Oxidation of Solid versus Liquid CHO Sourcesduring Exercise

BEATE PFEEFFER'. TRENT STELLINGWERFF 2, ERIC ZALTAS 2, and ASKER E. JEUKENDRUP'

'School of Sport and Exercise Sciences. Universio, qf Birninghani, Edgbaston. Birmingham. England.UNITED KINGDOM; and 2Nestl6 Research Center, Lausanne. SWVI7ZERLA)VD

ABSTRACT

PFEIFFER. B., T. STELLINGWERTF, E. ZALTAS, and A. E. JEUKENDRUP. Oxidation of Solid versus Liquid CHO Sources duringExercise. Med. Sc. SponesExerc., Vol. 42, No. 11. pp. 2030-2037,2010. The ingestion of CHO solutions has been shown to increase CHOoxidation and improve endurance performance. However, most studies have investigated C0 in solution, and sporting practice includesingestion of CHO in solid (e.g., energy bars) as wsell as in liquid form. It remains unknoNvn whether CHO in solid form is as effectivelyoxidized as CHO in solution. Purpose: To investigate exogenous CHO oxidation from Cl-1O provided in either solid (BAR) or solution(DRINK) form during cycling. Methods: Eight well-trained subjects (age = 31 -i 7 yr, mass = 73 ± 5 kg, height = 1.79 ]- 0.05 m, -69 :E 6 mL-kg'-min-') cycled at 58% ± 4% VO2,pn for 180 min while receiMinr one of the followinc three treatments in randomizedorder:. BAR plus water, DRINK, or water. The BAR and DRINK was delivered glucose + fructose (GLU + FRC) in a ratio of2:1 at a rate of1.55 g'min-', and fluid intake was matched between treatments. Results: During the final 2 h of exercise, overall mean exogenous CHOoxidation rate was -0.11 g-min-1 lower in BAR (95% confidence interval"= - 0.27 to 0.05 g-min- , P = 0.19) relative to DRINK, whereasexogenous CHO oxidation rates were 15% lower in BAR (P < 0.05) at 120, 135, and 150 min of exercise. Peak exogenous CHO oxidationrates were high in both conditions (BAR 1.25 ± 0.15 g-min-I and DRINK 1.34 L 0.27 g-min-1) but were not significantly different(P = 0.36) between treatments (mean difMerence = -0.9 --min-'. 95% confidence interval = -032 to 0.13 grin-). Conclusions:The present study demonstrates that a GLU + FRC mix administered as a solid BAR during cycling can lead to high mean and peakexogenous CHO oxidation rates (>1 g-miin-). The GLU + FRC mix ingested in the form of a solid BAR resulted in similar mean andpeak exogenous CHO oxidation rates and showed similar oxidation efficiencies as a DR-NK. These findings suggest that CHO from asolid BAR is effectively oxidized during exercise and can be a practical form of supplementation alongside other forms of CHO.Key Words: CHO INGESTION, SOLID AND LIQUID CHO, EXOGENOUS CR0 OXIDATION, CYCLING

1 ompetitive cyclists and triathletes routinely consumeCHO in different forms such as sports drinks, energybars, and CHO gels. The intake of CHO has been

shown to delay the onset of fatigue and improve endurancecapacity (4,6,12,23,26,27). However, the effect of differentforms of CHO intake (such as solids or solutions) on me-tabolism and performance is not fully understood.

The efficacy of CHOs in improving endurance perfor-mance is due, in part, to their capacity to be oxidized by theworking muscle. A mixture of glucose (GLU) and fruc-tose (FRC) has been shown to be oxidized at 20%7-50%higher rates than as GLU alone when ingested at high rates(1.5-2.4 g-min-') (15,16). Recently, it was also demon-strated that a similar GLU + FRC drink (1.8 g'min-1)

Address for correspondence: Asker E. Jeukendrup, Ph.D.. School of Sportand Exercise Sciences, University of Birmingham, Birmingham, EnglandB 15 2TT, United Kingdom; E-mail: A.E.Jeukendrup bharmac.uk.Submitted for publication April 2009.Accepted for publication March 2010.

0195-9131/10/4211-2030/0MEDICINE & SCIENCE IN SPORTS & EXERCISF4.Copyright © 2010 by the American College of Sports Medicine

DOI: 10.1249/MSS.0b0I3e3I8le0efc9

resulted in superior endurance performance compared withan isocaloric GLU drink (8,39).

