ntensetrai ni ng to avo id an upset
s to ma c hc a u s e db y u ndi gestedfood.
2. T F The ideal precompetitionmeal consistsof high-proteinfoods to ensureelevatedlevelsof
m u s c l ep ro te i nd u ring competi ti on.
3 . T F l t i s u n w i s eto e a t d uri ng hi gh-i ntensi tyaerobi cexerci se.
4. T F The glycemicindex indicatesthe number of caloriesin different forms of carbohydrate.
provi desthe most effecti vemeansto ra pidly
5 . T F C o n s u m i n gh i g h -g l ycemicarbohydrates
c
re p l e n i s hd e p l e te dgl ycogenfol l ow i ng i ntenseenduranceexerci se.
6 . T F Gl y c o g e nre s e rv e isn muscl eand l i ver usual l yrepl eni shw i thi nl 2 hoursw hen con sum ing
peri od.
h i g h -g l y c e m icca rb o hydrates
i n the i mmedi atepostexerci se
7 . T F Av o i d d ri n k i n g l i q u i dsi mmedi atel ybeforevi gorousexerci se
to mi ni mi zei ntesti n al
d i s tu rb a n c ea n d i mpai redexerci seperformance.
8 . T F P l a i nc o l d w a te r s e rvesasthe opti mal oral rehydrati onbeveragefor consumpti o ndur ing
exercise.
9 . T F S o u n dre s e a rc hs u pportsthe w i sdom of addi ng somesodi umto an oral rehydration
s o l u ti o n ,p a rti c u l a r l yduri ng prol ongedexerci sei n the heat.
1 0 . T F D ri n k i n ga c o n c e n t ratedsugardri nk before and duri ng exerci sefaci l i tatesrehyd r at ionand
enhancesexerciseperformance.
he r.reedto mair.rtainoptirnal nutrient intake to
sustain energy and tissue-buildingrequirements of regularphysicalactivity also requires
unique dietary modiflcationsto facilitateinte n s etra i n i n ga n dc o mpeti ti on.
THEPRECOMPETITION
MEAL
Athletes often compete in the morning following an
overnightfast.As noted out in Chapter l, considerabledepletion occursin carbohydratereservesover an 8- to 12-hour
period without eating,ever.r
if the personnormally follows
appropriatedietaryrecommendations.
precConsequently,
ompetitionnutrition takeson considerable
importance.Tfte
precompetitionmealshouldprovideadequatecarbohydrate
energyand ensureoptimal hydration.Within this framework,
fasting before competition or intense training makes no
sensephysiologicallybecauseit rapidly depletesliver and
muscleglycogen,which subsequently
impairs exerciseperformance.If a person trains or competesin the afternoon,
breakfastbecomesthe irnportantmeal to optimizeglycogen
reserves.For late afternoontraining or competition,lunch
becomesthe important source for topping off glycogen
stores.Considerthe following three factorswhen individr
izing th'eprecompetitionmeal plan:
l. Food preferences
2. Psychological
set
3. Food digestibility
As a generalrule,eliminatefoodshigh in lipid and prol
content on the day of competition becausethesefoods di1
slowly and remain in the digestivetract longerthan carbo
drate foods of similar energycontent.Timing of the preco
petition rnealalsodeserves
consideration.The increasedstt
and tension that usually accompanycompetition decre
blood flow to the digestivetract, depressingintestinalabso
tion. .It takes3 to 4 hoursto digest,absorb,and storeas rnu
preconpetition menl.
and liver glycogenn cnrbohyflrate-rich,
Proteinor Carbohydrate?
Many athletesbecomepsychologicallyaccustomedto, i
evendependon, the classic"steakand eggs"preconlpeti{
meal.Sucha mealmay satisrythe athlete,coach,and resl
rateur,but it providesno benefit to exerciseperformal
This qpe of meal,with its low carbohydratecontent,can I
der optimal exerciseperformance.
Thereare five reasonsfor modifuingor evenabolishing
thehigh-proteinprecompetitionrnealin favor of one high in
carbohYdrates:
l. Dietarycarbohydratesreplenishliver and muscleglycogen
depletionfrom the overnightfast.
2. Carbohydratesdigestand absorbmore rapidly than either
proteinsor lipids. They provide energyfasterand reduce
the feelingof fullnessfollowing a meal.
3 . A high-proteinmeal elevatesrestingmetabolisrnconsidermealowing to greater
ablymore than a high-carbohydrate
for digestion,absorp.ion,and assimienergyrequiremer.rts
lation. This additionalmetabolicheat potentiallystrair-rs
mechanismsand impairs hot
the body'sheat-dissipating
performance.
weatherexercise
L Proteinbreakdownfor energyfacilitatesdehydrationduring exercisebecausethe by-productsof amino acid breakdown require water for urinary excretion.For example,
approximately50 nll of water "accompanies"excretionof
e a ,- hgr at nof ur eain th e u ri n e .
5 . Carbohydrateservesas the primary energynutrient for
short-termanaerobicactivityand for prolonged,high-intensityaerobicexercise.
Rich
Makelt Carbolrydrate
The ideal precompetitilntneal maximizesmuscleand liver
glycogenstorageand providesglucosefor intestinalabsorptiott
The meal should:
duringexercise.
(3 to 5 g p e r kg of
L C o n t ain 150t o 300 g o f c a rb o h y d ra te
body massin eithersolidor liquid form)
2. Be consumed3 to 4 hours beforeexercising
3 . Contain relativelylittle fat and fiber to facilitategastric
emptyingand minimize gastrointestinaldistress
The importanceof preconrpetitionfeedingoccursonly if
the personmaintainsa nutritionally sound diet throughout
training.Pre-exercise
feedingscannotcorrectexistingnutr.itional deficienciesor inadequatenutrient intake in the weeks
beforecompetition.Chapter I I discusses
how enduranceathletescan augnlentprecompetitionglycoger.r
storagewith the
specificexercise/diet
loading.
rnodifications
of carboh."lrate
LIQUIDAND PREPACKAGED
BARS,
POWDERS,
AND MEALS
Commerciallypreparednutrition bars,powders,and liquid
rnealsoffer an alternativeapproachto precompetitionfeeding
or supplernentalfeedingsduring periodsof cornpetition.sT
Thesenutrient supplementsalsoeffectivelyenhanceenergy
al)d nutrient intakein training,particularlyif energyoutput
exceedsenergyintake from lack of interestor misnranagenreritof feedings.
tiquid Meals
,' l.iqui<1rnealstr',rovi)q
a high-carbohydrate
contentbut contain enoughlipid an{ protein to contributeto satiety.They
l
/
alsocan supply the personwith fluid becausethey existin liqno
uid form. A liquid meal digestsrapidly, leavingesser.rtially
residuein the intestinaltract.Liquid mealsproveparticularly
effectiveduring day-longsrvimmingand track meetsor durtournaments.It.rthesesituaing tennis,soccer,and basketball
has
little
time for (or interestin)
person
usually
the
tions,
practical
approachto supplementfood. Liquid mealsoffer a
ing caloric intake during the high-energyoutput phaseof
training. Athietescan also use liquid nutrition if they have
difficulty maintainingbody weight and as a readysourceof
caloriesto gain weight.
NutritionBars
Nutrition bars (called"energybars," "protein bars,"and "diet
bars") contain a relativelyhigh protein content that ranges
between10 and 30 g per bar.The tlpical 60-9bar contains25
g (100 kCal) ofcarbohydrate(equalarnourltsofstarch and
sugar),15 g (60 kCal)of protein,atrd5 g (a5 kCai)of lipid (3
g or 27 kCal of saturatedfat), with the remainingweight as
water. This representsabotrt49o/oof the bar's total 205 calofrom protein, and 220,6from
ries from carbohydrates,2go/o
lipid. The barsoften includevitaminsand minerals(30-50%o
of recomrnendedvalues),and somecontain dietarysupple(HMB). These
inents such as B-hydroxy-B-rnethyibutyrate
bars must be labeledas dietary supplements,rather than
foods.
While nutrition bars provide a relativelyeasyway to obtain important nutrients,they should not totally substitutefor
normal food intake becausetlrey lack the broad array of plant
found in food and containa relafibersand phytochemicals
fatty acids.As an addedwarning,
tively high levelof .saturated
sold asdietarysupplements;
no indethesebarsareger.rerally
by the Food and Drug Administration
pendent assessment
(FDA) or other federalor stateagencyexiststo validatethe labeling claimsfor macronutrientcontentand composition.
(o-called energybarscontaina greater proportion of
Jcarbohydrates while "diet" or "weight loss"barsare
lower in carbohydratecontent and higher in protein.
"Meal-replacementbars" havethe largestenergycontent
(240-310kCal).with proportionatelymore of the three
macronutrients."Protein bars" simplycontain a larger
amountof protein.
andDrinks
NutritionPowders
A high proteincontent,between10 and 50 g per serving,representsa uniqueaspectof nutrition powdersand drinks.They
alsocontainaddedvitarnins,minerals,and other clietarysupplement ingredients.The powderscot'uein cauistersor packets that readilyrnix with water (or other liquid); the drinks
conrepremixedin cans.Theseproductsoften serveas atr al-
236
Choices
and He'althful
Makinglr-rformed
ActivePerson:
Part3 OptimalNutritionior the Physically
asnrea]replilceternativeto nLltritionbars;theyarenrarl<eted
protein
ments,dietingaids,energyboosters,or concentrated
sources.
