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Nutr ition and Physical Ac tiv ity
Preexercise Feedings of Glucose, Fructose or Sucrose:Effects on
Substrate Depletion in Rats1'2ELIZABETH E. ADDIN GTON AND K
ATHARINE K. GRUNEW ALD3Dep artm en t o f F ood s an d N utritio n,
K an sa s S tate un iversity, M anha tta n, K S 66 506
ABSTRACT MaleWi st ar rats ( n = 72) t ra in ed t o runon a t re
adm illw e re f as ted ove rn igh t and f ed bygavage 3 m l
unsupplemen tedw ater or a solu tion con tainin g 2 g of glu cose,
fru ctose or sucrose 30 m in p rior t o exercise. S ix rats from
each dietary grou pw ere k illed a fter 0 , 1 o r 2 h o f exe rcise
. B lo od glu co se lev els d ecrea se d in all g ro up s o ver th
e 2-h ex ercise p erio d; h ow ev er, th ela rg es t d ec lin e in
b lo od g lu co se w as e xh ib ite d b y th e r ats fe d uns
upplemente d wate r (-3 2.1%), fo llowe d by th ose fe d fr uc to
se(-26.9% ), sucrose (-13.9% ) and glucose (-8.3% ). T he w
ater-fed con trol rats also had the largest increase in circulatin
gfr ee f atty a c id s ( + 2 15 .4%), fo llowe dby th os e fe d f
ru cto se ( + 12 0.8%), su cr ose ( + 6 9.2%) and g lu co se ( + 5
7.5%). T he fr uc to se -fe d anima ls e xh ib ite d th e g re ate
st d ep le tio n o f liv er g ly co ge n and th e sma lle st d ec
lin e in s ole us a nd r ed v astu s la te ra lis mu sc leg ly co
ge n o f th e r ats fe d th e d iffe re nt c ar bohydr ate s. T he
d ata in dic ate th at e xe rc is e-in du ce d c hang es in s ub
str ate le ve ls c an b em od ified b y th e ty pe of ca rb oh yd
rate g iv en p rio r to exe rcise . J . N utr. 1 17 : 5 93 -5 97 ,
19 87 .INDEXING KEY WORDS:g ly cogen exe rc is e carbohyd ra te
s
Human studies have shown that the major energysources used
during exercise are fatty acids, glucosefrom blood or muscle
glycogen and, to a lesser extent,am ino acids (1-5). The use of
these substrates is en-zym atically controlled and lim ited
according to the intensity (6) and duration (1, 7) of the exercise,
as well asthe subject's absorptive state (8) and previous level
oftraining (9, 10).Preexercise feedings m ay also affect substrate
utilization during exercise. G lucose ingestion prior to exercise
resulted in an increased rate of carbohydrate oxidation (11, 12),
stim ulatio n of gly cogen utilization (13)and rebound hypoglycem
ia (13-16). The effects of fructose feeding, however, were
different in that subjectsexhibited a slower rise in blood glucose
(13-16), lowerinsulin response (14-6)and less depletion of m
uscleglycogen (13, 14) than w hen glucose feeding w as tested.None
of the studies cited above compared the effectof sucrose to those
of glucose or fructose. Because sucrose consists of equimolar parts
of glucose and fructose and is readily available to exercising
subjects, itscom parison to other carbohydrates is warranted. In
addition, preexercise carbohydra te feeding s h ave not b
eenadequately ex am ined for their effects on liver gly cogen,which
is an im portant source of blood glucose.The present study was
designed to compare the ef
fects of preexercise feedings of glucose, fructose,, sucrose or
unsupplem ented water on glycogen contentsin the liver and several
selected muscles during a 2-hbout of exercise in trained rats. This
study shows thatthe changes in substrate levels that occur during
exercise can be m odified by the type of carbohydrate givenprior to
exercise and that those changes vary accordingto the tissue
studied.MATERIALS AND METHODS
S eve nty-tw o m ale w ean ling W istar rats (H SD : W I:BR,H ar
anSpragu e-D aw ley, In dia napo lis , IN ) w eig hin g 4 0-70 g
each were housed individually in stainless steelcages in a
temperature-controlled room (21 1C)maintained on a 12-h light-dark
cycle. The animals'T his s tu dy w as su pp orted b y the K an sas
A gric ultura l E xp erim en tS ta ti on , c on tr ib ut io n No. 8
6-4 00 -J .2A prelim inary report of these data w as presented at
the 1986 A n
nual M eeting of the F ederation of the American S ocieties for
E xperim ental Biology, St. Louis, M O. Addington, E. E . &
Grunewald,K . K . 11986| P reexercise feedings of glucose,
fructose, or sucrose:effects on fuel hom eostasis in rats. F ed. P
roc. 45: 972 (abs.).3T o w hom co rres po nd en ce sh ou ld be ad
dres sed .0022-3166/87 $3.00 1987 American Institute of N utrition.