However, these high exogenous CHO oxidation ratesduring exercise only seem to occur with high rates of CHOintake (>1.5 g-min-') (14). In the studies previously men-tioned, this was done by administering CHO solutions>10%. Although available data concerning athletes' actualCHO intake during competitions are limited, the use of suchconcentrated drinks seems not to be common practice.Sporting practice includes ingestion of CHO in solid (e,g..energy bar), liquid (e.g., sports drink), and gel forms, and amore varied intake of CHO sources seems to be a moreconvenient way for athletes to ingest CHO in large amounts.However, whether it is possible to extrapolate the positiveexogenous CHO oxidation and performance findings fromdrinks to other forms of CHO, intake such as the ingestionof a solid food, is not yet clear.

To date, only a few studies have investigated the effect ofsolid CHO or solid CHO-rich food on metabolism duringexercise and endurance performiance. Studies have shownincreased performance with the ingestion of solid CHOcompared with a placebo (3,10,13) and similar performanceimprovements when the same amount of CHO was ingestedin the form of solids compared with liquids (3,9,12,26). Incontrast, the study by Rauch et al. (32) reported enhanced fat

2030

metabolism and impaired endurance performance with aCHO bar compared with a drink. However, the CHO bar inthis study delivered only -29 g of CHO per hour comparedwith a delivery rate of 70 g of CHO per hour with the drink.Whether the same amount of CHO ingested in solid form hasthe same effect on CHO and fat metabolism and especiallyexogenous CHO oxidation during exercise is less clear (22,33).

To the best of our knowledge, CHO oxidation rates andefficiency from the ingestion of solid food have never beenexamined. It is well established that solid food is emptiedslower from the stomach compared with liquids while atrest. This delayed gastric emptying with solid food is be-cause of the increased particle size (13,40) as well as fatand fiber (10,11,37,40). Thus, it can be hypothesized that asolid CHO source could be oxidized at lower rates comparedwith liquid CHO sources. We therefore set out to study theexogenous CHO oxidation of a CHO mixture (GLU/FRC ina ratio of 2:1) given in the form of an energy bar (BAR)combined with plain water or a CHO solution (DRINK).

METHODS

Subjects. Eight well-trained male endurance cyclists/triathletes (age -31 ± 7 yr, mass = 73 ± 5 kg, height = 1.79 ±0.05 m, VO2m,, = 69 ± 6 mL-kg-'-min-1) volunteered toparticipate in this study. Subjects trained at least three timesa week for more than 2 h-d-1 and had been involved inendurance training for at least 2 yr. All subjects were healthyas assessed by a general health questionnaire. Exclusioncriteria for the study were a diagnosis of metabolic or in-testinal disorders, smoking, associated cycling injuries,regular consumption of medication, and donation of bloodin the previous 3 wk. All subjects were informed of thepurpose, practical details, and risks associated with theprocedures before giving their written informed consent toparticipate. The study was approved by the School of Sportand Exercise Sciences ethics subcommittee, University ofBirmingham, Birmingham, UK.

Preliminary testing. At least 1 wk before the start ofthe experimental trials, an incremental cycle test to volitionalexhaustion was performed to determine maximal poweroutput (Wma,) and maximal oxygen consumption (VQO2mx)on an electromagnetically braked cycle ergometer (LodeExcalibur Sport, Groningen, The Netherlands). On arrival atthe laboratory, body mass (Seca Alpha, Hamburg, Germany)and height were recorded. Subjects then started cycling for3 min at 95 W, followed by incremental steps of 35 W every3 min until exhaustion. Wam, was determined by the fol-lowing formula: Wmax = Wout + [(t/180)35], where Wo,, isthe power output (W) during the last completed stage and t isthe time (s) in the final stage. HR was recorded continuouslyby a radiotelemetry HR monitor (625X; Polar, Kempele,Finland). Wmn values were used to determine the 50%Wm,ax, which was later used in the experimental trials. Res-piratory gas measurements were obtained using the Douglasbag technique. Douglas bags were collected for 1 min during

the final stage and were analyzed using a gas analyzer(1400; Servomex, Crowborough, Sussex, England).

Experimental design. Each subject completed threeexercise trials that consisted of 180 min of cycling at 50%Wmax while ingesting 1.55 g of CHO-min-i (GLU and FRCin the ratio of 2:1) in the form of a 10.75% maltodextrin plusFRC drink (DRINK), or an isocarbohydrate energy bar plusplain water (BAR), or plain water (WAT). The order of thetrials was randomly assigned and separated by at least 5 d.

