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Carbohydrate-Protein Intake and Recovery from Endurance Exercise: Is Chocolate Milk the Answer?

Saunders, Michael John PhD, FACSM

Current Sports Medicine Reports: July 2011 - Volume 10 - Issue 4 - p 203-210
doi: 10.1249/JSR.0b013e318223ccb4
Nutrition & Ergogenic Aids
Free

Postexercise carbohydrate intake has been shown to augment recovery from heavy aerobic exercise. The effects of carbohydrate and protein coingestion (CHO + Pro) have been investigated more recently, including the potential influences of chocolate milk. Some studies report that CHO + Pro beverages (including chocolate milk) enhance subsequent exercise performance versus carbohydrate beverages, although others have reported no positive effects. The putative efficacy of CHO + Pro could be due to influences on glycogen resynthesis, protein turnover, rehydration, attenuations in muscle disruption, or perhaps a combination of these factors. However, there are inconsistencies in the literature regarding the effects of CHO + Pro on these factors, and the mechanisms explaining potential influences of CHO + Pro are not defined clearly. Further research is required to address these limitations, but present evidence suggests that CHO + Pro beverages may positively influence recovery under some exercise conditions, and chocolate milk is likely a good recovery beverage for lactose-tolerant endurance athletes.

Department of Kinesiology, James Madison University, Harrisonburg, VA

Address for correspondence: Michael John Saunders, PhD, FACSM, Department of Kinesiology, James Madison University, 261 Bluestone Dr., MSC 2302, Harrisonburg, VA 22807 (E-mail: saundemj@jmu.edu).

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Introduction

Endurance athletes regularly engage in training sessions and competitive events lasting several hours with vast energetic requirements (>600 to 1,000 kcal·h−1). The process of recovery from such heavy exercise is highly complex, including aspects of fuel replenishment, muscle repair, and rehydration. Nutrient intake is known to influence various aspects of postexercise recovery. For example, carbohydrate intake after exercise augments glycogen resynthesis, with maximal levels occurring at ingestion rates of approximately 1 g·kg−1 body weight·h−1 (20). The timing of carbohydrate intake also seems to be important because feedings commencing within 30 min after exercise result in substantially higher rates of glycogen synthesis versus when intake is delayed (19). As a result, endurance athletes generally are encouraged to consume relatively high-carbohydrate diets, with an emphasis on carbohydrate intake immediately after heavy exercise.

It is logical for athletes to implement nutritional strategies that promote recovery during competition scenarios with compressed recovery time (i.e., soccer/tennis tournaments, track/swim meets that include multiple heats, or bicycle stage races). Interventions to support recovery also are useful for athletes during periods of heavy training in which they do not wish to sacrifice training quality (i.e., training camps or intensified blocks of training) (15). During the past decade, numerous studies have reported that coingestion of carbohydrate and protein (CHO + Pro) may enhance postexercise recovery compared with carbohydrate alone. In particular, chocolate milk's potential as a recovery beverage recently has received considerable media attention. Relatively few peer-reviewed studies have examined directly the effects of chocolate milk on recovery from endurance exercise, in comparison with carbohydrate (i.e., Gilson et al. (13), Karp et al. (23), and Thomas et al. (42)). However, chocolate milk has a variety of nutritional characteristics that suggest the capability to be a good recovery beverage for endurance athletes. In addition to the natural sugars present in "regular" milk (∼12 g of lactose per 8 oz), chocolate milk has added sugars such as sucrose, high-fructose corn syrup, or other sweeteners (depending on the brand) (43). As a result, carbohydrate constitutes the majority of calories provided in chocolate milk, resulting in high rates of muscle glycogen replenishment (12). Milk contains high-quality proteins (i.e., casein and whey), which include a substantial proportion of branched-chain amino acids such as leucine, which have important roles in protein synthesis and muscle metabolism (35). Protein also has been reported to attenuate various markers of muscle disruption after heavy endurance exercise, which may influence short-term recovery of muscle function/performance compared with carbohydrate alone (14,41). Decreases in oxidative stress also have been attributed to cocoa-based beverages because of the presence of flavonoids in chocolate (50). The fat content of chocolate milk varies with product types (i.e., skim, low fat, whole milk). Assuming athletes consume adequate dietary fat, there seems to be little rationale for increasing fat content in a postexercise recovery beverage, particularly if this results in lowered carbohydrate/protein intake to maintain caloric intake. Thus, most studies examining the effect of chocolate milk intake on postexercise recovery have used reduced-fat or nonfat milk beverages. Lastly, chocolate milk has a relatively high electrolyte content (including sodium, potassium, and calcium), which may play an important role in postexercise rehydration (40,47). The purpose of this review was to discuss representative research that has investigated the efficacy of CHO + Pro coingestion (and chocolate milk in particular) on recovery after heavy endurance exercise.