R eceived: 28 A pril 1986. A ccepted: 4 N ovem ber 1986.
59 3
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59 4 A DD IN GTON A ND GRU NEWALDw ere fed a nonpurified diet (5
001 Rodent LaboratoryChow, Ralston Purina, St. Louis, MO) and water
adlibitum thro ughout the study .When the rats weighed
approximately 200 g, theybeg an a training pro gram on a zero -g
rade treadmill (Ra-diotrol Treadmill, 1/2 hp, Boston Gears, Quincy,
MA )for 2 w k prior to the planned day of killing. A progressiv e
ex ercise schedule w as fo llowed until animals co uldrun
comfortably at 18 rn/min for 30 min/d. At the endof the training
period, the rats w ere w eighed and fastedovernight (12-16 h). Rats
were not exercised the daybe fore the trial s.On the day of the
trials, the rats were given preex-ercise feedings by gastric lavage
30 min prior to exercise. The feeding solutions were 3 ml distilled
w atercontaining no additional carbohydrate (co ntrol) or 2 gof
glucose, fructose or sucrose, Thirty minutes afterreceiving the
feedings, animals were placed on thetreadmill and exercised at 18
m/min. This speed allowed all animals to comple te their desig
nated runningtimes without undue fatigue or injury. Within
eachdietary group, six rats were killed at each of the following
time intervals: prior to exercise (30 min afterfeeding) or after 1
or 2 h of exercise.A fter exercising for the allotted period of
time, ratsw ere anesthetized w ith sodium pentobarbital (50 mg/kg)
and 8-10 ml of blood was drawn via cardiac puncture, allowed to
clot and centrifuged at 5000 x g for12 min. Serum was frozen at -
18Cor later analysiso f g luc ose, free fatty acids and blo od urea
nitro gen. Thepyramidal lo be of the liver and right hindlimb
musclesincluding the soleus and red and w hite portions of thevas
tus late ralis were identifi ed as pre vious ly de sc ribed(17).
Great care was taken to isolate consistently the
same muscle tissue in each rat. Tissues were excisedas quickly
as possible, weighed to the nearest 0.001 g,frozen in liquid
nitrogen and stored at - 18Cuntilanalys is f or g lycogen.B lo od g
luco se was dete rmined by the g luco se o xidas emethod (Sigma Kit
No. 510, Sigma Chemical Co., St.Louis, MO). Muscle and liv er
glycogen w ere measuredby the phenol-sulfuric acid method for small
tissuesamples (1 8). B lo od free fatty acids (FFA ) w ere assay
edby a co lo rimetrie micromethod based on the fo rmatio nof FFA Cu
soaps (19). Blood urea nitrogen (BU N) wasmeasured by a
colorimetrie procedure (Sigma Kit N o.535, Sigma Chemical
Co.).Statistical differences w ere assessed by least significant
differences tests follo wing significant (P
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CA R BOHY D RA TE FEEDIN GS A N D EX ER CIS E 595f ed f ructose
( + 120.8% ), sucrose ( + 6 9.2% ) and glucose(+ 57.5%).BUN levels
w ere sim ilar 30 m in af ter feeding butincreased in all groups
during the ex ercise period. T hegreatest increase in B UN levels w
as m anifested by thefructose-fed rats ( + 41.9%), follow ed by
those f ed un-supplem ented w ater ( + 39.1% ), glucose ( + 16.1% )
andsucro se ( + 1.9% ).Liver and muscle glycogen. The effects of
preexer-cise feedings on liv er and m uscle gly cogen contents
areshow n in T able 2. T here w ere no dif ferences betw eengroups
in liv er gly cogen 30 m in af ter the f eedings. H owever, rats in
all groups exhibited a decline in liv er glycogen during the ex
ercise period. T he greatest declinein liv er gly cogen w as ex
hibited by the rats fed fructoseor unsupplem ented w ater, w here
nearly half of the initial gly cogen lev els had been depleted
during the 2-hrun. A sm aller decline in liv er gly cogen w as
noted forrats f ed g lucose or sucrose.Ef fects of ex ercise and
carbohy drate f eedings on glycogen contents in three dif ferent ty
pes of m uscle tissuew ere also ex am ined. N o consistent ef fects
on m usclegly cogen w ere noted in the w hite portion of the
vastuslateralis. How ever, m uscle gly cogen lev els w ere reduced
during exercise in the soleus and red portion ofthe v astus
lateralis m uscle tissues of rats in all dietarygroups. T he f
ructose-f ed rats ex hibited the sm allest decline in gly cogen
contents in those tissues.