Experimental treatments. Because the BAR con-tained not only CHO but also fat and protein, the CHOtreatments were not isocaloric (Table IA). The BAR alsoconsisted of different CHO (Table IB). CHO consisted of0.67 of GLU and GLU polymers and 0.33 of FRC (per BAR,5 g as crystalline FRC and 9 g contained in cane syrup). Toquantify exogenous CHO oxidation, com-derived maltodex-trin (GLUCIDEX 19; Roquette, Lestrem, France) and FRC(Krystar 300; AE Stanley Manufacturing Co., Decatur, IL),which have a high natural abundance of 13C (-11.228%oand - 10.725%o vs Pee Dee Bellemnitella, respectively), wereused for the preparation of DRINK. To ensure a similaramount of sodium in the BAR and in the DRINK, sodiumin the form of sodium chloride was added to the DRINK(500 mg of sodium per liter). The BAR consisted predomi-nantly of ingredients with a high natural abundance of 13C(72%). Enrichment of all CHO sources of the BAR is shownin Table lB. The 13C enrichment of the ingested CHO wasdetermined by elemental analyzer-isotope ratio mass spec-trometry (EA-IRMS; Europa Scientific GEO 20-20, Crewe, UK).

Diet and activity before testing. Subjects were askedto record their food intake and activity patterns for 24 h be-fore the first exercise trial and were then instructed to followthe same diet and activities before the next two trials. Theinstructions also contained advice to follow a diet rich inCHO. It was made sure that CHO intake was >4 g.kg- 1

body weight, which, in combination with light training orrest, would allow participants to start the trials with adequatemuscle glycogen stores. Diet was assessed with 24-h recallsthe days before the rest of the trials, and CHO intake ofall participants was confirmed to be above 4 g.kg-1 bodyweight. In addition, subjects were instructed to refrain fromstrenuous exercise and drinking any alcohol in the 24 h be-fore the exercise trials. Furthermore, subjects were instructedto perform an intense training session ("glycogen-depletingexercise bout") 3-7 d before each experimental trial in anattempt to reduce any 13C-enriched glycogen stores. Sub-jects were also instructed not to consume products with ahigh natural abundance of 13C: CHO derived from C4 plants

TABLE IA. Nutrition facts for both CHO treatments.

Per Bar (65 g) Per 400 mL of CHO Drink

Energy (kcal) 230 172Protein (g) 8CHO (g) 43 43Fat (g) 2Fiber (g) 2Sodium (mg) 200 200

CHO OXIDATION FROM SOLID VS LIQUID

MR*0

all.

Medicine & Science in Sports & Exercisea 2031

TABLE lB. Enrichment of different CHO sources of the BAR.

Mean 6'3 CV-PBD ('4 % CHO

High enriched CHO Cane syrupMaltodextrinFRO -11-05 72

Low enriched CHO Crisp riceOat branBrown rice flour -27.15 28'

"5% starch.

(maize and sugarcane) had to be avoided at least I wk beforeeach experimental trial to reduce the background shift(change in 13 C) from endogenous substrate stores.

Protocol. The subjects arrived in the laboratory in themorning (between 6:00 and 9:00 a.m.) after an overnight fast(10-12 h). All experimental trials were performed at thesame time of the day to avoid circadian variance. On arrival,subjects were weighed before a 20-gauge Teflon catheter(Venflon; BD, Plymouth, UK) was inserted into an ante-cubital vein of an arm and attached to a three-way stopcock(Sims Portex, Kingsmead, UK) to allow for repeated bloodsampling during exercise. The cannula was kept patent byflushing with 1.0-1.5 mL of isotonic saline (0.9%; BD) aftereach blood sample collection. The subjects then mounted acycle ergometer, and a resting breath sample was collectedin a 10-niL tube (Exetainer; Labco Ltd., Brow Works,High Wycombe, UK), which was filled directly from amixing chamber to determine the 13 C/12 C ratio in the ex-pired air. A resting blood sample (10 mL) was collected andstored on ice until centrifugation. Subjects then started a180-min exercise bout at a work rate equivalent to 50%WVmax (58% ± 4%VO 2 MaJ,. Additional blood samples weredrawn at 15-min intervals until the cessation of exercise. Atthe same 15-min intervals, expiratory breath samples werealso collected. During the first 2 min, expired air was sam-pled into Douglas bags. Douglas bag samples were analyzedas described above, and oxygen consumption (VO2), carbondioxide production (-VCO 2), and RER were determined.Within the last 60 s of each 3-min period, Exetainertubes were filled in duplicate for breath 13C/12C ratio asdescribed above.