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CHO + PRO Intake and Glycogen Replenishment

During the past 20 yr, multiple studies have reported that CHO + Pro enhances glycogen resynthesis to a greater extent than carbohydrate alone (see Betts and Williams (6) for a recent review). In some instances, CHO + Pro treatments contained greater total macronutrient content, so it is difficult to directly attribute these observations to protein per se. However, a few of these studies have reported faster glycogen replenishment with CHO + Pro versus isocaloric carbohydrate beverages (2,18). Higher glycogen resynthesis rates with CHO + Pro ingestion could be related to elevated insulin levels, particularly when protein is fed at a rate of ≥0.3 g·kg−1·h−1 (6). However, Ivy et al. (18) reported increased glycogen replenishment rates with CHO + Pro without altered insulin levels, suggesting an insulin-independent effect such as increased glycogen synthase activity resulting from amino acids' stimulation of the mammalian target of rapamycin (mTOR) pathway (17). Although the evidence above is intriguing, numerous other investigators have observed no improvements in glycogen replenishment with CHO + Pro administration (see Betts and Williams (6)), even in the presence of increased insulin levels (7,21,44). Various differences between methodologies may explain the discrepancies between studies, and the length of the recovery period and the amount of carbohydrate/protein consumed in postexercise feedings may be particularly relevant factors. Treatment differences are likely to be negligible when examined over relatively long recovery periods because glycogen replenishment can be complete within 24 h when an athlete is fed an adequate amount of carbohydrate (39). Betts and Williams (6) concluded in a recent review that there is little evidence that glycogen resynthesis rates are augmented when protein is added to carbohydrate beverages provided at rates of ≥1 g·kg−1 BW·h−1 but may be accelerated when protein is added to lower carbohydrate doses (Fig. 1).

FIGURE 1

FIGURE 1

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Chocolate Milk and Glycogen Replenishment

Few studies have directly examined the effects of chocolate milk intake on glycogen resynthesis rates after endurance exercise. Chocolate milk has a macronutrient composition that is similar to that of other CHO + Pro beverages that have been reported to enhance glycogen resynthesis (i.e., a 500-kcal serving of low-fat chocolate milk contains approximately 1.2 g·kg−1 of carbohydrate and 0.4 g·kg−1 of protein for a 70-kg athlete (13)). Thus, it seems logical that chocolate milk may promote glycogen resynthesis at rates that are similar or possibly higher than calorically matched carbohydrate beverages, depending on the timing/amount of feedings after exercise. In support of this hypothesis, Karfonta et al. (22) recently presented data indicating that chocolate milk intake provided greater glycogen replenishment than an isocaloric (1.2 g·kg−1) carbohydrate beverage during a 1-h recovery period after heavy endurance exercise. In addition, Ferguson-Stegall et al. (12) reported that glycogen resynthesis during 4 h of recovery from heavy aerobic exercise with chocolate milk intake (30.6 μmol·g−1 wet weight) tended to be greater than when an isocaloric carbohydrate beverage was consumed (23.6 μmol·g−1 wet weight), although there was no statistically significant difference between treatments (P = 0.06).