DISCUSSIONT he ef fects of feeding v arious carbohy drates prior
toex ercise hav e been com pared in sev eral hum an studiesw ith
dif fering results. In tw o studies (13, 14) f ructose-fed subjects
exhibited a sm aller reduction in m usclegly cogen during ex ercise
than those fed glucose, but nodif ferences w ere observ ed betw een
those treatm ents ina third study (16). T he lack of agreem ent m
ight be attributed to dif ferences in ex perim ental protocol
andalso to the fact that dif ferent m uscles w ere studied.Only one
m uscle w as exam ined in each study .In our trial the ef fects of
preex ercise carbohy dratef eedings w ere determ ined on gly cogen
contents in threedif ferent ty pes of m uscle during exercise in
rats. T hem uscles studied w ere the soleus, w hich has predom
inantly red type I slow tw itch ox idativ e m uscle f ibers,and the
vastus lateralis, w hich has both red and w hiteportions consisting
prim arily of red ty pe H a f ast tw itchox idativ e gly coly tic
and w hite type lib fast tw itch gly -
coly tic f ibers, respectiv ely (21).W e found that rats run on
a treadm ill for 2 h ex hibiteda gly cogen reduction in m uscles
that contained predom inantly red f ibers, i.e., the soleus and red
portion of the w hite v astus m uscle. T hese observationsh av e b
een rep orted p rev io usly f or treadm ill-ex ercisedrats (2 2).W
e further found that the decline in m uscle gly cogen
TABL E 2E ffe ct of pr ee xe rcise ca rb oh ydr ate fee din gs o
n g ly co ge n co nten ts in live r, so le usand red and w hite
vastus lateralis m uscles in rats du ring 2 h of exercise1
TissueLiver,m g/g w ettControlFructoseSucroseGlucoseSoleus,
m g/g w ettControlFructoseSucroseGlucoseRed
v astus, m g/g w ettControlFructoseSucroseGlucoseW hite
v astus, m g/g w ettControlFructoseSucroseGlucose05.62
.99"6.13 .99"7.66 .99"7.04 .00'3.35 .36"b2.91 .36"3.97 .36'4.06
.37b7.57 .09"7.70 .11"9.67 .99"8.97 .13*4.50 .92"3.98 .93"4.59
.92"4.99 0.93"Hours
ofunning12.26
.00a3.71 .02"4.98 .02'b7.67 .99"2.25 .37"2.98 .37"b4.08 .37"3.51
.36bc6.31 .01"6.67 .15"8.40 .03"9.00 .003.71 .934.12 .95"6.15
.95"4.50 0.92"22.92
.00"3.17 .026.66 .03b4.97 .99"b2.43 .37"2.86 .37"3.01 .382.84
.36"5.90 .01"7.40 .025.48 .13"5.96 .99"2.61 .93"4.09 .95'b5.51
.96"b3.95 0.92bPercent
change-48
13.05-29.4
3.02+20.04-20.8'V alues are m eans S EMf or six rats. M eans w
ithin each group colum n not sharing a comm on superscript letter
are signif icantly dif ferent,P < 0.05. R ats w ere fed the
preexercise feedings 30 m in before tim e 0.