During the first 2-3 rmin of exercise, subjects ingestedan initial bolus of one of the three experimental treatments:400 mL of water (WAT), 400 mL of water plus one energybar (BAR), 400 mL of 10.75% CHO drink (DRINK).Thereafter, a beverage volume of 200 mL and 32.5 g of BARwas provided every 15 min. The total fluid intake during theexercise bout was 2.6 L (867 mL-h-1), whereas the totalCHO intake was 280 g (93 g-h-1). All exercise tests wereperformed under normal and standard environmental con-ditions (16'C-24°C dry bulb temperature and 50oo-60%relative humidity). During the exercise trials, subjects werecooled with standing floor fans to minimize thermal stress.

Questionnaires. Every 30 min during the exercisebout, subjects were requested to verbally answer a shortquestionnaire to directly assess gastrointestinal (G1) toler-ance. GI symptoms were scored on a 10-point scale (0 = noproblem at all and 9 = the worst it has ever been). A score >4

was registered as serious. RPE were collected using a 6- to20-point Borg scale (1).

Analyses. All blood samples were collected into pre-chilled test tubes containing EDTA and centrifuged at 2300gfor 10 min at 4°C. Aliquots of the plasma were frozenand stored at -25°C until further analysis. Plasma sampleswere analyzed enzymatically for GLU (Glucose HIK.; ABXDiagmostics, UK), lactate (ABX Diagnostics, Shefford. UK),and free fatty acid (FFA. NEFA-C Kit; Alpha Laboratories,Eastleigh, Hampshire, UK) concentrations on a semiautomaticanalyzer (Cobas Mira S-Plus; ABX Diagnostics). Insulin wasanalyzed by ELISA (DRG Ultrasensitive Insulin ELISA;DRG Instruments GmbH, Marburg. Germany). Breath sam-ples were analyzed for "3C/'C ratio by continuous-flowIRIvIS (GC, Trace GC Ultra; IRMS, Delta Plus XP; bothThermo Finnigan. HeFrts, UK). From indirect calorimetry(-VO 2 and VCO2 ) and stable isotope measurements (breathI3C/T2 C ratio), rates of total fat, total CHO, and exogenousCHO oxidation were calculated.

Calculations. From VO, and -TCO 2 (L-min-'), CHOand fat oxidation rates (g'min-') were calculated usingstoichiometric equations (19), with the assumption thatprotein oxidation during exercise was negligible.

CHO oxidation = 4.21 VCO, + 2.962 1v02 [IIfat oxidation = 1.695VO2 + 1.701 VCO 2 [2]

The isotopic enrichment was expressed as 8 per mil dif-ference between the 13C/12C ratio of the sample and a knownlaboratory reference standard according to the formula ofCraig (7):

j11C = [[mpe - 1] x 102per rail

The 81"C was then related to an international standard(Pee Dee Bellemnitella).

In the CHO trials, the rate of exogenous CHO oxidationwas calculated using the following formula (31):

(5Exp - Exp,g'\ (1•exogenous CHO oxidation = IVCO 2 -Exp,k ,~

in which 8Exp is the '3 C enrichment of expired air duringexercise at different time points, 85ng is the 13C enrichmentof the ingested CHO solution, 8 Expbk, is the 13C enrichmentof expired air in the WAT trial (background) at differenttime points, and k is the amount of CO 2 (L) produced by theoxidation of I g of GLU (k = 0.7467 L of CO2 "g-I GLU).

To determine the enrichment of the BAR, the mean valueof the enrichments of all CHO sources according to theircontent in the BAR was calculated first. The exogenousCHO oxidation calculated with this approach would assumethat all CHO are oxidized at the same rate and lead to anoverestimation (maximum exogenous CHO oxidation). Thelimitation of this approach is that 28% of CHO in the BARshow low 3-C enrichment. Approximately 18% of the low

2032 Official Journal of the American College of Sports Medicine http://wwrvw.acsrn-rnsse.org

13 C-CHO (-5% of total CHO) consists of starch (Table 1B).Because insoluble starch (24% amylose and 76% amylo-pectin) has been shown to be oxidized at (approximately30%) lower rates as GLU (36), the small amounts of starchcould lead to an overestimation of exogenous CHO oxida-tion of -3%. To account for the possible overestimation,a second enrichment value (minimum exogenous CHO ox-idation) was used, assuming that starch was not oxidized atall. Both enrichment values were then averaged and used forcalculations (enrichment= -15.2).