Nutrient compositioltof powdersand drinksvariescol.tsiclerablyfrom nr-ttritionbars.For one thing, nr-rtritionbars
to providetextureand
containat Ieast15 g of carbohydrates
taste,whereaspowdersand drinks do not. This accountsfor
the relativelyhigh protein content of powdersand drinks.
Nutrition powdersand drinks ger.rerally
contain fewercalories
per servingthan do bars,but this can vary for a powder,dependingon the liquid usedfor mixing.
A recentsurveyof high-performanceCanadianathletes
Flindicated that 88.4%reportedtaking one or more dietarysupplementsduring the previous6 months.Of the
supplements used, sport drinks (22.4%), sport bars
(14J%),multivitaminsand minerals(13.5%),proteinsupplements(9.0%),and vitaminC(6.4%)were the mostfrequently reported.24
about 45
The recommendedservingof a powder averages
g, the sameamount as a nutrition bar (rninusits water content), but wide variation existsin this recommendation.A t1picalservingof a high-proteinpolvdermix containsabout l0 g
ofcarbohydrate(two thirds assugar),30g ofprotein, and 2 g
of calories
of lipid. This amountsto a total of 178kCal or 23o/o
from carbohydrate,67o/ofrom protein, and l0o/ofrom lipid.
Thus,when mixed in water,thesepowderednutrient supplements far exceedthe recommendedprotein intakepercentage
percentand fall belowrecommendedlipid and carbohydrate
ages.A drink typically containsslightlymore carbohydrate
and lessprotein than doesa powder.
As with nutritiorl bars,the FDA'or other federalor state
agencymakesno independentassessment
of the validityof labelingclaimsfor macronutrientcontentand composition.
TABt.lt
8.1 providesthe macronutrientconrpositiontbr
(rapidstompackaged
liquid food supplements
comnrerciaily
distress),
ach emptyingwith low residueand gastrointestinal
high-carbohydratedrinks, and "high-energy"bars typically
advocatedfor physicallyactiveindividuals.Prudent use of
besomeof thesesupplementscan replenishglycogenreserves
fore and afterhigh-intensityexerciseand competition,especiallybecausean athlete'sappetitefor "normal" food rvanes.
CARBOHYDRATE
FEEDING
BEFORE,
DURING,AND
FOTTOWING
INTENSE
EXERCISE
The "vulnerability"of the body'sglycogenstoresduring intense,prolongedexercisehas focusedconsiderableresearch
on potential benefitsof carbohydratefeedingsimmediately
beforeand durins exercise.'fhis
hasalsoinch.rded
waysto op-
recov
in the postexerclse
repler-rishrr.rent
timize cartrohydrrrte
ery period.
CarbohydrateFeedingsBefore
Exercise
Confusionexistsabout potentialendurancebenefitsof pre
nr'rtritior
in exercise
ingestionof simplesugars.Son-re
exercise
carbo
absorbed
rapidly
arguethat consuminghigh-glycernic,
nega
(see
exercising
I
l'rour
before
Fig. 8.3) within
hydrates
two
ways:
performance
in
one
of
tively affectsendurance
1 lnducing an overshootin insulin from the rapid rise ir
blood sugar.Insulin excesscausesa relativehypoglycemii
(reboundhlpoglycemia).Blood sugarreductionimpair:
centralnervoussystemfr.rnctionduring exerciseto pro
ducea fatiguingeffect.
2. Facilitatingglucoseinflux into muscie(through largein
sulin release)to increasecarbohydratecatabolismfor en'
ergy in exercise.At the satne time, high insulin levell
inhibit lipolysis,rvhichreducesfree fatty acid mobilizatior
fron'r;rdiposetissue.Both atlgmentedcarbohydratebreakfat mobilizationcontributeto prenadown and depressed
ture glycogendepletionand earlyfatigue.
in the late 1970sindicatedthat drinking a high\
Research
concentratedsugarsolution30 minutesbeforeexerciseprecipitatedearlyfatiguein enduranceactivities.For exalnple,enduranceon a bicycleergometerdeclined197orvhensubjects
consumeda 300-mL solution containing75 g of glucose3C
minutesbeforeexerciseconiparedwith riding time preceded
by the samevolumeof plain wateror a liquid neal of protein.
consurningtl.reconParadoxically,
lipid, and carbohydrate.rT
centratedpre-eventsugardrink (in contrastto drinkingplain
This ocwater)prematurelydepletedmuscleglycogenreserve's.
curred becausethe dramaticrise in blood sr'rgarwithin 5 to 10
minutes after ingestionproduced an overshootir.rir.rsulin
hlperinsulinemia)folreleasefrom the pancreas(accentuated
lowed by a rapid decline in blood sugar (rebound hlpoAt the
glycemia)as glucosemoved rapidly into muscle.3t''n'
sametime, insulin inhibited fat mobilizationfor energy,an
effectthat can ]astfor severalhours after consunringa concentherefore,intramusculal
tratedsugarsolution.During exercise,
to a greaterdegreethan ttndernorcarbohydrate
catabolized
rnal conditions.This increasedthe rateof glycogendepletion.
Thesenegativeresearchfindings seemimpressivelnd
yet subsequentinvestigations
their explanationreasonable,
or patients
replicatedin healthysubjectsr're'26'32
havenot beer-r
glucoseingestion
In fact,pre-exercise
with type I diabetes.T0
increased
muscleglucoseuptakebfi reducedliver glucoseoutput during exercise,
to conserveliver glycogen."f'he discrepancy amo:lg studieshas ncl clear explanation.One way to
simelinrinateany potentiallbr negativeeffectsof pre-exercise
exerbefore
60
minutes
at
least
sugars
is
consume
them
to
;rie
hornlonal
cising.This providessuftlcienttime to re-establish
dif'
individual
In
begins.
all
likelihood,
exercise
balancebefore
ferencesexistin the responseto specificcarbohydrateingestion beforeexerciseand the subsequentinsulin release.The
LllaPtCro
'-
r\ucr'(rv'rur
o-
'
Supplementsin Liquidand SolidForm
Carbohydrate
8.1 Composition
of Commercial
TABLE
SportsNutritionand"Metabolic0ptimizer"
kCalper
B',Y'l19_'
SportsNutrition
GatorPro
Nutrament
SportShake
SegoVery
Go
Sustacal
Ensure
EnduraOptimizer
Me tabololll
ProOptibol
Mu sc leP ep
ProteinRepairFormula
8;ozSeryino
g
Carbohydrate,
360
240
310
180
190
240
254
279
258
266
261
200
s8(6s%)
34(s7%)
4s (s8%)
30(67%)
27(s6%)
33(ss%)
35 (54o/o)
s7 (82%)
40 (62%)
44 (56o11
4s {6s% )
26 (s2% )
g
Protein,
Lipid,9
7 ( ', t 7 %)
6.5 (25o/o)
10 (29%)
2.5 (13o/o)
3 (13%)
5 . 5 ( 2 'l o / o )
e (32%)
<1 (2o/o)
2 (7o/o)
2 ( 7 %)
'l (3Vo)
1.5 (8o/,)
16 (18o/o)
1"1(18o/o)
11 (13% )
9 (20o/o)
1s (31% )
14.5(24%)
e (14%)
11 (16% )
20 (31%)
18 (27%)
18 (28% )
20 (40%)
HighCarbohydrateBeverages
Beverage
type
Carbohydrate,
ServingSize,oz
Carbohydrate.
9 peroz
% Carbohydrate
GatorLode
Carbop lex
Exceed
Carbo Fire
U ltra Fu el
Carbo Power
& glucose
Maltodextrin
Maltodextrin
& sucrose
Maltodextrin
fructose
polymers,
Glucose,
Maltodextrin
high-fructose
cornsyrup
Maltodextrin,
tz
5.9
7.1
7.1
7.1
6.25
7.9
20
24
24
24
23
't8
SportsEnergyBars
Bar
PowerBar
Exceed
SportsBar
Ed g e bar
K-Trainer
Tiger5port
Th u nderB ar
UltraFuel
Cl i fBa r
Ga to rB ar
Forza
BTUStoker
PRBar
5ize,oz
TotalkCal
2 .2 5
2 .9
2 .5
2 .2 5
2 .3
2 .2 5
4 .8 7
2 .4
2 .2 5
2 .5
2 .6
225
280
234
220
230
220
290
252
zzv
t.o
25 1
2s2
190
32
to
-9:!:Y_9:9:r
(75%
42
s3 (76%
44 (75o/o
4O (730/o
40 (70%
41 (74o/o
1OO(82%
52 (80o/o
48 (B7o/o
45 (78o/o
46 U3o/o
1e (40%)
Protein,g
Lipid,9
10 (17% )
12 (17% )
10(17% )
10(18% )
11(19o/o)
10 (18% )
1s (12%)
s (8% )
3 (s% )
10 (18% )
10 (16% )
14 (30%)
2 \B %)
2 (7o/o)
2 (8%)
2 (gqo)
3 (11 o/ o)
2 (8%)
3 (6%)
3 (12%l
2 (8%)
1(4%)
3 (11o/ o)
6 (30%)
Carboh y d r a t e o n t e n t o f c o m m o n fo o d s:4 ch o co la te ch ip co o kie s =289;l cupWheati es(1oz):239;l appl e=2l g;l cupappl ei ui ce=299;l banan a:279
Proteincontentofcommonfoods:l cup milk = 8 g;3 oz bakedsalnon = 21 g;1 cupcooked peas= 16 g;3 ozsteak = 22 g;1 large egg = 69.
individual'spre-exercise
glucoseor glycogenstatusplaysa role
includingthe glycemicindex of food ingested(seepage240).
