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BRIEF COMMUNICATION / COMMUNICATION BRE`VE
Improved endurance capacity following chocolate milk consumption compared with 2 commercially available sport drinks Kevin Thomas, Penelope Morris, and Emma Stevenson
Abstract: This study examined the effects of 3 recovery drinks on endurance performance following glycogen-depleting exercise. Nine trained male cyclists performed 3 experimental trials, in a randomized counter-balanced order, consisting of a glycogen-depleting trial, a 4-h recovery period, and a cycle to exhaustion at 70% power at maximal oxygen uptake. At 0 and 2 h into the recovery period, participants consumed chocolate milk (CM), a carbohydrate replacement drink (CR), or a fluid replacement drink (FR). Participants cycled 51% and 43% longer after ingesting CM (32 ± 11 min) than after ingesting CR (21 ± 8 min) or FR (23 ± 8 min). CM is an effective recovery aid after prolonged endurance exercise for subsequent exercise at low-moderate intensities. Key words: sports drinks, glycogen resynthesis, protein, performance, milk. Re´sume´ : Cette e´tude analyse les effets de 3 boissons de re´cupe´ration sur la performance en endurance a` la suite d’un exercice ayant e´puise´ les re´serves de glycoge`ne. Neuf cyclistes masculins entraıˆne´s participent a` 3 se´ances expe´rimentales selon un ordre ale´atoire contrebalance´ et consistant en un exercice d’e´puisement des re´serves de glycoge`ne suivi d’une re´cupe´ration d’une dure´e de 4 h et d’une e´preuve mene´e a` 70 % de la Pmax jusqu’a` e´puisement sur une bicyclette. Au de´but de la pe´riode de re´cupe´ration et deux heures plus tard, les sujets boivent du lait au chocolat (« CM ») ou une boisson de re´hydratation sucre´e (« CR ») ou une boisson de re´hydratation (« FR »). Comparativement aux dure´es des efforts apre`s avoir bu du CR (21 ± 8 min) et du FR (23 ± 8 min), les sujets ame´liorent de 51 % et de 43 % respectivement la dure´e de leur effort apre`s avoir bu du CM (32 ± 11 min). Le lait au chocolat est une boisson ame´liorant la re´cupe´ration conse´cutive a` un exercice d’endurance et pre´alable a` un effort d’intensite´ faible a` mode´re´e. Mots-cle´s : boissons pour sportifs, resynthe`se du glycoge`ne, prote´ines, performance, lait. [Traduit par la Re´daction]
Introduction Prolonged moderate-intensity exercise causes significant muscle glycogen depletion, which leads to diminution in performance. The resynthesis of glycogen postexercise is largely influenced by the type, amount, and timing of nutrient intake (Ivy 2005). Glycogen resynthesis is more rapid if carbohydrate is consumed immediately after exercise (Ivy et al. 1988; Friedman et al. 1991). The addition of protein to carbohydrate has been shown to positively affect glycogen resynthesis (van Loon et al. 2000; Ivy et al. 2002; Williams Received 2 April 2008. Accepted 13 November 2008. Published on the NRC Research Press Web site at apnm.nrc.ca on 23 January 2009. K. Thomas1 and E. Stevenson. School of Psychology and Sports Sciences, Northumbria University, Newcastle-upon-Tyne, NE1 8ST, UK. P. Morris. Mars UK Ltd, Dundee Road, Slough, Berkshire, SL1 4JX, UK. 1Corresponding
author (e-mail:
[email protected]).
Appl. Physiol. Nutr. Metab. 34: 78–82 (2009)
et al. 2003; Berardi et al. 2006) and subsequent same-day endurance performance (Ivy et al. 2002) when supplementation occurred immediately and 2 h postexercise. Further, Rowlands et al. (2008) reported a delayed performance benefit (60 h) of consuming a protein-enriched high-carbohydrate diet postexercise, but no short-term effect (15 h), compared with a carbohydrate-rich diet. The performance benefits of adding protein to a carbohydrate-rich postexercise bolus are unclear, with some studies reporting a positive effect (Niles et al. 2001; Ivy et al. 2002; Saunders et al. 2004) and some reporting no effect (Betts et al. 2005; Millard-Stafford et al. 2005; Betts et al. 2007; Rowlands et al. 2007). Flavoured milk has a favourable carbohydrate and protein content, and could potentially be used as a postexercise recovery aid. Karp et al. (2006) examined the effects of chocolate milk (CM), a fluid replacement drink (FR), and a carbohydrate replacement drink (CR) on a submaximal endurance exercise cycle in a glycogen-depleted state. With CM, subjects cycled 49% longer than with CR, and no difference was found between CM and FR (Karp et al. 2006). In CM and CR, the amounts of carbohydrate were matched; however, the drinks were not isocaloric. As a result, the en-
doi:10.1139/H08-137
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Thomas et al.
hanced performance with CM may have been due to the greater number of calories provided. This study aims to resolve this discrepancy by investigating the effect of a similar range of drinks on recovery of endurance exercise capacity after glycogen-depleting exercise, using isocaloric supplements. It was hypothesized that there would be no difference between CR and CM when matched for calorific content.