Endogenous CHO oxidation was calculated by subtract-ing exogenous CHO oxidation from total CHO oxidation. Amethodological consideration when using 13C0 2 in expired

air to calculate exogenous substrate oxidation is the tempo-rary fixing of 13 C0 2 in the bicarbonate pool, in which anamount of CO 2 arising from CHO and fat oxidation is re-tained (34). However, during exercise, the turnover of thispool increases several fold so that a physiological steady-state condition will occur relatively rapidly and 13C0 2 in theexpired air will be equilibrated with the 13 C02 /HI1

3C0 2 pool,respectively. Recovery of 13 CO2 from oxidation will ap-proach 100% after 60 min of exercise when the dilution inthe bicarbonate pool becomes negligible (30,34). As a con-sequence of this, all calculations on substrate oxidation wereperformed during the last 120 min of exercise (60-180 min).The oxidation efficiency was determined as the percentageof the ingested CHO that was oxidized and was calculatedby dividing exogenous CHO oxidation rate by the CHO

ingestion rate and then multiplying it by 100.Statistical analyses. A two-way ANOVA (treatment x

time) for repeated measures was used to compare differencesin substrate utilization and in blood metabolites among thethree trials. A Tukey post hoc test was applied where a

significant F-ratio was detected. Mean values were calcu-lated without the use of a statistical model that includes allsampling variance. Paired-sample t-tests were applied whentwo mean values were compared.

Values in text, tables, and figure are presented asmean ± SD. Differences between treatments are presented

as mean differences with 95% confidence interval. Statisti-cal significance was set at P < 0.05. All statistical analyseswere performed using SPSS 15 for Windows (SPSS, Inc.,Chicago, IL).

RESULTS

VO2, RER, total CHO, and fat oxidation. Vo 2 , RER,total CHO, and fat oxidation rates during the 60- to 180-minexercise period are shown in Table 2. There was no signif-icant difference in V02 among the three experimental trials.RER was significantly (P < 0.01) lower in the WAT trialcompared with the two CHO ingestion trials, but it was notsignificantly different between the BAR and DRINK trials.Correspondingly, CHO oxidation was not significantly dif-ferent between the two CHO trials, but it was significantlyhigher compared with WAT (P < 0.01). The mean totalCHO oxidation rates during the last 120 min of exercisewere 1.64 ± 0.56, 2.26 ± 0.31, and 2.22 ± 0.43 g'min-1 forWAT, BAR, and DRINK, respectively. Fat oxidation wassignificantly higher in the WAT trial than after CHO in-gestion (P < 0.01). Mean fat oxidation rates during thelast 120 min of exercise were 0.89 ± 0.29, 0.58 ± 0.21, and0.57 ± 0.22 g'min-1 for WAT, BAR, and DRINK, respec-tively. Fat oxidation did not differ significantly betweenthe BAR and DRINK trials.

Exogenous CHO oxidation, endogenous CHOoxidation, and oxidation efficiency. Exogenous CHOoxidation rates gradually increased over time and leveledoff after 135 min with the ingestion of the DRINK (Fig. 1).

With the BAR, a plateau was reached after 75 min followedby a second significant increase detected after 150 minof exercise. Both treatments lead to high peak exogenousCHO oxidation rates (BAR 1.25 ± 0.15 g.min-' andDRINK 1.34 ± 0.27 g'min-1), which were not significantly(P = 0.36) different between treatments (mean difference =-0.9 g.min-1, 95% confidence interval = -0.32 to0.13 g'min- 1). The total amount of the ingested CHO thatwas oxidized during the entire 180-min exercise was not

TABLE 2. Oxygen uptake (V02), RER, total CHO oxidation (CHOtotaj), total fat oxidation (Fat It.1), endogenous CHO oxidation, and exogenous CHO oxidation with ingestion of WAT, BAR,

and DRINK.

Trial Time (min) V02 (L-min-') RER CHOt.at, (g-min-1) Fatt.t., (g-min-1) Endogenous CHO (g.min-1) Exogenous CHO (g.min-1)

WAT 60-90 2.91 t 0.20 0.83 ± 0.05 1.77 ± 0.53 0.81 ± 0.26 1.77 - 0.5390-120 2.98 ± 0.22 0.82 ± 0.06 1.64 _i 0.62 0.90 ± 0.30 1.64 - 0.62

120-150 3.02 ± 0.24 0.82 ± 0.06 1.61 ± 0.60 0.93 ± 0.30 1.61 - 0.60150-180 3.00 ± 0.23 0.81 ± 0.06 1.54±• 0.60 0.95 ± 0.31 1.54 0.60