Pre-exercise Fructose: Not a Cood Alternative
Fructoseabsorbsmore slowlyfrom the gut than eitherglucose
or sucrose.This car-rses
only minimal insulin responsewith essentiallyno declinein blood glucose.Theseobservationshave
stimulateddebateabout the possiblebenefitsof fructoseas an
imrnediatepre-exercise
carbohydrate
fuel sourcefcrr
exogenous
prolongedexercise.The theoreticalrationalefor fructoseuse
appearsplausible,but its exercisebenefitsremaiu incorlclu'sive.
Frou a practicalstandpoint,consumillga high-fructosebeverageoften producesgastrointestinal
distress(vomiting and diarrhea); this negativelyaffectsexerciseperformance.Once
absorbedby the small intestine,fructosemust first enLerthe
liver for conversionto glucose.T'hisfurther limits how quickly
fructosebecomesavailableasan energysource.
Feedings
During
Carbohydrate
Exercise
liver glycoHigh-intensityaerobicexercise
for I hour decreases
55o/o,
whereas
a
2-hour
strenulus
workout
alrnost
gen by about
totally depletesthe glycogenclntettt of the liver and exercised
muscles(ntusclefibers). Even maximal, repetitive 1- to 5with periodsof lower
minute bouts of exerciseinterspersed
occurs
ice hockey, field
exercise-as
in
soccer,
intensity
lower
handball,
and
tennis-dramatically
hockey,European
glycogen
mental
reserves.-'7'83
Physicaiand
liver and muscle
performanceirnproveswith carbohydratesupplementation
238
part 3 Optinral Nutrition for the PhysicallyActive Person:Making Infornredancl HealthftrlChoices
OxygenConsumptim
The addition of protein to the carduring exercise.2'ee'r03'r06
beverage(4:I ratio of carbohydrateto
bohydrate-containing
pr ot ein ) m a y a u g me n tti m e to fa ti g ueand reducemuscl e
damagecomparedto supplementationduring exercisewith
carbohydrateonly.78Carbohydratefeedings during proat greaterinlongedexercise
alsoallorvindividualsto exercise
tensity,althoughtheir perceptionof physicaleffort remains
no differentthan for a placebogroup.nt
No AbnormallnsulinResponse
DuringExercise
ri
ti
Consuming high-glycemicsugarsduring exercisedoes rot
augmentthe insulin response(and possibleresultinghypoglycemia)that cor.rldoccur with sugarconsumption in the
pre-e-xercise
condition. This occurs becausesympathetic
nervoussystemhormonesin exerciseinhibit insulin release.
Concurrently,exerciseaugmentsglucoseuptakeby muscleso
the exogenousglucosemovesinto thesecellswith a lower insulinrequirement.
each
Consumingabout60 g of liquid or solidcarbohydrates
hour benefitshigh-intensity,long-duration(= I h) aerobicexerAs
ciseand repetitiveshortboutsof near-maximaleffort.3'ts'aa'58
discussedin Chapter 5, sustainedexerciseat or belorv50% of
maximum intensityreliesprimarily on energyfrom fat oxidation, with relativelylittle demandon carbohydratebreakdown.
This levelo[exercisedoesnot tax glycogenreservesto a degree
that would limit endurance.On the otherhand,glucosefeedings
provide supplementarycarbohydrateduring intenseexercise
greatly.In fact,mixwhen glycogendemandfor energyincreases
at
turesof glucose,fructose,and sucroseingestedsimultaneously
a high rate (between1.8and 2.4g ' min r) resultin a20 to 55o/o
higherexogenous
carboltydrate
oxidationratepeakashigh as1.7
carbohydrate)
g ' min-r (with reducedoxidationof endogenous
comparedwith ingestionof an isocaloricamount of glucose.t2'43'76'77
Exogenouscarbohydrateintake during exercise
providesthe following two benefits:
l. Sparesmuscleglycogen,particularlyin the type I, slowtwitch musclefibers,becausethe ingestedglucosepowers
el
exercise.se
This ele2. Maintainsa more optimal levelof blood glucose.
vatesplasmainsulin levelsand lowerscortisol and growth
lightheadedness'
hormone levelsand preventsheadache,
nausea,and other symptomsof centralnervoussystem
distress.r0'63'10e
Blood glucosemaintenancealsosupplies
muscleswith glucosewhen glycogenreservesdepletein
the laterstages
ofprolongedexercise.rs'34
rtculE 8.1showsthat trainingstatusdoesnot alterthe ability to oxidizeglucoseduring exercise
when the trainedand untrained exerciseat the samerelativeintensity.Seventrained
cyclistsand sevenuntrained subjectsexercisedfor 2 hours at
600loofaerobic capacity.At the onsetofexercise,eachsubject
consumed8 mL ' kg-t of an 8olonaturallylabeledIIiC] -glucose
solutionwith 2 mL ' kgl of the fluid ingestedevery20 minutes
thereafter.Total exogenousI t3C]-glucose
use(3.2kCal ' min ')
higherabsoluteoxywassirnilarin both groupsdespitea 24o/o
(36 vs.29 mL 02 ' Lg-' '
gen uptakein the trained .subjects
min-r; Flc. 8.1A)and highertotalfat oxidation.(About1.5-l'7
, nn^
O
29 mL kg-1.min'l
36 mL.kg-1.min'1
o
=^
itrtr
N'=
Eo
J ( .o
dlL
-o
P o-
8!* 10c0
a
Untrained
Trained
Fat IOral carbohydrates[jEndogenous carbohydrale
s
g^ o )s . E
.d EE
iET
*sr
6
E
lrr
Untrained
Trained
FIGU R E8.1 r A , A bsol ute(kJ per 60- m inof exer cise)
and
B, relative(%) contributionsof substratesto energyexpendi ture i n endurancetrai ned and unt r ained m en. ( *) indicatesstati sti cal l ysi gni fi cantdi ffer ence bet ween t r ained
and untrai ned.Mul ti pl y by 0.239to conver t kilojoulest o
ki l ocal ori es.(FromJeukendrupA E, et al. Exogenousglucoseoxi dati on duri ng exerci sei n endur ance- t r ainedand
untrainedsubjects.J Appl Physiol1997;83:835.)
g [6.0-to 6.8kCal] per minute represents
the upperlimit fbr oxidizingexogenouscarbohydrate.43'4s'r0o'ror)
Equivalencein exogenousgr ;coseusebetweentrainedand untrainedsubjects
occurredevenwith a relativelysmallercontribution of exogenous and endogenouscarbohydrateto the higher total energy
expenditureof the trainedsubjects(Fig.8.l B). This suggest
that carbohydrateabsorptionfron.rthe gastrointestinal
tract
into the circulationlimits ingestedcarbohydrate's
catabolisrn
rateduring exercise
independentoftraining state.
o not substituteexogenousfructosefor glucoseduring prolongedexercise,becauselessfructoseoxidizes
when consumingequivalentamountsof both sugars.s3
A DistinctErgogenic
Advantage
in Intense
Aerobic
Exercise
Carbohydrate
feedingduring exercise
at 60 to 807oofaerobic
capacitypostponesfatigueby l5 to 30 minutes,with perform-
_I
ChapterB Nutritionalconsiderations
for IntenseTrainingand Sportscompetition
ancermproyementgenerallyrangingbetween15 and 35o/o.
This effect,potentiallyimportant in marathonrunntng, occurs becausefatiguein weli-nourishedindivrdualsusuallybecomesnoticeablewithin 2 hoursof intenseexercise.
A person
can ward off fatiguear.rdextendendurancewith a sir.rgle
concentratedcarbohydratefeedingapproximately30 minutesbefore anticipatedfatigue.FtcuRE,
8.2 showsthat this f'eeding
the levelofblood glucose,which thensustainsthe enrestores
ergyneedsof activemuscles.