Materials and methods Participants Nine male trained cyclists participated in the study. Their mean (±SD) age, stature, mass, maximal oxygen uptake (V_ O2 max), and it’s associated power (Pmax), were 25.4 ± 8.0 years, 180.3 ± 8.3 cm, 72.8 ± 8.4 kg, 4.3 ± 0.4 Lmin–1, and 333 ± 43 W, respectively. After institutional ethics approval, all participants provided written informed consent and completed a pre-exercise medical screening questionnaire. Procedures Each participant completed an incremental exercise test followed by 3 experimental trials (described below). Prior to each visit to the laboratory, participants were instructed to refrain from strenuous exercise (>24 h) and to arrive in a fully rested, hydrated state. Trials were conducted at the same time of day to minimize diurnal variation. Participants completed a 3-day food diary prior to the first experimental trial, and reported to the laboratory between 0800 and 0900 hours after an overnight fast. On subsequent experimental trials, participants were asked to replicate their dietary intake for the 24 h before each session (Table 1). All testing was conducted using a cycle ergometer (Monark 824E, Vansbro, Sweden). Heart rate was monitored using a short-range telemetric monitor (Polar Electro, Finland). Blood lactate was determined from 25 mL samples of capillary blood, collected in heparinised capillary tubes, and immediately analysed for lactate concentration (Analox P-GM7 Micro-stat, Analox Instruments Ltd., London, U.K.). Total body water was assessed using bioelectrical impedance analysis (Bodystat 1500, Bodystat, Isle of Man, U.K.). This method has previously been endorsed for use with healthy adult subjects (Kyle et al. 2004). Incremental exercise test After a self-determined warm-up, participants cycled at a chosen pedal cadence (85–100 rmin–1) at a power output of 100 W, increased by 50 W every 2 min until volitional exhaustion. Participants were instructed to maintain their pedal cadence throughout the test, and the chosen cadence was used on all subsequent test occasions. Expired gas was measured breath by breath, using a cardiopulmonary exercise system (Quark b2, Cosmed, Rome, Italy). Maximal oxygen uptake was determined as the maximum 60 s mean V_ O2 (Hill et al. 2003). Power at V_ O2 max (Pmax) was defined as the power output of the final completed stage. Experimental trials Participants completed 3 experimental trials, separated by at least 1 week, in which they consumed CM (Mars Refuel chocolate milk, Mars Inc., U.K.), FR (Gatorade, Chicago,
79 Table 1. Mean composition of subjects’ diets (% of total kilocalories), from 3-day food diary and for the 24-h period prior to each trial. Dietary content Energy (kcal) Carbohydrate (%) Protein (%) Fat (%)
Mean 3-d intake 2402±606 49±7 20±5 31±7
Mean 24-h intake (prior to testing day) 2557±698 50±10 22±10 28±8
Ill.) or CR (Endurox R4, PacificHealth Laboratories, Woodbridge, N.J.). The volume of CR was calculated to yield 1 g carbohydratekg–1 body mass. An isovolumetric amount of FR was provided. The volume of CM was calculated to provide a caloric content identical to CR (Table 2). The order of trials was randomized and fully counter-balanced. Participants were permitted to drink water ad libitum throughout the experimental trials. The volume consumed (mL) was recorded. A schematic of the experimental trial design is shown in Fig. 1. Each aspect of the experimental trial is described below. Glycogen-depletion trial The protocol employed has been described elsewhere (Jentjens and Jeukendrup 2003). In brief, the trial consisted of alternating 2 min intervals (60%–90% Pmax) and recovery bouts (50% Pmax). Participants began at 90% Pmax and cycled, alternating 2 min periods, until they could no longer maintain their chosen cadence. The intensity of the interval was decreased in 10% increments each time pedal cadence decreased by approximately 10 rmin–1 over a 30 s time period. The trial ended when participants could not maintain their chosen rmin–1 at 60% Pmax. Blood lactate, body mass, and bioelectrical impedance analysis measurements were taken pre- and post-trial. Recovery period Participants rested in the laboratory for a 4-h recovery period. Within 60 s after the glycogen-depletion trial and 2 h into recovery, participants were provided with a recovery drink. Participants completed visual analogue scales at 30-min intervals throughout the recovery