60-180 2.97 ± 0.22 0.82 ± 0.05 1.64 ± 0.58 0.89 ± 0.29 1.64 ± 0.50BAR 60-90 2.95 ± 0.24 0.88 + 0 .0 3 b 2.23 ± 0.25b 0.57 . 0.19b 1.33 ± 0.27b 0.90 ± 0.12

90-120 2.98 •± 0.24 0.88 ± 0.03b 2.24 ± 0 .3 1b 0.58 ± 0.19b 1.26 ± 0.31b 0.98 ± 0.15

120-150 2.99 : 0.25 0.88 ± 0.03 b 2.25 ± 0.32' 0.58 ± 0,21b 1.21 0.28ab 1.04 ± 0.13

150-180 3.03 ± 0.23 0 ,88 ± 0.04 b 2.27 ± 0.36b 0.59 ± 0.23b 1.09 0.31b 1.17 ± 0.11

60-180 2.99 ± 0.24 0.88 ± 0.03b 2.26 ± 0.31b 0.58 ± 0.21b 1.23 0.29 b 1.03 ± 0.11

DRINK 60-90 2.91 ± 0.21 0.89 ± 0 .0 4b 2.22 ± 0.46b 0.55 ± 0.23b 1.28 ±0.38b 0.96 ± 0.13

90-120 2.93 ± 0.22 0.89 ± 0.04b 2.24 ± 0.41b 0.56 ± 0.22b 1.14 ±035" 1.12 ± 0.13

120-150 2.95 ± 0.23 0.88 ± 0.04b 2.23 ± 0.40" 0.57 - 0.21b 1.01 ± 0.31b 1.24 ± 0.16

150-180 2.95 ± 0.24 0.88 k 0.04b 2.19 ± 0.47b 0.59 ± 0.22b 0.88 ± 0.33b 1.30 ± 0.22

60-180 2.94 ± 0.22 0.88 ± 0.04b 2.22 ± 0,43b 0.57 ± 0.22b 1.12 ±0.34b 1.14 ± 0.16

Values are means ± SD."a Significant difference between CHO treatments.b Significantly different from WAT.

Medicine & Science in Sports & Exercise® 2033CHO OXIDATION FROM SOLID VS LIQUID

S 1.2-

Lo) -0

S0.80

o 0.6-

OK 0.4-04 -- 0- BAR

0.2 - DRINK

0.0V0 15 30 45 60 75 00 105 120 135 150 165 I00

Time (min)

FIGURE 1-Exogenous CHO oxidation (g-min-1) during exercise iithingestion of BAR and DRINK. 'Exogenous CHO oxidation in DRINKtrial significantly greater than that in BAR trial (P < 0.05).

significantly less with the BAR compared with the DRINK(-16.0 g, 95% confidence interval = -42.4 to 10.2 g,P - 0.19). Consequently, the difference in oxidation effi-ciency was not significant between the BAR and DRINKtrials (-7.1%, confidence limit ±10.5%, P = 0.15).

A treatment x time effect was detected between the CHOtrials (P = 0.001). Significantly higher exogenous CHOoxidation rates from the DRINK compared with the BARwere detected at three intermediate time points (120, 135,and 150 min). However, the mean exogenous CHO oxida-tion rates during the final 120 min of exercise were high forboth treatments (1.03 ± 0.11 and 1.14 ± 0.16 g-min-' forBAR and DRINK, respectively) and not significantly dif-ferent (-0.11 g.min-1, 95% confidence interval= -0.27 to0.05 g.min-', P = 0.19; Table 2).

Along with higher exogenous CHO oxidation rates be-tween 120 and 150 min, endogenous CHO oxidation rateswith the DRINK ingestion were significantly lower duringthis period as well. However, no statistically significantdifferences were found in mean endogenous CHO oxida-tion rates during the last 120 min of exercise between theBAR and the DRINK trials (0.11 g-min-1, 95% confidenceinterval = -0.09 to 0.32 g.min-1). Compared with the WATtrial, endogenous CHO oxidation was significantly lowerin both forms of CHO ingestion during the last 120 minof exercise (P = 0.02; Table 2).