Endurancebenefitsfrom carbohydratefeedingsbecome
apparentat about 75%oof aerobiccapacity.When exerciseinitially exceedsthis intensity,an individual must reduceintenduring the final stagesto maintain the
sity to the 75o/olevel
benefitsfrom carbohydrateintake.r8Repeatedfeedingsof
solid carbohydrate(43 g sucrosewith 400 rnl water) at the beg i n n i n g and at 1, 2, an d 3 h o u rs d u ri n g e x e rc i s emai ntai n
blood glucoseand slowsglycogendepletionduring 4 hours of
cycling.Maintaining blood giucoseand glycogenreservesalso
enhanceshigh-intensityexerciseperflormance
to exhaustionat
the end of the activity.r's'73'8s
The winner of a marathonrun is
usuallythe athletewho sustainshigh-intensityaerobiceffort
a n dsp rint st o t he t inis h .
REPLENISHING
GTYCOCEN
RESERVES:
REFUELING
FORTHE
NEXTBOUTOF INTENSE
TRAININGOR COMPETITION
All carbohydratesdo not digestand absorb at the sarnerate.
Plant starchcomposedprimarily of amyloserepresents
a resistantcarbohydratebecauseofits relativelyslow hydrolysis
rate.Conversely,starchwith relativelyhigh amylopectincontent digestsand absorbsmore rapidly.
The GlycemicIndex
i
asa relative(qualitative)indicatorof
Theglycenic index seryes
how cnrbohydrate-containing
food affectsbloodglucoselevels.
The rise in blood sugar-terme d the glycenricresponse-isdetermined after ingestinga food containing 50 g of a digestible
carbohydrate(total carbohydrateminus fiber) and comparing it over a 2-hour period with a "standard" for carbohydrate
(usuallywhite bread or glucose)with an assignedvalue of
The glycemicindex expresses
100.7'r07
the percentageof total
areaunder the blood glucoseresponsecurve for a specific
food comparedwith glucose(t,tc.8.3). Thus, a fbod with a
glycernicindex of 45 indicatesthat ingesting50 g of the food
raisesblood glucoseconcentrationsto levelsthat reach450lo
of that reachedwith 50 g of glucose.The glycemicindex provides a more usefulphysiologicconceptthan simply classi$.ing a carbohydrateon the basisof its chemicalconfiguratior-r
as simple or complex,sugaror starch,or availableor unavail-
=
tr
:
q,
4.5
o
o
o
J 4.0
(5
tr
o
o
(g
tr
6 3.5
s0.
o
()
o
o
o
to
o
E
o
o
o
0
30
60
9 0 1 2 0 ' t3 5 1 5 0 1 8 0 1 9 5 210
ExerciseTime,min
FIGU RE8. 2 r A v er ag ep l a s m ag l u c o s ec o n c e n tra ti o nduri n g p rolonged high- in te n s i tya e ro b i ce x e rc i s ew h e n subj e cts co ns um edeit her a p l a c e b oo r g l u c o s ep o l y m e r (3 g
p e r kg body m as s in a 5 0 % s o l u ti o n ). (Mo d i fi e d from
C o g g a nA R, Coy le E F .M e ta b o l i s ma n d p e rfo rma n cefol l o w i n g c ar bohy dr at ei n g e s ti o n l a te i n e x e rc i s e Med
.
S ci
SportsExerc1989;21:59.)
FIGU R E8.3 r Generalresponseof i ntesti nalgl ucoseabsorpti on fol l ow i ng feedi ng of foods w i th ei ther ( A) low
gl ycemi ci ndex or (B ) hi gh gl ycemi ci ndex suchas g lucose.
The l ow gl ycemi cfood absorbsat a sl ow er rate th r oughout the ful l l ength of the smal l i ntesti ne to pro duce a
more gradualri sei n bl ood gl ucose.
f
240
l vcti veperson:
P a rt3 O p ti m a lN u tri ti o nfo r th ep hysi calA
Maki ngInformed
and H eal thful
C hoi ces
I ' ' " ' l i l t 'l
i'r"
t "i
he glycemicindexis a functionof glucose
appearance
I in the systemic
circulationand its uptakeby peripheral
tissues,which is influencedby the propertiesof the carbo_
hydrate-containing
food. Forexample,
the food,samylose
to amylopectinratio and its fiber and fat content influ_
enceintestinalglucoseabsorption,whereasthe orotein
contentof the food may augmentinsulinrelease
to facili_
tate tissueglucoseuptake.Ts
f
able.One internationallisting of glycemicindexvaluescontainsnearly1300entriesthat representvaluesofmore than 750
differentfood types.28
Differencesin valuesexistwithin the literature,dependingon the laboratoryand exactfood type evaluated (e.g.,slightvariationsin tlpe of whitebread,rice, and
potatoesusedasthe standardof comparison).Do not view the
glycenticindex as an unwaveringstandardbecause
consiclerablevariabilityexistsamong individualsconsuminga specific
carbohydrate-containing
food. A high glycemicindex rating
doesnot necessarily
indicatepoor nutritionalqr-ralitv.68
For ex_
am ple,c a rro tsb, ro w nri c e ,a n d c o rn ,w i t h thei rri ch ouanti ti es
of health-protective
micronutrients,phytochemicals,
and dietaryfiber,haverelativelyhigh indices.
Individual differencesin responsero how food is digested
and its preparationand ripenessaffectthe glycemicindex. For
example,a ripe bananahas a higher glycemicindex than a
"greener"banana.Once foods are combined(i.e.,
a ripe ba_
nana eatenwith three flavorsof ice creamtoppedwith nuts
and chocolatefudge),the meal'sglycemicindex for that com-
bination of foodsdiffels fronr the glycemicindex for the separateitems.
The revised glycemic index listing also includes the
glycemicload associated
with the specifiedservingsizesof difquanferentfoods.Whereasthe glycemicindexcompareseqr.ral
food, the glycemicload
tities of a carbohydrate-containing
quantifiesthe overallglycemiceffectof a typicalportionof food.
This represents
the productof the amount of availablecarbohydratein that servingand the glycemicindex of the food. A high
glycemicload reflectsa greaterexpectedelevationin blood glucoseand a greaterinsulinrelease.
An increased
risk for tlpe 2 diabetesand coronaryheartdiseasecoincideswith the chronic
consumptionof a diet with a high glycemicload.nr'50
Not All Carbohydrates
Are Equal
r-tcuntB.4Aliststhe glycemicindexfor common itemsin various food groupings.Inset table (B) givesexamplesof highand l ow -gl ycemi c i ndex meal s of sim ilar calor ie and
macronutrient composition. For easyidentification, we have
placed foods into high, rnedium, and low categoriesfor
glycemicindex.Interestingly,a food's index rating doesnot
dependsimply on its ciassification
asa "sirnple"(mono- and
"complex"
(starchand fiber) carbohydrate.
disaccharides)
or
This is becausethe plant starchin white rice and potatoeshas
a higherglycemicindex than the simplesugars(particr-rlarly
fructose)in applesand peaches.
A food'sfiber contentslows
(e.g.,peas,beans,and
digestionrate;thus, many vegetables
other legumes)have a low glycemicindex. Ingestinglipids
and proteins tends to slow the passageof food into the srnail
intestine,thus reducingthe glycemicindex of the meal'saccompanyingcarbohydratecontent. Clearly,the most rapid
methodto repleni;hglycogenaJterexerciseis to conxtmefoods
A Heh gtycemic
F I G UR E 8 .4 r
A.
G ly c em i ci n d e x c a te gorizationof common
food sourcesof carbohydrates.B. Thistable
givesexamplesof high
and lo w g l y c e mi c index di e ts th a t c o n tain the sameamounts
of energy and macronut r ien ts a n d d e ri v e
50% oI e n e rg y fro m
c ar bohy d ra te (C H o )
and 30 % o f e n e rg y
f r om l i p i d . (D i e ts
f r om B r a n d -Mi l l e r J ,
Foster-PowellK. l/utr
Today 1999;34:64.)
clucosg
100
Carrotg
92
Hon6y
87
g0
Corn tlakes
Whola meal brsad T2
White rlce
72
Now potaloes
70
White br8ad
69
Shreddedwhest
6Z
Brown rico
66
B€et8
64
Ralslns
64
Bananag
62
Moderate glycemic
Corn
Sucrose
All-bran
Potato{hlps
Peas
Whilo pssta
Oatmeal
Sweet polatoes
Wholo wheat pasta
Oranges
59
59
Sl
5.1
5l
50
49
48
42
40
Low glycemic
Apples
Fish sticks
Butter beans
Navy bean6
Kidney beans
Lentils
Sauaage
Fructose
Peanuta
Breakfast
30 g Corn Ftakes
1 banana
1 sticewhote meat bread
1 tsp margarine
75
30
12
Snack
'1crumpet
1 tsp margarine
9.9
7.8
3.8
6.4
L!nch
2 sticeswhote'meatbread
2 tsp margarina
25 g cheese
1 cup diced cantatoupe
7.6
t0.4
Breakfast
30 g Att-Bran
1 diced peach
1 sticegrain bread
1 tsp margarine
1 tsp jetly
Lunch
2 sticesgrainbread
2 tsp margarine
25 g cheese
1 appte
Snack
200 g tow-fat fruit yogurt
Dinner
120g teansteak
'1cup of mashedpotatoes
1/2 cup of carrots
'l/2 cup of greenbeans
50 g broccoti
Dinner
'120g lean mincedbeef
1 cup boiied pasta
1 cup of tomato and onion sauce
Greensatadwith vinaigrette
12.1
1.7
0.6
4.5
3.6
34
8
1
6-4
2.5
0.6
10
14
7.1
1.9
712
39.0
I
39
38
35
31
29
29
Total
20
t3
For eoch diet, the carbohydrate choicesore maximized for differences between the two diets.