period to monitor their psychological responses to the drink provided. A set of 11 mood and appetite questions were presented, in the form of ‘‘How [word] do you feel?’’, where the word was clear-headed, energetic, friendly, full, happy, hungry, jittery, nauseous, relaxed, or thirsty. Participants were also asked to rate their desire to eat. Each question required the participant to mark their current perception on a 100 mm scale, anchored at either end with appropriate polarized labels (e.g., not at all vs. very much so). Endurance capacity trial Following the recovery period, participants completed a cycle to exhaustion at 70% Pmax (Fallowfield and Williams 1997). Heart rate was recorded every 5 min, and rating of perceived exertion every 10 min. Participants were given no feedback regarding elapsed time, and were instructed to remain seated throughout the trial. Pedal cadence was monitored constantly, and participants were given a warning Published by NRC Research Press
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Appl. Physiol. Nutr. Metab. Vol. 34, 2009 Table 2. Nutritional content of the postexercise recovery beverages. Drink content Volume (mL) Carbohydrate (g) Protein (g) Fat (g) Energy (kcal) Sodium (mg) Potassium (mg)
Chocolate milk 459.2±52.6 62.9±7.2 14.2±1.6 9.2±1.0 394.9±45.2 312.2±35.8
Fluid replacement drink 526.3±60.4 30.7±3.5 0 0 109.5±12.6 242.2±27.8 90.0±79.1
Carbohydrate replacement drink 526.3±60.4 72.5±8.3 18.9±2.2 1.6±0.2 394.9±45.2 321.1±36.8 173.7±19.9
Note: Values are means ± standard deviation.
Fig. 1. A schematic of the experimental trial structure. BIA, bioelectrical impedance analysis.
when cadence dropped by ‡10 rmin–1 for more than 20 s. The second time this occurred, the trial was terminated and time to exhaustion (tlim) was recorded. Blood lactate, body mass, and bioelectrical impedance analysis measurements were taken pre- and post-trial. Data analysis Descriptive data are expressed as means (±SD). Significance was set at a < 0.05 for all analyses. Participants’ 3-day food diaries were analysed for macronutrient content using Microdiet software (Downlee Systems Ltd., High Peak, U.K.). Experimental trial data were analysed using a simple repeated-measures analysis of variance (ANOVA). Simple contrasts were planned a priori in the case of a significant main effect, and CM was the reference category. Effect sizes were calculated using omega squared (u2) for main effects and r for contrasts (Field 2005). Statistical analysis was conducted using SPSS (SPSS Inc., version 11.5, Chicago, Ill.) and Microsoft Excel.
Results Descriptive statistics are presented in Table 3. There were no differences in exercise time for the initial glycogendepletion cycle (CM, 54 ± 24 min; CR, 69 ± 42 min; FR, 75 ± 30 min; F(2, 16) = 1.9; p = 0.18). Analysis revealed a main effect for tlim in the endurance capacity cycle (F(2, 16) = 9.8; p = 0.002; u2 = 0.21). Contrasts revealed that tlim with CM (32 ± 11 min) was longer than with either FR (23 ± 8 min; F(1, 8) = 11.2; p = 0.01; r = 0.76) or
CR (21 ± 8 min; F(1, 8) = 11.8; p = 0.01; r = 0.76) (Fig. 2). There were differences between the amounts of water consumed during the endurance capacity cycle (CM, 514 ± 257 mL; FR, 239 ± 158 mL; CR, 389 ± 169 mL). Mauchly’s test indicated that the assumption of sphericity had been violated (c2(2) = 9.8); therefore, degrees of freedom were corrected using Greenhouse-Geisser estimates of sphericity (3 = 0.57). The results showed a main effect (F(1.1, 9.1) = 6.9; p = 0.03; u2 = 0.19), with contrast analysis revealing a difference between water consumed during the CM and FR endurance capacity cycles (F(1, 8) = 31.1; p = 0.001; r = 0.89). There was, however, no difference between the total water consumed during the recovery period (CM, 597 ± 462 mL; FR, 544 ± 534 mL; CR, 628 ± 344 mL), and there were no differences in total body water between or within trials. There were no differences in any of the mood or appetite parameters measured during the 4-h recovery period following the glycogen-depleting exercise. There was a trend for feelings of fullness to be higher and feelings of hunger to be lower following CM immediately post-drink and at 30 min, compared with CR and FR (p = 0.08). There were no other within-subject differences for any of the other variables measured, including blood lactate, heart rate, rating of perceived exertion, and body mass (Table 3).