Plasma metabolites. Plasma GLU and insulin con-centrations at rest and during 180 min of exercise are shownin Figures 2 and 3, respectively. Resting plasma GLU con-centrations before the onset of exercise were not signifi-cantly different among trials. During exercise in the WATtrial, plasma GLU concentration gradually declined from5.12 ± 0.67 to 4.27 ± 0.76 mmol-L-1. However, plasmaGLU concentrations were maintained throughout the exer-cise period in the BAR and DRINK trials and were signifi-cantly greater than the WAT trial during the last I h ofexercise. Significantly greater plasma GLU concentrations

a

00 4

a.

Tim 1110Timc (minin

FIGURE 2-Plasma GLU concentration during exercise with ingestionof NVAT, BAR, and DRINK. 'Plasma GLU concentration in DRINKtrial significantly greater than that in BAR trial. bPlasma GLU con-centration in BAR trial significantly greater than that in WAT trial.'Plasma GLU concentration in DRINK significantly greater than thatin NVAT.

during the DRINK trial compared with the BAR trial werefound after 45 and 75 min, respectively (P = 0.03 andP = 0.004. respectively).

Resting plasma insulin concentrations were similar be-tween treatments. Plasma insulin concentration increasedsignificantly in both CHO trials, reaching peak values (19.6+ 6.8 and 19.6 ± 8.8 mmol-L-i for BAR and DRINK, re-"spectively) after 30 min, followed by a decline over time.During both CHO trials, insulin concentrations were signif-icantly higher than during the WAT trial at all exercise timepoints. There was no significant difference in plasma insulinconcentrations between the two different forms of CHOintake.

Plasma lactate concentrations were not significantlydifferent at rest (1.01 ± 0.33, 0.94 + 0.17, and 0.98 ±0.28 mmol-L-1 for WAT, BAR, and DRINK, respectively)

31-

3-

:1 L _ --0--BARSDRINKC, 15 30 45 60 75 90 105 120 135 150 165 ISO

Time (min)

FIGURE 3-Plasma insulin concentration during exercise with inges-tion of WAT, BAR, and DRINK. bPiasma insulin concentration in BARtrial was significantly greater than that in NVAT trial. 'Plasma insu-lin concentration in DRINK trial wýas significantly greater than thatin NVAT.

2034 Official Journal of the American College of Sports Medicine http:/wýww.acsm-mnsse.org

8-

0

.4

ab

6- ab

lC C

MINI!WATm BAR

Hour of Exercise

FIGURE 4-NMean perceived stomach fullness during the first, second,and third hours of exercise. 'Alean perceived stomach fullness in BARtrial significantly greater than that in DRINK trial. bMean perceivedstomach fullness in BAR trial significantly greater than that in WATtrial. 'Mean perceived stomach fullness in DRINK trial significantlygreater than that in WAT trial.

and increased significantly in the first 15 min during all threetrials (P = 0.02). In the first hour of exercise, plasma lactateconcentrations were higher within the CHO trials than withthe WAT trial, reaching statistical significance for theDRINK at 45 and 60 min (P = 0.04 and P - 0.02, respec-tively) and for the BAR at 45 min of exercise (P = 0.04).

Plasma FFA concentrations were not significantly differ-ent before the onset of exercise (244 ± 118, 256 ± 169, and234 ± 168 immol.L-1 for WAT, BAR, and DRINK, re-spectively). Concentrations of FFA in plasma increasedduring the WAT trial and were significantly higher thanduring the CHO trials at all time points. No significant dif-ferences occurred between the CHO trials or over timeduring exercise.

GI symptoms, perceived fullness, and RPE. Nosevere GI problems were recorded in any of the trials. Meanperceived stomach fullness (Fig. 4) during each hour wassignificantly higher with the ingestion of DRINK than withWAT and was significantly higher with the BAR than withboth other treatments (P < 0.05). RPE during the last halfhour of exercise were 13 ± 2, 12 ± 2, and 12 ± I in the WAT,BAR, and DRINK trials, respectively.

DISCUSSION

The present study demonstrates that a GLU + FRC mixadministered either as a solid BAR or as DRINK is oxidizedefficiently and leads to high peak and mean exogenous CHOoxidation rates during exercise. Previously, it was thoughtthat despite high intake rates (>1.5 g-min- 1) of single-sourceCHO drinks, exogenous CHO oxidation during exercise waslimited to peak oxidation rates of -1 g-min- 1 (20). In con-trast, when drinks containing multiple transportable CHOare administered, 20%-50% higher oxidation rates withpeak values of up to 1.75 g-min- 1 have been reported

•h

CHO OXIDATION FROM SOLID VS LIQUID Mudicine & Science in Sports & Exercises 2035

(15,16). In the present study, administration of both forms ofGLU + FRC (BAR and DRINK) resulted in exogenousCHO oxidation rates greater than I g-min- 1 (1.25 ± 0.05 vs1.34 ± 0.09 g-min-1, respectively).