Snack
290 g watermeton
I cup of reduced-fatmitk
throughout day
4.7
1,1
2.7
Snack
'l sLicegrainfruit loaf
1 tsp margarine
Snack
4 plain sweet biscuits
3Z
4
2
24
8
14
15
14
5.1
1.9
212
69.8
Snack
'1orange
1 cup of reduced-fatmilk
throughoutday
Total
for Intense
Trainingand SportsCompetition 241
Considerations
ChapterB Nutritional
glycenicinLlices
with nroderate-to-high
ratherthanfoodsrated
et,enif thereplenishntent
los,tt'Ie'20'e4r02
nlealclfltaittsa small
atnoutftof lipid and protein.r2In fact,theadditionof liquid
proteinto the carbohydratesupplementmay evenenhance
the magnitudeof glycogenresynthesis.r'
During the first 2
hours of recovery,when muscleglycogencontentis at its
lowestlevel,consurninga glucosepolyrnersolution(low osmolalitl') restorcsglycogennore rapidly than an energyequivalentsolution of tnonomerswith high osmolality.6T
This beneficialeffectof low-osrnolalitysolutionson glycogen
replenishrnent
probablyresultsfrom two factors:(l) more
rapidgastricenlptyingand glucosedeliverl,to the small intestineand (2) augmentedpostexercise^stimulated
non-insulin-dependent
glucoseuptakeby the muscles.
The addition
ofr.-arginineto a carbohydrate-containing
beverageoffersno
additionalbenefitto carbohydrate
repler,,:hrnent.7a
The needfor glycogenin previouslyactiventuscleaug,
rnentsglycogenrcsynthesis
Whel"r
in the postexercise
period.65
footl becornesavailableafter exercise,the follorvingfour f'actorsfacilitatecellularuptakeofglucose:
1. Hormonal milieu reflectedby elevatedinsulin
2. Iucreasedtissuesensitivityto insulin and other transporter proteins(e.g.,GLUT I and GLUT 4, membersof a
farnily of facilitativemonosaccharitle
transportersthat
mediatemuch of glucosetransportactivity)
3 . 'Lowcatecholamine
levels
4 . Increasedactivityofa specificform ofthe glycogen-storing enzymeglycogensynthase
To speedglycogenreplenislunent
afterhiensetrahing or competitiltl, oneshouldconsttnrcIigh -glycglnlcs6rl,ol
ry(lrate-rich.foods
assootlaspossible.Follow this practicaladviceto rapidly restore
depleted
glycogenreselres:Witlin l5 minutesafterstoppingexercise,consurne50 to 75 g (2-3 oz, or 1.0-1.5g per kg body
mass)of high- to moderate-glycemic
Continue
carbohydrates.
eating50 to 75 g ofcarbohydrateevely 2 hours until achieving
5 ()0t o 700g ( 7- l0 g p e r k g o f b o d yn ra s s,,r
a l arg,e
) u n ri le a ti rrg
trrgh-carbohydratc'
meal.If immediatelyingestingcarbol.rydrate
atterexerciseprovesimpractical,an alternativestrategyinvolves
eating.rneals
containing2.5g of high-glycemiccarbohyclrate
per
xg Dodymassat 2,4,6,8,and22 hourspostexercise.
Thisreplenishesglycogento levelssirnilarto thoseachievedwitlr the sar.ne
protocolbegunimmec-liately
postexercise..'a
Insulin-stimulating
Effectof Protein
Ingestion
in Recovery:
Doeslt
AugmentGlycogen
tieplenishment?
Consumir.rg
an amino acid-proteinmixture of wheat protein
hydrolysate
with freeleucineand pfienvlalanine(0.4g . ]<g' .
h-r) in a carbohyclrate-containir.rg
beverage(0.g S . t[-t . 1-t,
facilitatesutore ntuscleglycogen,to.ogi without gastrointestinaldiscol.r'rfort
thar.ringestinga carbohydrate-oniybeverageof the santeconcentration.e'l
appearsto
Tl-risaclvatrtage
relateto tl'reinsulinotropiceffectof a higher-levelof plasrtra
atlrinoacicls.e-r'1ro
The ber.refit
of atlded orotein and/or arnino
acicls(and the associated
incrcasedi,rs,-ilinr.'lease)on glyco-
gen replenishment,however,is no greaterthan achievedby
simply addingadditionalcarbohidlsls to the recoverysupplement.TsFor exampie,trained athletesattainedglycogen
synthesisratesequivalentto thoservith a glucoseplus protein
supplemer-rt
with a carbohydrate-only
intakeof 1.2g ' kg-r .
h-r.eaSupplements
weregiven at 30-rninuteir-rtervais
over a
5-hour recoveryperiod. This replenishmentprotocol produced maximal glycogenresynthesis.
Additional intake of
proteinor amino acids,while increasing
the insulinrespotlse,
doesnot increasethe rate ofglycogensynthesis.
What ls the OptimalApproach?
Research
hasaddressed
the followingquestior.r:
Is it betterto
consume large meals or more frequent snacks of highglycemiccarbohydratesto optin-rizegiycogenreplenishrnent?
Orrestudy compared24-hour carbohydratereplenishment
with two pattenlsof consumingan energy-equivalent
rnealof
( 1) "gorging"on a singlelarge
high-glycemiccarbohydrates:
r.neal,r'ith its greaterincrementalglucoseand insulin responseor (2) "nibbling" on frequentsmallersnacks,which
producesa more stableglucoseand insulin response.r3
The
two stylesof eating produced no differencein final glycogen
levels.Tl'resefindings indicate that individuals should eat
high-glycemiccarbohydrates
following intenseexercise;
the
frequencyof the mealsand snacksshould dovetailwith a person'sappetiteand the availabilityof fbod after exercising.
ClycogenReplenishment
TakesTime
Avoid legumes,fructose,and milk productswhen rapidly replenishingglycogenreserves
because
of their slowratesof intestinalabsorption.More-rapid glycogenresynthesistakes
placeif the personremainsinactivedr-rringrecovery.'7
With
intake,glycogenstoresreplenishat about
optimal carbohydrate
5 to 7o/oper hour. Thus,evenunderthe bestof circurttstartces,
it
takesat least20 hours to re-establishglycogenstoresfollowing a
glycogen- depleting exercisebout.
Optimal glycogenreplenishrnentber.refits
individualsinvolvedin ( 1) regularintensetraining,(2) tournamentconrpetition with qualifying rounds, or (3) competitive events
with only I or 2 daysfor recuperation.
scheduled
Beforecurrent
methodsfor establishing
a wrestler'sminimal wrestlingweight
(seeChapterl4), wrestlerswho lost considerable
glycogen(and
rvater)using food and fluid restrictionbeforethe weigh-in to
"make weight" alsobenefitedfrom a proper glycogenreplenishn.rentstrategy.ao
For collegiatewrestlers,short-term weight
lossthroughenergyrestrictionwithout dehydrationalsoimpairedanaerobicexercise
capacity.Tl
Arraerobicperfonxance
recoveredtcl r.retrr-baseline
valueswhen theseathletesthen consun.redmealscontaining75o/o
carbohydrate(21 kCal per kg
body mass)overthe next 5 hours.No improverrentoccurredif
the refeedingdiet containedonly 45%ocarbolrydrate.
Even
without full glycogenreplenishrr.r
ent, sonle repler.ri.sl'rn-rent
in
recoverybenefitselrdurancein the r.rextexercisebout. For example,replenishingcirrbohydrateafteronly a 4-hour recovery
period from glycogen-depleting
exelciseyields better enclurancein subsequer.rt
exercisethan a sinrilarscenariowith no
carbohydrate
eatenin recovery.
242
C hoi ces
Part 3 Optimal Nutrilion for the PhysicallirActive Person Mal < i ng
Informed
and H eal thful
Choosethe RightFormof
Carbohydrate
To evaluatethe influenceof a carbohydrate'sstructure on
glycogenreplenishment,eight male cyclistsdecreasedthe
glycogencontentof the vastuslateralismusclewith 60-minutes of cycling at 75o/oVOrn,o*followed by six 1-minute
sprintsat l25o/oVOr,,.u*.ou
Twelvehouls aFterthe glycogendepletingexercise,
they consumeda 3000-kCalmeal (ratio of
650/0:200/0'.150/0
carbohydrateto lipid to protein).A solution
of ghrcose,maltodextrin (glucosepolymer), waxy starch
( 100o/o
amylopectin),or resistantstarch( l00o/oamylose)provided all of the recoverymeal'scarbohydrate.Musclebiopsie.s
24 hours into recovery(r.lc.8.5) reveaieda lower glycogenrepletion level from the resistantstarchmeal (high amylose
l"
r
r:t
l ::', " ,r' :it ri ]
'
::4.