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CHO + PRO Intake and Protein Turnover

Relatively few studies have investigated the influences of nutrient intake on protein turnover during/after prolonged aerobic exercise. Protein degradation seems to be increased during endurance exercise (24), and amino acid oxidation may be greater during the later stages of prolonged exercise (16). Protein breakdown is elevated after endurance exercise without nutrient intake, and total net protein balance may remain in a negative state for hours after exercise (38). The coingestion of CHO + Pro positively influences protein turnover during and after endurance exercise and results in a more positive protein balance compared with carbohydrate-only feedings (16,24-26) (Fig. 2). Some studies have attributed these effects to improved whole-body protein synthesis with CHO + Pro (25,26), whereas others have concluded that a decrease in protein breakdown was responsible for the improved protein balance (16,24). Howarth et al. (16) recently reported that CHO + Pro ingestion after endurance exercise increased mixed skeletal muscle fractional synthetic rate compared with carbohydrate alone and speculated that this response was at least partially due to increased mitochondrial protein synthesis.

FIGURE 2

FIGURE 2

On the basis of current evidence, it seems that CHO + Pro ingestion after endurance exercise improves protein balance via reduced protein breakdown and/or increased protein synthesis. The functional relevance of these effects and their implications with respect to specific training-induced adaptations remain to be elucidated. However, the influences of CHO + Pro on protein turnover are generally considered to be advantageous to skeletal muscle during recovery (16).

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Chocolate Milk and Protein Turnover

The author is not aware of any published studies of chocolate milk that are directly comparable to the investigations discussed above. However, ingestion of milk (or milk proteins) has been shown to promote positive changes in protein turnover after resistance exercise (48). Milk protein contains approximately 80% casein and 20% whey protein by mass (48). Thus, it has a relatively high proportion of leucine, an amino acid that plays an important role in the translational control of protein synthesis (35). Casein provides a relatively slow, prolonged rise in postprandial amino acid levels, which may suppress proteolysis (8,11). Whey provides a more rapid rise in amino acid levels, which has been associated with increased protein synthesis (8,11). As a result, milk generally is considered a desirable source of protein and reportedly promotes higher protein synthesis rates and a more positive net protein balance than soy protein consumption after resistance training (48).

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CHO + PRO Intake and Indices of Muscle Disruption

Heavy endurance exercise results in mechanical and biochemical stresses to skeletal muscle. As a result, endurance athletes may exhibit elevated biomarkers of sarcolemmal damage (i.e., plasma/serum levels of creatine kinase (CK), myoglobin (Mb), lactate dehydrogenase (LDH)), increased ratings of muscle soreness, and impaired muscle function after exercise (i.e., Greer et al. (14)). Numerous investigators have examined the potential influences of CHO + Pro intake on these indices of recovery. A considerable number of studies have reported that CHO + Pro (or branched chain amino acids [BCAA]) ingestion may attenuate postexercise CK levels after endurance exercise, compared with carbohydrate alone (9,13,14,27,32,37,41). Some of these studies also reported reduced levels of other biomarkers, such as Mb (37) or LDH (9,32) with CHO + Pro ingestion. In contrast to these findings, numerous other studies have reported no effect of CHO + Pro on similar markers of muscle disruption (i.e., Betts et al. (5) and Millard-Stafford et al. (30)).

The practical relevance of reduced CK levels (or other biomarkers) has been questioned because it can be poorly correlated with direct markers of muscle damage or muscle function (46). For this reason, it is important to note that protein supplementation also has been reported to ameliorate changes in muscle soreness (14,27,32) and muscle function (14,41) after heavy exercise. Although currently unsupported by biopsy-obtained markers of muscle damage, these findings suggest that (at least in some instances) CHO + Pro intake may positively influence functionally relevant indicators of muscular impairment. Nutritionally induced improvements in muscle function could logically result in enhanced performance in subsequent whole-body exercise, as discussed later in this review. However, the effects of CHO + Pro ingestion on soreness/function have been inconsistently observed in the literature, with some studies reporting no effects on similar indices of muscle recovery (5,13).