To the best of our knowledge, this is the first time that theoxidation of a solid CHO food has been studied during ex-ercise. Technical difficulties are probably the explanation forthe lack of studies that measured CHO oxidation from awhole food. Food naturally consists of several CHO andoften other ingredients with different natural enrichments in13C, making the calculation of exogenous CHO oxidationvery complex. As described earlier, the BAR used in thecurrent study contained -5% starch, which has been shownto be oxidized at approximately 30% lower rates as GLU(36) and would have resulted in a possible overestimation ofexogenous CHO oxidation rates of -3%. This was avoidedby using an average enrichment, which assumed that only50% starch was oxidized. Furthermore, it has repeatedlybeen documented that endogenous protein oxidation canaccount for 1%-6% of the energy yield during enduranceexercise (38). Endogenous protein oxidation would havebeen reduced by CHO and protein feeding. On the otherhand, protein from the BAR could have contributed to pro-tein oxidation to a small extent. However, error in total CHOand fat oxidation would have been small and with a minimalamount of protein ingested similar in both conditions. Fur-thermore, the protein-containing sources in the BAR showeda low 13C enrichment (-27.2), and an overestimation ofexogenous CHO oxidation through oxidation of 13C fromprotein is therefore not possible.

With the ingestion of CHO in the form of a solid BAR, weexpected that peak exogenous CHO oxidation and oxidationefficiency might be lower than the values obtained from aDRINK because of the increased particle size, fat, and fibercontent, which cause slower gastric emptying comparedwith liquids (10,11,13,37,40). Slower gastric emptying rateswould lead to decreased oxidation efficiency and lower ox-idation rates. However, the difference in oxidation efficiencybetween BAR and DRINK was only -7% and was notstatistically significant. Differences in peak oxidation rateswere also not significantly different between both treat-ments. These results suggest that gastric emptying rates werenot substantially influenced by the form of CHO intake. Thismight be due to the relatively low fat and fiber content (both3%) of the BAR. Gastric emptying might also have beensupported by the fluid volume administered in the study(867 mL-h-1). Gastric volume and moderate-intensity ex-ercise are potent factors to enhance gastric emptying(2,5,25,28,29). Furthermore, the bolus of water administeredat the onset of exercise with the BAR could have helped todeliver much CHO to the intestine, which can explain therapid increase in oxidation rates.

The rapid increase in exogenous CHO oxidation rateswith both treatments goes in line with a similarly quick risein plasma GLU and insulin concentrations with both treat-ments. This is in agreement with previous studies, which

CHO OXIDATION FROM SOLID VS LIQUIDMedicine & Science in Sports & Exercises 2035

demonstrated a similar GLU and insulin response from solidversus liquid at the onset of exercise (2 1,33). A - 15% lower

oxidation rate for the BAR compared with the DRINK wasdetected between 120 and 150 min of exercise. These find-

ings are supported by the study of Robergs et al. (33), which

found significantly lower blood GLU concentrations from a

bar at one time point (80 min). Both of these studies show a

more phasic time course for CHO oxidation and blood GLU

from solids compared with liquids, respectively. In the cur-

rent study, after lower oxidation rates from the BAR be-

tween 2 and 2.5 h, oxidation rates rise to values >1 g-min-.which were not significantly different compared with the

DRINK. This pattern is evident not only in the average data

but also in individual oxidation rates from seven of eightparticipants, a phenomenon that was not seen in data fromdrinks.

Whether the observed lower exogenous CHO oxidationrates between 2 and 2.5 h could lead to a relevant differencein performance is difficult to answer now. The exact rela-

tionship between increased exogenous CHO oxidation and

exercise performance is still unclear. Dose-response studies

in which the effect of ingestion of different amounts of CHOon performance was investigated have resulted in equivocalfindings (for review, see Jeukendrup [17]). Although, inthese studies, exogenous CHO oxidation was not measured.it can be assumed that oxidation increased with increasingintake (in the range used in these studies) (17). This in-creasing intake, however, was not always linked to im-

proved performance (23,24). No studies have directlyinvestigated the link between increasing exogenous CHOoxidation and performance. Because dose responses havenot delivered a clear picture, it is likely that substantial

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CHO OXIDATION FROM SOLID VS LIQUIDMedicine & Science in Sports & Exercises 2037

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Oxidation of Solid versus Liquid CHO Sources during Exercise

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