Discussion The aim of the study was to assess the effect of 3 beverPublished by NRC Research Press
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Table 3. Influence of recovery drink on selected variables. Variable Mean heart rate* (beatsmin–1) Mean RPE*
Chocolate milk 171±14 16.5±1.3
Fluid replacement drink 174±9 17.3±1.2
Carbohydrate replacement drink 166±16 16.6±1.5
Blood lactate (mmolL–1) Preglycogen depletion Postglycogen depletion Precapacity trial Postcapacity trial
1.4±0.4 4.1±2.7 1.8±0.4 4.5±2.5
1.9±0.7 3.5±1.5 2.0±0.5 4.0±1.4
1.8±0.6 4.7±3.5 1.8±0.5 5.0±2.8
Body mass (kg) Preglycogen depletion Postglycogen depletion Precapacity trial Postcapacity trial
72.6±8.3 72.4±8.5 72.5±8.4 72.4±8.3
72.8±7.9 72.5±7.8 72.4±8.3 72.2±8.3
72.6±8.3 72.5±8.3 72.5±8.7 72.4±8.7
Note: Values are means ± standard deviation. RPE, rating of perceived exertion. *Heart rate and RPE during endurance capacity trial.
Fig. 2. Time to exhaustion during endurance capacity trial, following ingestion of 3 different recovery drinks. CM, chocolate milk; FR, fluid replacement drink; CR, carbohydrate replacement drink. *, Significantly different from chocolate milk.
ages consumed during recovery from prolonged exercise on subsequent endurance capacity in cycling. Participants cycled 51% and 43% longer after ingesting CM than after ingesting CR and FR, respectively. The amount of water ingested in the endurance capacity cycle was higher with CM than with FR. There were no other within-subject differences for any of the variables measured. Participants cycled longer after CM ingestion than after CR ingestion, despite the beverages being isocaloric. This difference could be attributable to differences in carbohydrate type and (or) fat content between beverages. These 2 factors are discussed below. Carbohydrate type All of the carbohydrates in CM (glucose, fructose, sucrose, and lactose) and CR (glucose, fructose, and maltodextrin) have high solution osmolarity and similar peak oxidation rates during exercise (Wallis et al. 2005). The absence of sucrose in CR, however, may have affected liver glycogen repletion. Casey et al. (2000) investigated the effect of carbohydrate ingestion on liver and muscle glycogen resynthesis and exercise capacity, and showed that sucrose ingestion resulted in greater liver glycogen re-
pletion than glucose (25 ± 5 g vs. 13 ± 8 g). Although there was no statistical difference in mean exercise time (glucose, 40 ± 5 min; sucrose, 46 ± 6 min), there was a modest association between the change in liver glycogen content and subsequent exercise capacity (r = 0.53; p < 0.05). In this study, differential changes in liver glycogen content might have mediated the availability of blood glucose and, subsequently, affected tlim in the endurance capacity trial. Fat content The higher fat content of CM, compared with CR (Table 2), could have resulted in increased circulating free fatty acid concentrations during the cycle to exhaustion. Improvements in endurance capacity at exercise intensities (60%–75%, V_ O2 max) similar to our study have been previously reported when plasma free fatty acid concentrations and subsequent oxidation are elevated (Pitsiladis et al. 1999; Stevenson et al. 2005). Conversely, reported enhancements in endurance capacity using CR supplementation have been at higher submaximal exercise intensities (85%, V_ O2 max) (Williams et al. 1999, 2003). It is plausible that CMismosteffective for postexercise recovery forlow-moderate endurance exercise, due to its greater fat content, and that CR is most effective for higher-intensity endurance exercise performance, due to a greater reliance on carbohydrate as a fuel source. Whether CM is effective for recovery for higher endurance exercise intensities is not known. These conclusions are speculative, as no measures of substrate oxidation were undertaken. Further research is warranted. The participants consumed similar amounts of water during the recovery period (CM, 597 ± 462 mL; FR, 544 ± 534 mL; CR, 628 ± 344 mL), and there was no difference between total body water or body mass at any stage during the experimental trials (Table 3). Participants, however, consumed more water in the endurance capacity trial after CM than after FR (CM, 514 ± 257 mL; FR, 239 ± 158 mL). Participants were permitted to drink water ad libitium during the experimental trial in an attempt to maintain a euhydrated state throughout. The differences in water consumption between endurance capacity trials could have influenced the results. Future research should ensure a similar hydration Published by NRC Research Press
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state, measured with more accurate methods than the ones employed in the current study. Using isocaloric beverages, this study supports the original conclusion of Karp et al. (2006), that CM could be used as an effective means of recovery from prolonged endurance exercise. This conclusion applies to subsequent exercise at low-moderate (70% Pmax) exercise intensities. The mechanisms for this enhancement require further investigation.
Acknowledgements This study was funded by Mars U.K. Ltd. and Runner’s World Magazine (U.K.).
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