Although there are noted inconsistencies, the evidence here suggests a broad potential for postexercise protein supplementation to enhance muscle recovery after heavy endurance exercise. It is difficult to assess the influence of protein per se from these studies because many compared treatments that were not isocaloric, and some have used CHO + Pro treatments that contained relevant ingredients (such as antioxidants), which were not present in the comparison beverages. In addition, the wide-ranging exercise protocols used in these studies and varied nutrient ingestion amounts/timing make it difficult to provide generalizations regarding the conditions in which a positive effect from CHO + Pro on markers of muscle recovery would be expected. However, a potential factor in the inconsistent effects of CHO + Pro on muscle recovery could be the magnitude of muscle disruption elicited by exercise. Betts et al. (5) observed that most (although not all) studies showing significant effects with CHO + Pro have reported postexercise serum/plasma CK levels of 250 to 600 U·L−1, whereas studies reporting no effects typically have reported values exceeding 1,000 U·L−1. The authors speculated that CHO + Pro supplementation may only be effective at attenuating less severe degrees of exercise-induced muscle damage. Luden et al. (27) reported that postexercise CHO + Pro ingestion was most effective at attenuating CK changes among collegiate cross-country runners who performed the highest mileages during a 6-d intervention period (i.e., those who incurred the greatest increases in CK levels without protein supplementation). Although this could be interpreted to refute the speculation of Betts et al., the posttraining CK values in the higher mileage runners were consistent with the "less severe" range indicated by Betts et al. (5). In addition, the apparent lack of efficacy of CHO + Pro for the lower mileage runners in the study of Luden et al. was potentially a consequence of these subjects experiencing negligible muscle disruption in the "control" condition, which negated any possible treatment effects from being observed. Although highly speculative, these observations potentially could explain some of the varied findings in the literature because studies with very small or large changes in muscle disruption markers (which would be influenced by the specific exercise protocols used and the training status of the subjects studied) may be less likely to observe significant effects with CHO + Pro. Further study is warranted to determine the specific circumstances in which CHO + Pro ingestion may be efficacious at influencing markers of muscle disruption in endurance athletes, as well as establishing the functional relevance of some of these potential changes.

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Chocolate Milk and Muscle Disruption Markers

Few published studies have assessed the effects of milk beverages on markers of muscle disruption after endurance exercise. Cade et al. (9) examined recovery in competitive swimmers when consuming a sucrose solution or milk protein supplement after training and reported faster recovery of CK and LDH with the milk protein supplement. In addition to potential influences of milk protein on muscle disruption markers, chocolate milk has been proposed to influence oxidative stress because of the presence of cocoa-derived flavonoids (50). However, the flavonoid content of commercial beverages varies considerably between products because of variations in processing procedures, which may lead to considerable losses in these compounds (29). McBrier et al. (29) reported that a cocoa-based CHO + Pro beverage attenuated postexercise soreness after downhill treadmill running but had no influence on biomarkers of muscle damage and inflammation. Gilson et al. (13) reported that postexercise chocolate milk intake reduced serum CK levels compared with an isocaloric carbohydrate treatment during 4 d of intensified soccer training. However, other markers of muscle recovery (such as peak isometric muscle force and muscle soreness and energy ratings) were not different between beverages. In addition, a recent study from Ferguson-Stegall et al. (12) reported no treatment differences in markers of muscle disruption (including plasma CK and Mb) between chocolate milk and an isocaloric carbohydrate supplement.

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CHO + PRO Intake and Rehydration

Fluid losses from sweat result in dehydration during heavy exercise and can impair physiological function and performance in subsequent exercise, as reviewed previously in this journal (10). Traditional sports drinks provide fluids to replace those lost from sweat, along with carbohydrates and electrolytes to provide fuel and promote fluid absorption/retention. There is some evidence that protein/amino acids may enhance intestinal absorption of sodium and water, particularly when the solution contains adequate sodium to promote amino acid cotransport (45). However, there also is evidence that high protein doses (i.e., 120 g·L−1 (28)) may slow gastric emptying rates. Few studies have examined the effects of more typical postexercise doses of CHO + Pro intake on hydration, but Seifert et al. (36) reported that a CHO + Pro beverage (60 g·L−1 of carbohydrate + 15 g·L−1 of protein) improved fluid retention during recovery by 15% compared with a carbohydrate-matched beverage. These beverages were not matched on all ingredients (because commercial products were compared), and thus, there were small differences in electrolyte content and other ingredients that also could have influenced these findings.