,'t : |
t
.,
', ; r '
'
.
'i : l
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,
.9
o
3
b zoo
.a
.--...
- . -, .t
,r:r;:!i
. .'.,
, ,.; ,
---.
-,t.--',
*.',
tl:
!
ED
0
rso
F
c,
qt
crt
c
(E
FI
o 100
c
o
ct)
o
o
.=
( r so l
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=
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$p*q
CarbohydrateTrial
F I G URE8. 5 r C h a n g e si n mu s c l eg l y c o g enw i th vari ous
c ar bohy dr atefe e d i n g so f s i mi l a re n e rg yc o ntent i n the 24hour per iod fo l l o w i n g g l y c o g e n -d e p l e ti ngexerci se.(* )
Denot ess ign i fi c a n tl yl o w e r v a l u eth a n g l u cose,mal todext r in, and wax y s ta rc h .(F ro mi o z s i A C , e t a l . The i nfl uence
of starchstructureon glycogenresynthesis
and subsequent
cyclingperformance.lnt J SportsMed 1996;17:373.)
content,low glycernicindex) than fiom mealswith the other,
carbohydrates.
Keepingto the p.rrsmore rapidlyhydrolyz.ed
imrnediate
recoveryperiod
intake
in
the
scribedcarbohydrate
procluces
rnoredesirableglycogenreplenishnentthan letting
athleteseatthe amounttheywish.
THEGLYCEMIC
INDEXAND
PRE.TXERCISE
FEEDINGS
Usethe glycemicinclexto formulatethe immediatepre-exercise f-eeding.
The ideal meal immediatelybefore exercising
should provide a sourceof glucoseto maintain blood sr"rgar
and sustainmusclemetabolism;it also should not triggera
spike in insulin release.A relativelynormal plasmainsulin
level theoreticallypreservesblood gh-rcose
availabilityand optimizesfat rnobilizationand catabolismwhile sparingglycogen reserves.As mentioned previously,consuming simple
sugars(concentrated
high-glycemiccarbohydrates)
immediately before exercisingcausesblood sugar to rise rapidly
(glycemicresponse),often triggeringexcessive
insulin release
(insulinemic response).The resultingreboundhlpoglycernia,
fat catabolism,and possibleearlydepletionof glycodepressed
negativelyaffectenduranceexerciseperformance.
gen reserves
In contrast,consuminglow-glycemicindex fbocls(starch
prorvith high amylosecontent)immediatelybeforeexercise
vides a relativelyslow rate of giucoseabsorptioninto the
blood. This elirninatesany possibleinsulin surge,while a
steadysupply of "slow-release"glucosebecomesavailable
This effecttheofrom the digestivetract asexerciseprogresses.
reticallyprovesbeneficialduring long-term,intenseexercise,
particularlyin unusualeyentssuch as oceanswimming where
the practicality9f consurningcarbohydrateduring exerciseremainsa challenge.r05
Severalstudiessupport the wisdom of consuminglow(starchwith high amylosecontentor
glycemiccarbohydrates
moderate-gl1'cemic
carbohydratervith high dietary fiber content) in the immediate45- to 60-minuteperiod beforeexercise,which allowsfor a slowerrate of glucoseabsorption;this
reducesthe potential rebound glycernicresponse.For trained
cyclistswho performedhigh-intensityaerobicexercise,a preexerciselow-glycemicmealof lentilssignificantlyextendedendurance over that with feedings of either glucose or a
high-glycemicmeal of potatoesof equivalentcarbohydrate
content.rrHigher blood giucoselevelsnearthe end stagesof
exerciseaccompaniedthe low-glycemic,pre-exercisef'eedit'tg.22'33
Despiteinducing ;rotentiallyfavorablealterationsin
blood glucoseand tht catabolism,all researchhasnot observed
ergogenicbenefitsfrom low-glycemicpre-exercise
carbohydrates.26'r2'88
Furtherstudy of the topic seemswarranted.
AND
ELECTROLYTES,
GLUCOSE,
WATERUPTAKE
As we discussin Chapter10,fluid ingestionbefbreand durir.rg
exerciseminimizesthe detrimentaieffectsof dehydrationon
cardiovascular
dynamics,temperatureregulation,and exercise
C h a p te B
r N u t ri ti onal
C onsi derati ons
for Intense
Trai ni ngand S ports
C ompeti ti on 243
performance.Adding carbohydrateto the oral rehydration beverageprovidesadditional glucoseenergyfor exerciseasglycogen
deplete.Addhry electrolytesto tlte rehydrtrtionbe'yerage
reserves
ntaintainsthe thirst mechanisnend reducesthe risko.fhyponaffemia(seeChapterl0). The coachand athletesshouldwork
togetherto deterrninethe optimaI fluid/carbohydratemixture
and voluu.reto minimize fatigueand prevent dehydration.
Concernfocuseson the dual observationsthat a largefluid
volumeintakeimpairscarbohydrateuptake,while a concentratedsugar/electrolyte
solution irnpairsfluid replacement,
ImportantConsiderations
Stornachemptyingrate greatlyaffectsfluid and nutrient absorptionby the small intestine.FIGURE
8.6 illustratesimportant factorsthat influencegastricemptying.Little negative
effectof exercise
on gastricernpryingoccursLrrtilan intensity
of about 75o/oof maximurn, after which, entptying rate
slows.ae
Gastricvolume, holvever,greatlyinfluencesgastric
emptying becauseernptyingrate declinesexponentiallyas
fluid volune decreases.
A majorfactor to speedgastricetrrprying and contpensate
any
inhibitory effectsof tlte beverttge's
for
carbohydrate
contentinvolvesmaintainirrga relotivelylnrge
stomach
lluid yolume.
PracticalRecommendations
Consuming400 to 600 mL of fluid 20 rninutesbefbreexercise
optirnizesthe beneficialeffectof increasedstomachvolume
on fluid and nutrient passageinto the small intestine.
Regularlyingesting150to 250 mL of fluid (at l5-minute intervals)throughout exercisecontinually replenishesfluid
passedinto the intestine;this maintainsa relativelylargeand
constantgastricvolume.a'23''7'o'
This protocoldeliversabout
I L of fluid per hour to the small intestine,a volume that
meetsthe needsof most enduranceathietes.Prior researchin-
dicatedthat colderfluid emptiedfrom the stomachmore rapidly than fluid at room temperature,yet fluid temperature
doesnot exert a major influenceduring exercise.Beverages
containingalcoholor caffeineir.rducea diuretic effect(alcohol
most pronounced),which facilitateswater loss.Both beveragesare contraindicatedfor fluid replacement.
ConsiderFluidConcentration
Concern existsabout the potential negativeeffectof sugar
drinks on water absorptionfrom the digestivetract. Gastric
enrptyingslowswhen ingestedfluids eithercontainincreased
of particlesin solution(osmolality)or possess
concentrations
Rehydrationbeverages
hypertonicto
high caloric content.8'72
plasma(>280 rnOsm' kg-r) retardnet fluid uptakeby the intestine. This negativelyaffectsprolonged exercisein hot
weather,when adequatefluid intake and absorption play
prime rolesin the participant'shealthand safety.The negasugarr.nolecules
on gastricelnptytive effectof concentrated
i ng i s di mi ni shed(and pl asmavol ume mai ntai ned )if t he
drink containsa short-chainglucosepolyrner (maltodextrin)
rather than simplesugars.Short-chainpolymers(3-20 glucoseunits) derivedfrorn cornstarchbreakdownreducethe
number of particlesin solution.Fewerparticlesfacilitatewater movementfrom the stomachfor intestinalabsorption.
Adding a small anount of glucoseand sodiurn(glucose
being the more important factor) to the oral rehydrationsolution doesnot negativelyaffectgastricemptying. It also facilitates lluid uptake by the intestinal lumen becauserapid
cotransportof glucose-sodiumacrossthe intestinalntucosa
stimulateswater'spassiveuptake by osmotic action.2e'30'8r
effectively,
and the additionaiglucoseupWater replenishes
This glucose
take contributesto blood glucosemaintenance.
then sparesmuscleand liver glycogen,and/or providesblood
glucoseshould glycogenreservesfall during the later stageof
exercise.
IntestinalFluidAbsorption
GastricEmptying
. Carbohydrate:
lowto moderate
levelof glucose+ sodium
increasesfluid absorption
. Sodium:lowto moderate
level
increases
fluidabsorption
hypotonic
to
' Osmolality:
isotonic
fluidscontaining
NaClandglucoseincrease
fluidabsorption
. Volume:increased
volume
emptying
rate
increases
. Caloriccontent:increased
energy
emptyingrate
contentdecreases
. Osmalityi
solute
increased
concentralion
decreases
emptyingrate
. Exercise:
intensity
exceeding
rate
of 75"/oof maximumdecreases
emptingrate
. pH:markeddeviations
from7.0
decreaseemptyingrate
. Hydration
level:dehydration
gastricemptyingand
decreases
risk
increases of gastrointestinal
distress
FIGURE8. 5 r M ajor f a c to rsth a t a ffe c t g a s tri ce m p tyi ng(stomach)and fl ui d absorpti on(smal li ntesti ne).