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59 6 A DDINGTON A ND GRUN EW A LDcould be m odif ied by the ty
pe of carbohy drate fed priorto ex ercise. T he f ructose-f ed rats
had a sm aller declinein soleus and red v astus gly cogen than
those f ed glucoseor sucrose, suggesting that f ructose f eeding is
associated w ith a sparing ef fect on gly cogen use in thosemu
scles d urin g ex ercise.T he sparing of m uscle gly cogen in the f
ructose-fedrats m ight be attributed to increased utiliz ation of
otheren erg y so urces d urin g ex ercise. L iv er g ly co gen an d
b lo odglucose lev els decreased m ore f or the f ructose-f ed
ratsthan f or those f ed the other carbohy drates, suggestingthat f
ructose f eeding prior to ex ercise has dif ferent effects on the
av ailability or utilization of those fuelsources.A lthough it is
dif f icult to explain the observ ed eff ects, the energy prov ided
by f ructose m ay hav e becom eavailable at a low er rate than that
from the other carbohy drates. Fructose disappears m ore slow ly
from therat intestine than m ost other m onosaccharides (23) andis
m etaboliz ed principally in the liv er (24) so that littleenters
the general circulation. Feeding studies haveshow n that f ructose
su ppresses liv er gly cogen sy nthesis(25) and increases gluco
neogenesis (2 6).T he fructose-f ed rats also had the greatest rise
incirculating FFA of the rats fed the dif f erent carbohydrates. T
hese f indings m ight be ex plained by the low erinsulin response
to f ructose than glucose f eedings (27).It is w ell k now n that
insulin suppresses the m obiliz ation of fatty acids from adipose
tissue, and m ore fattyacids m ay have been available for energy in
rats fedfructose prior to ex ercise. Hum an studies hav e show
nthat the uptak e of FFA by exercising m uscle is proportional to
serum concentration (1, 2).N ew sholm e and S tart (28) found that
serum FFA levels decreased f ollow ing an oral glucose load; how ev
er,w e did not observ e any dif ferences w hen com paringFFA lev
els of w ater-fed rats w ith those of rats f ed thedif ferent
carbohy drates 30 m in af ter feedings. It is possible that the
trained condition of our rats altered theirre sp on se to th e carb
oh yd rate f eed in gs. E xercise train in gin rats has been found
to increase lipoly sis during a fast(29), alter insulin sensitiv
ity (30, 31) and dim inish therate of gly cogen depletion during ex
ercise (32).T he ef fects of sucrose w ere com pared w ith those
ofglucose and fructose in our study . During the ex erciseperiod,
the sucrose-f ed rats ex hibited changes in serumglucose and FFA
interm ediate betw een those of rats fedglucose and f ructose. How
ever, the sucrose-fed ratstended to have the highest m easured lev
els of bloodglucose throughout the ex ercise period. T he reason
forthis f inding is not clear; in a prev ious study (33) ratsfed
2-g m eals containing sucrose had a greater rise inblood reducing
sugar lev els than rats fed sim ilar m ealsco ntain in g g lu co se
o r f ru cto se.Our inv estigation conf irm s m any prev ious hum
anand rodent studies show ing that ex ercise is accompanied by a
progressiv e decrease in blood glucose (1,34), liv er gly cogen
(22) and m uscle gly cogen (14, 22, 35)
and a concom itant rise in circulating FFA (1, 16, 34).B ut w e
also found that these changes could be m odif iedby the type of
carbohydrate giv en 30 m in prior to exercise. O f the
carbohydrates tested, f ructose w as associated w ith the greatest
sparing of m uscle gly cogen;and w e postulate that this m ay have
occurred at theex pense of alternativ e fuel sources such as liv er
glycogen and FFA .B ecause there is som e indication that endurance
m aybe enhanced by a greater av ailability of fatty acids (34,36),
future studies m ight be directed tow ard a com parison of the ef
fects of preex ercise carbohy drate f eedingso n th is cap acity
.
ACKNOWLEDGMENTSW e thank Dallas Johnson, Departm ent of S
tatistics,K ansas S tate Univ ersity , for his help w ith
statisticalprocedures and K az uk o S ak am oto and K om alam
Jariajf or their assistance during the ex ercise trials.
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