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Chocolate Milk and Rehydration

The macronutrient composition and electrolyte content of milk/chocolate milk suggest the potential to be an effective rehydration solution. A few recent studies have directly investigated this hypothesis. Shirreffs et al. (40) reported that subjects consuming low-fat milk (in an amount equal to 150% of sweat loss during exercise) had lower urine output during 5 h of recovery compared with when consuming equal amounts of water or a carbohydrate-electrolyte beverage. As a result, milk intake resulted in positive or euhydrated fluid balance throughout the recovery period. In contrast, negative fluid balance occurred within 1 h of recovery with the water or carbohydrate solutions. The authors speculated that the high electrolyte content of milk, particularly potassium, was probably responsible for this result, although differences in gastric emptying also may have influenced these findings. A subsequent study by the same group (47) reported similar findings, with milk consumption maintaining net positive fluid balance during 3 h of recovery, resulting in fluid changes that tended (P = 0.051) to be less than those with a carbohydrate-electrolyte beverage.

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CHO + PRO Intake and Subsequent Exercise Performance

From the standpoint of external validity, the most useful assessment of recovery for athletes is their subsequent exercise performance. Numerous studies during the past decade have examined the influence of CHO + Pro coingestion on performance in subsequent endurance exercise. Many of these studies have indicated enhanced performance with CHO + Pro intake versus carbohydrate-only treatments (i.e., Berardi et al. (1), Rowlands et al. (33), Skillen et al. (41), and Williams et al. (49)). One of the earliest studies of this topic (49) reported relatively large positive effects because time to exhaustion at ∼85% V˙O2peak (performed 4 h after an initial session of heavy exercise) was ∼50% longer when subjects received CHO + Pro during the recovery period, compared with carbohydrate. However, the beverages in this study were not matched for carbohydrate content or calories, so it was not possible to ascertain whether these effects were directly attributable to protein. Subsequent studies have compared carbohydrate and CHO + Pro recovery treatments using varied methodologies. Many of the studies have concluded no significant differences in subsequent performance between treatment beverages (i.e., Berardi et al. (2), Betts et al. (3,4), Luden et al. (27), Millard-Stafford et al. (30), Romano-Ely et al. (32), and Rowlands et al. (34)). However, three recent studies are noteworthy because they reported that CHO + Pro ingestion improved subsequent exercise performance compared with isocaloric carbohydrate treatments (1,33,41). Berardi et al. (1) provided subjects with multiple feedings of CHO + Pro or carbohydrate during a 6-h period of recovery after a maximal-effort time trial of 1 h. Maintenance of performance during a second time trial was significantly improved in subjects who consumed CHO + Pro because declines in average power output and distance traveled (−3.9 ± 6.5 W, −0.30 ± 0.50 km) were significantly less than those in subjects who consumed an isocaloric carbohydrate beverage (−16.5 ± 6.7 W, −1.05 ± 0.44 km). Similarly, Skillen et al. (41) observed that CHO + Pro intake prevented declines in time to fatigue (at 85% V˙O2peak), which were observed with carbohydrate supplementation during consecutive-day exercise sessions. The consecutive days of heavy exercise resulted in worsening levels of vertical jump height and fatigue when the subjects received carbohydrate; CHO + Pro ingestion partially attenuated these declines. Rowlands et al. (33) had cyclists complete three high-intensity rides during a 4-d period. The initial session included 2.5 h of aerobic intervals, with repeat-sprint tests performed on the second and fourth days of the intervention. During a 4-h period of recovery after the first and second rides, subjects consumed a control diet including 2.1 g·kg−1 BW·h−1 carbohydrate or an isocaloric protein-enriched diet (1.4 g·kg−1 BW·h−1 carbohydrate, 0.7 g·kg−1 BW·h−1 protein). Sprint test performance was not different between carbohydrate and CHO + Pro on day 2 (average sprint power = 318 ± 16 and 318 ± 17 W, respectively) but was significantly enhanced with CHO + Pro on day 4 (351 ± 17 vs 338 ± 16 W).