244
C hoi ce s
and H eal thful
Pa rt3 Op ti m a lN u tri ti o nfo r th e P hysi cal lAycti veP ersonMaki ngInformed
Rehydrationsolutionsthat combinetwo differenttrans(glucose,fructose,sLlcrose,
portablecarbohydratesubstrates
or maltodextrins)producegreaterwater uptakeat a particuiar
intestinallumen osmolalitythan solutionscontainingonly
one substratefrom enhancedsoluteflux (and thus water flux)
from the intestine(prc.8.7).The secondsubstratestimuiates
more intestinaltransport mechanisms,thus facilitatingnet
water absorptionby osmosis.
BENEFIT
OF SODIUM.Adding moderateamountsof
POTENTIAL
(the
space)to the
sodium
most abundantion in the ex:tracellular
doesnot
glucose
absorption
and
ingestedfluid minimally affects
E
o
o
0
T
I
e
j
-100
o optimizewater and carbohydrateabsorption,consolution that
I srme a 6% carbohydrate-electrolyte
combinesfructoseand sucrose,eachtransportedby separate noncompetitivepathwaYs.
f
alterthe contributionofingestedglucoseto the totalenergyyiel
The extrasodiun-r(0.5-0.7g per L
in prolongedexercise.3l'36'sa
does,however,helpto maintainplasmasodium concentration
This effectbenefitsultraenduranceathletesat risk for hypona
tremia. Hyponatremiaoccursfrom a largesweat-sodiumlos
coupled with drinking copious amounts of plain water
Maintainingplasmaosmolalitywith addedsodium in the rehy
dration beveragealso reducesurine output and sustainsthr
sodium-dependentosmotic drive to drink. Thesefactorspro
mote continuedfluid intakeand fluid retentionduring recov
of i
the optimal characteristics
Chapter10 discusses
ery.55'se'toa
dehydration.
rehydrationbeveragefollowing exercise-induced
.j
c
o
-200
ORAL
RECOMMENDED
BEVERAGE:
REHYDRATION
DRINKS
THESPORTS
EVALUATING
o)
()
o
o
-300
q)
't
beverageconsumetlwhilt
A 5 to 8o/ocarbolrydrate-electrolyte
exercisingin the heat helpsto regulatetemperatureand Jluia
balanceas effectivelyas plain water. As an addedborus, thi:
drink maintainsglucosemetabolism(providesan intestinaldeglycogenduring
Iiveryrate of 5.0 kCal ' min-t) and preserves
Consumingthis solution in recover
prolongedexercise.s6'57'82
from prolonged exercisein a warm environment also improvesendurancecapacityfor subsequentexercise.
-400
o
o
o -500
E
L
c)
o
=
-600
o
z
-700
260
270
280
290
iS#
Meanosmolal
300
:i;_t"*,
310
320
sesment,
F I G UR E8 .7 r N e t w a te r m o v e me ntrel atedto mean osm olali ty i n th e i n te s ti n a lte s t s e g m e nt.W ater absorpti on
f r om t h e i n te s ti n es h o w sa s a n e g a t i veval ue (hi gh negat iv e v a l u e s i n d i c a te g re a te r a b s o rpti on)w hi l e secreti on
int o t h e i n te s ti n a ll u me n w o u l d s h o w as a posi ti veval ue.
f he pu rp l e /i n e s h o w sth e re l a ti o n s hi pamong the three
t es t s o l u ti o n s c o n ta i n i n g o n e tra nsportabl e substrate,
while the orange /ine refersto six solutionscontainingtwo
mi ddl e l i ne (red)
.
or t hr e e tra n s p o rta b l es u b s tra te sThe
r epr es e n tsth e re l a ti o n s h i pa m o n g al l test sol uti ons.For
as osmoeac ht e s t s o l u ti o n ,n e t w a te r a b s o rp ti oni ncreases
lalit yde c re a s e sH. o w e v e r,fo r a n y o s mol al i tyval ue,greater
net wa te r a b s o rp ti o n o c c u rsfro m the gut i nto the body
wit h so l u ti o n sc o n ta i n i n g mo re th a n one transportabl e
s ubs t ra te (F
. ro mS h i X, e t a l . E ffe c tso f carbohydratetype
and c o n c e n tra ti o na n d s o l u ti o n o s m ol al i tyon w ater absorption.Med SciSportsExerc1996;27:1607.)
1.
2.
3.
4.
5.
Tastesgood
A bosrbsrapi dl y
dist r ess
C ausesl i ttl e or no gastroi nte st inal
fl ui d v olum e and osm olalit y
Mai ntai nsextracel l ul ar
Offersthe potenti al to enhanceexer ciseper f or m ance
To determinethe percentagecarbohydratein a drink' divide carbohydratecontent (in grams)by fluid volume (in milliliters) a,'d multiply by 100. For example, 80 g of
carbohydratein I L (1000mL) of water providesan B%osol-r
tion. Environmentaland exerciseconditions interactto influencethe optimal compositionof the rehydrationsolution
Fluid replenishmentbecomesof utmost importanceto health
and safetywhen intenseaerobiceffort in hot, humid weathe
lastsbetween30 and 60 minutes.Under theseconditions'we
ChapterB NutritionalConsiderations
for IntenseTrainingand SportsCompetition
recommenda more dilute carbohydrate-electrolyte
solution
containingless than 5olocarbohydrate.In cooler weather,
when dehydrationis not a major foctor,a more concentrated
of l5o/ocarbohydratesuffices.Little differenceexists
beverage
amongliquid glucose,sucrose,or starchas the preferredin_
gestedcarbohydratefuel sourceduring exercise.
Optirnalcarbohydratereplenishmentrangesbetween30
and 60 g (about 1-2 oz)per hour. r,rurc g.2 con-.lpares
the car_
bohydrateand mineral contentand osmolalityofpopular fluid
z4s
plcune8.8 presentsa generalguideline
repiacementbeverages.
for fluid intakeeachhour during exercisefor a givenamount of
carbohydraterepienishment.Although a tradeoffexisrsbe_
t'weencarbohydrateingestionand gastricemptying,the ston_
ach ernptiesup to lZ00 mL of water per hour, even when
drinking an 87o carbohydratesolution. However, 1000mL
(about I quart) offluid consumedper hour probablyrepresentsthe optimal voiume to offsetdehydration,becauselarger
fluid intakescan causegastrointestinal
discomfort.
TABTE
8,2 comparisonof various Beveragesusedby Athletesto Replace
Fluid Lost in Exercise
Beverages
Flavors
CHO(onc
(y"l
CHOSource
GATORADE"
Thirst
Lemon-lime,lemonade,S/G(powder)
Stokely-Van
Quencher
fruit punch,orange,
Camp,Inc.,a subsidiary citruscooler
S/Gsyrup
of the euakerOats
solids(liquid)
Company
Exceed"
RossLaboratories
Lemon-lime,
orange
Quickick'
CramerProducts,
Inc.
Lemon-lime,
fruit punch, FtS
Sqwincher,
the Activity
Drink
Universal
Products,
Inc
L e m o n - lim e ,fr u it p u n ch ,
le m o n a d e , o r a n g e ,
grape, strawberry,
g r a p e fr u it
GlF
10-K
Beverage
Products,
Inc.
L e m o n - lim e ,o r a n g e ,
fr u it p u n ch ,
lemonade, iced tea
SIG/F
USAWet
TexasWet, Inc
L e m o n - lim e ,o r a n g e , fr u it S
p u n cn
Coca-Cola
Coca-Cola.
USA
Re g u la r ,Cla ssic,Ch e r r y
HF CS/S
Sprite
Coca-Cola,
USA
L e m o n - lim e
HF CS/S
G polymers/F
6.8
tr. d
-
s port aLY T E
9.2
VitaminC,chloride,
phosphorus
trace
10
61
)1
tow "
HFCS/M
873
HF CS
8-9
S/G/F
6.3
FCS/S
I- t
M/F
6
M/F
6
M /F IG
t.>
510
Chloride,
phosphorus
450
Phosphorus
600-715
tow"
33
55
55
69s
P hosphorus,
vi tami n C
P hosphorus,cal ci um,
i ron, vi tami ns C
and A , ni aci n, ri bofl avi n, thi ami ne
I
10
60
> /r
aServing
size,8 fluid oz.
bDepends
on watersource.