The studies above indicate that CHO + Pro may have positive influences on whole-body recovery/performance that are independent of caloric differences between treatments. The specific mechanisms that may explain these effects are currently unknown but could be related to one or more of the factors discussed previously. Despite the potential efficacy of CHO + Pro on recovery, numerous studies have reported no differences in subsequent exercise performance when athletes receive carbohydrate or CHO + Pro treatments after exercise, and the reasons for these discrepancies between studies are poorly understood. It is possible that variations between exercise/nutritional protocols may explain some of the differences between studies. For example, the duration, intensity, and type of exercise (i.e., proportional contributions of eccentric/concentric contractions) may affect the extent of glycogen depletion, muscle damage, dehydration, and other factors after exercise. This could influence the potential efficacy of nutritional interventions to influence postexercise recovery. Similarly, differences in the nutritional composition of beverages, timing of feedings, and duration of the recovery period also would influence subsequent exercise performance. The existing data do not provide obvious generalizations regarding the effect of these factors, and further study is required to determine the specific exercise/nutritional conditions in which CHO + Pro ingestion may enhance performance in subsequent exercise.

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Chocolate Milk and Subsequent Exercise Performance

The effects of chocolate milk ingestion on subsequent endurance performance have been directly examined in a few studies, as summarized in the Table. Karp et al. (23) assessed performance using a time-to-exhaustion test conducted 4 h after a 1-h session of intense aerobic cycling intervals. Chocolate milk consumption resulted in significantly longer exercise time versus a CHO + Pro beverage containing similar amounts of carbohydrate and protein. However, chocolate milk produced only equal performance results compared with a carbohydrate beverage containing no protein and less than half the carbohydrate and calories of chocolate milk, making interpretation of these findings difficult (23). Thomas et al. (42) conducted a subsequent study that used similar protocols and reported that chocolate milk enhanced time to exhaustion compared with both the carbohydrate and CHO + Pro comparison beverages. Pritchett et al. (31) reported that chocolate milk and a CHO + Pro beverage of similar macronutrient composition produced similar effects on subsequent exercise performance. Other studies have reported no influences of chocolate milk (or other milk beverages) on subsequent exercise performance compared with carbohydrate beverages (13,47). However, both of these studies reported positive influences of CHO + Pro on other indices of recovery (attenuated CK levels and enhanced fluid balance, respectively), and it is unclear whether the initial exercise sessions in either study were adequately demanding to elicit impairments in subsequent performance in either treatment condition. A recent study by Ferguson-Stegall et al. (12) is notable for examining the effects of chocolate milk intake during recovery on subsequent time trial performance (rather than time to fatigue), in comparison with an isocaloric carbohydrate beverage and a noncaloric placebo. As shown in the Table, chocolate milk intake resulted in significantly faster performance in a subsequent 40-km time trial (performed 4 h after a prolonged session of endurance exercise) compared with either comparison beverage. Thus, the data available to date suggest that chocolate milk may provide similar or perhaps enhanced performance in subsequent exercise when compared with carbohydrate or other CHO + Pro beverages.

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Conclusions

As illustrated in the literature reviewed above, there is a variety of evidence indicating that CHO + Pro ingestion and chocolate milk in particular may promote postexercise recovery and enhance subsequent exercise performance when compared with carbohydrate alone. These effects could be the result of positive influences on glycogen resynthesis, protein turnover, muscle disruption, rehydration, or a combination of these factors. Although it seems that CHO + Pro beverages (and chocolate milk) may have the potential to improve recovery under some exercise conditions, the topic remains controversial because of substantial inconsistencies in findings across the research literature to date and a lack of clearly defined mechanisms to explain the potential influences of CHO + Pro. Further research is warranted to address some of the limitations present in the existing literature and to provide generalizations regarding the potential roles of CHO + Pro ingestion on these variables. Notwithstanding the caveats mentioned above, it is worth noting that the overwhelming majority of studies in this area have concluded that CHO + Pro ingestion provides postexercise recovery that is at least equal to that of more traditional carbohydrate beverages. Despite the relative paucity of studies investigating chocolate milk, most have revealed favorable results with respect to its influence on postexercise recovery. Chocolate milk has been noted for its high availability, relatively low cost, and widely appreciated taste (13). With the exception of individuals with lactose intolerance, there seem to be no contraindications to consuming chocolate milk for recovery (35). Thus, although greater clarity is required before specific recommendations can be provided, present evidence suggests that chocolate milk is a good choice as a recovery beverage for endurance athletes.

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Acknowledgments

The author has received prior research funding from the National Dairy Council and from sports nutrition corporations. He has served on a scientific advisory committee for the National Dairy Council and has received fees and travel reimbursement for work related to this role.

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