5 = sucrose;
F = fructose;G = glucose;
HFCS-high fructose corn syrup; M = maltodextin
Calcium,
chloride,
phosphorus
trace
Water
H y dra Cha r g e
60
28
15
E v erlas t
280-360
Chloride,
calcium,
magnesium,
phosphorus
j 0.2
11.8
B reak t hro u g h
Chloride,
phosphorus
4)
ot
1 0 . 7 - 1. 3
Flslc
Cytomax
25
Osmolality
{m o i m 'l - 1 )
phosphorus,
Chloride,
470
calcium,
magnestum,
vitaminC
HF CS/S
10K
110
4.7
orange, grape,
le m o n a d e
Orangejuice
All-Sport
Potassium OtherMinerals
andVitamins
(rs)
7.2
Cranberry
juicecocktail
PowerAde
Sodium
(ms)
100
150
45
20
rrace
100
50
890
246
Pa rt 3 Op t im al Nut r it ion f or t he Phy s ic allyA c t i v e P e r s o n :M a k i n g l n f o r m e d a n d l " ': a l t h f u l C h o i c e s
paci tyi n tw o groupsof 10 young men match edf br aer obic
capaci tyw ho consumedei ther a hi gh-car bohydr at ediet
(65% kcal from carbohyclrate)or high-fat diet (6270kCal
!
| 'rN ' l. " .
from lipid) for 7 weeks.Eachgroup trainedfor 60 to 70 min. ..,-,. :: .. ...,1.:..,..:.1:,1,-J-ll.::.::l
utesat 50 to 85% o[aerobiccapacity,3daysper weekduring
weeks 1 to 3 and 4 days per week during weeks 4 to 7.
1. M onit or de h y d ra ti o n ra te fro m c h a n g e si n body
Fol l ow i ng 7 w eeksof trai ni ng, the gror.rpconsum ir r gt he
weight. Requireurination before postexercise
body
diet. FICURTI
high-fat diet switchedto the high-carbohydrate
weight det e rm i n a ti o n .E a c hp o u n d o f w ei ght l oss
8.9 displays the exerciseperformance for both groups.
c or r es pondto
s 4 5 0 m L (1 5 f I o z ) o f d e h y d rati on.
2. Dr ink f luids a t th e s a meo r s o m e w h a tg re ater rate
Enduranceresultswereclear:the group consumingthe highas t heir es t i m a te dd e p l e ti o n (o r a t l e a s t at a rate
carbohydratediet performed significantly better after 7
c los e t o 80 % o f th e s w e a ti n g ra te ) d u r i ng proweeksof trainingthan the group consumingthe high-fatdiet
longed ex e rc i s ew i th a c c o m p a n y i n gc a rd i ovascul ar
(102.4 vs. 65.2 mi nutes). W hen the hi gh-f at diet gr oup
stress,high metabolicheat, and dehydration.
switchedto the high-carbohydrate
diet during week8, only a
3. E ndur anc ea th l e te s c a n m e e t b o th c a rb ohydrate
improvement
in
small
additional
endurance
of 11.5minutes
(30-60 g per hour) and fluid requirementsby drinkoccurred.
C
onsequentl
y,
total
overal
l
endurance
im pr oveing dur ing e a c h h o u r 6 2 5 to 1 2 5 0 mL (average
period
rnent
over
the
8-week
reached
1|5o/o
for
the
high-fat
about 250 mL every 15 minutes)of a beveragethat
diet grollp, whereasendurancefor the group on the highcontains4 to 8% carbohvdrate.
carbohydratediet improvedby 194o/o.
The insettableshows
daily energyand nutrient intakesbefore the experimental
treatment(habitualdiet) and during the 7-weekexperimental diet. The high-fat diet producedsuboptimaladaptations
in enduranceperformance,
which werenot fully remediedby
switchingto a high-carbohydrate
diet.
Subsequentresearchliom the samelaboratoryfailedto
demonstrate
any endurance-enhancing
effectof a high-fatdiet
Debateconcernsthe wisdom of maintaininga high-fat diet
(or evenfasting)during training or beforeendurancecomcontainingonly moderatecarbohydrate(l5o/ototal kCal) in
petition.2r'2s'6r'e6'e7
rats,regardless
of th . ir trainingstatus.For sedentary
humans,
Adaptationsto high-fatdietsconsistently
show a shift in substrateusetoward higher fat oxidation in
maintair.ringeitl.rera low or high dietaryfat intake for 4 weeks
exercise.ra'38're'87'l1r
Proponentsof high-fatdietsarguethat a
did not affectn.raximalor submaximalaerobicexerciseperIong-term increasein dietary fat stimulatesfat burning by
formance.6e
A 6-day exposureto a high-fat, low-carbohydrate
augmentingthe capacityto mobilize and catabolizethis endiet, followed by I day of carbohydraterestorationwith a
e r gy nut r ient . A n y fa t-b u rn i n ge n h a n c e me nshoul
t
high-carbohydratediet increasedfat oxidation during prod conserve glycogen reservesand/or contribute to improved
longedsubmaximalexercise.This carbohydrate-sparing
effect
endurancecapacityunder low-glycogenconditions.To indid not enhance1-hour time trial performancefollowing 4
vestigatepossiblebenefits,researchcomparedendurancecahoursof continuouscycling.t6
1 , ,, ,; :.. /' :l t. i l ti :i f: f:' .i ,d ; i :,l nl 5i g1' ,:
.,' ..r-i ; :i : ' ,,.1' ' ' ;;' '
j ;.' i -; ;, r.' .,;.-..1
1 :i .j
'
';"lf-a'a'
HIGH.FATVERSUS
LOW.FATDIETS
FORENDURANCE
TRAININGAND
EXERCISE
PERFORMANCE
.s-.$ 2o/.
ci
4o/"
/rrgi oz
iT p ez t
"
ii
:l
E'#l
FIG URE8. 8 I F lu i dv o l u meto i n g e s te a c hh o u r to obtai n the noted amount of carbohydrate(C H O).(Modi fi e df r om Coyle
EF.Montain 5J.Benefitsof fluid replacement.Med SciSportsExerc1992;24:5324.)
Trainirrgand SportsCompetition 247
ChapterB NutritionalConsiderations
for Intense
Weeks1
Week8
@
@
10
20
30
40
50
60
70
80
90
100
110
Ti me to E xhausti on, mi n
Daily intake of energyand nutrients in the subjects' habitualdiet and during
7 weeks on an experimentaldiet
H abi tual di et
Units
CHO
Lipid
Experimental diet
CHO
Lipid
Energy,E
MJ
tJ.o
11.9
14.3
13.7.1
Protein
E/"
13.2
105.0
14.3
'101
.0
14.6
123.0
16.s"i
133.0"i
Ey"
44.2
386.0
4.7
53.4
373.0
11.0
2.3
34.3
118.0
'10.0
2.6
39.0
94.0
75.0
2.2.t
2.2t
62.0't
217.01
31.0
29.0
26.0
44.0.t
4.4
4.3
4.0
11.2.t
0.39
0.43
0.53
0.62't
Carbohydrate
s
g , kg bodyw1-1
Simplesugars
Dietaryfiber
Lipid
Ek
^
rr,-l
C-/o
I
Cholesterol
mg . tr43-1
EssentialFA
E-/o
P/S ratio
o3.u
546.0
6.8
7.0
4.3
22.0.t
177.01
2 4'+
Values
'signilicanily
aremeans;
different
between
thehabitual
andtheexperimental
diet;
difterent
between
thetwoexperimental
diets;
MJ,megajoule;
E%,percent
of totalenergy
fsignilicantly
F IGU R E8 .9 r Effe c tso f a h i gh-carbohydrate(C H O)versusa hi gh-fat di et on enduranceperformance.The group consumingthe high-fat diet for 7 weeksswitchedto
th e h i g h -C H Od i e t d u ri n g w e ek 8. The endurancetest consi stedof pedal i nga bi cycl e
ergometerat the desiredrate. The insettable comparesthe averagedaily energyand
n u tri e n t i n ta k e sd u ri n g th e h abi tual and experi mentaldi ets.P /5rati o, pol yunsaturated-to-saturatedfatty acid ratio. (From HelgeiW et al. Interactionof training and
diet on metabolismand enduranceduring exercisein man.J Physiol1996;492:293.)
A high-fat diet stimulatesadaptiverespoltses
that augment fat catabolism,yet reliableresearchhas not demon_
strated consistentexerciseor trairlir.tgbeneflts from this
dietary modification. Colnpror.nised
training capacityand
sylrptoms of lethargy,increasedfatigue,and higher ratir.rgs
of
perceivedexertior.rusuallyaccolnpanyexerciservhensubsisting on a high-fatdietla'38's6
One must carefuliyconsiderthe
Potentialdetrimentalheaithriskswhen recommendinga diet
tvith 600/oof total caloriesfrom lipid. This concernrnaypr.ove
unwarrantedfor athleteswith high levelsof daily energyex-
penditure.Increasingthe det'spercentage
oflipid caloriesto
50o/ofor physicallyactiveindividualswho maintain a stable
body weight doesnot adverselyaff-ectheart diseaserisk factors,includingplasmalipoproteinprofiles.e'48
Overall,availabl e researchdoes r)ot support the popul ar n ot ioir t hat
whiie increasingfat intake abovea
reducing carbol-rydrate
30% leveloptimizesthe metabolic"zone" for enduranceperConversely,significantrestrictionof dietary
formance.sO'es
fat intake below recomn.rended
levelsalso impairs endurance
exercise
Derforrnance.Js'o4'ns