Recent research has suggested that milk may be an effective recovery beverage (12). Studies comparing time to fatigue during a second exercise bout after an earlier session and recovery with either chocolate milk or a carbohydrate replacement beverage have reported that run and cycling times are similar or longer with chocolate milk (7,10,14). In particular, low fat chocolate milk contains carbohydrates, protein, and other nutrients, such as branched chain amino acids, that may aid in recovery (6). Its carbohydrate content is similar to that of many common sports drinks, making it a good source of sugars that can be used by skeletal muscle to synthesize glycogen and rebuild depleted stores during postexercise recovery (6). In addition, it is a rich source of casein and whey, which are digested and absorbed slowly (2). This allows blood amino acid concentrations to remain elevated (16), which may permit extended muscle anabolism during the recovery period (6,16,20).
Current recommendations for maximizing glycogen resynthesis suggest consuming either 1.2 g carbohydrate per kilogram of body weight immediately after exercise and each hour after or as multiple small meals for 4–6 hours postexercise or 0.8 g·kg−1·h−1 in combination with amino acids or protein (6). Although solid and liquid supplements are equally effective (8), liquids may be preferred if appetite is suppressed during the first few hours postexercise. Liquids also provide fluid to aid in rehydration, and the high concentrations of electrolytes in milk (sodium and potassium) help replenish those lost through sweating during exercise. For example, studies comparing the effectiveness of milk, water, and carbohydrate replacement drinks at promoting rehydration after cycling in the heat have found that participants remained in positive fluid and potassium balance only on consumption of milk (13,18). Low fat chocolate milk is also inexpensive and readily available and is the most popular milk flavor among children and adults (15).
To date, most of the studies examining low fat chocolate milk as a recovery beverage have used male cyclists (7,14); little is known about responses among women or athletes participating in collegiate-level team sports. However, similar results may be obtained with athletes participating in sports that require short bursts of speed interspersed with longer periods of slower running. Research with male and female soccer and basketball players has demonstrated significant improvements in time to exhaustion when carbohydrate is consumed during and between workouts designed to simulate the intermittent, high-intensity running that occurs in these sports (9,19). This finding is particularly important because fatigue at the end of a game or during a second practice session limits performance at a time that is critical for making game-deciding plays or maximizing skill acquisition and rehearsal. Further, there is limited evidence about how athletes respond in a practical setting. Previous studies of chocolate milk have involved fasted subjects (7,14); in contrast, athletes usually perform exercise in a fed, non–glycogen depleted state before consuming a recovery beverage. Thus, the purpose of the current project was to compare the effectiveness of low fat chocolate milk and a carbohydrate–electrolyte beverage (CE) on recovery between morning and afternoon preseason practice sessions among male and female Division III collegiate soccer players and to use these data to help practitioners tailor recommendations for using chocolate milk during recovery in collegiate team sport athletes.
Experimental Approach to the Problem
The study was conducted using a randomized, crossover design with 13 athletes, and it took place in late summer during the teams' preseason training camps. This design has been used in other similar studies to reduce between-subject variability (7,14,20): The subjects are randomly assigned to receive 1 of the 2 beverages during the first treatment day and receive the alternative on the second day, allowing each person to serve as his or her own control (4). Before the 2 experimental trials, the subjects reported to the laboratory for a preliminary visit to complete a 20-m shuttle run to fatigue to estimate maximal oxygen uptake (V̇O2max). The intervention occurred on 2 days during the middle of the preseason separated by a 2-day washout. During the intervention days, the subjects completed their morning practice and received either low fat chocolate milk or an equivalent volume of CE. The subjects then completed their afternoon practice followed by a 20-m shuttle run to fatigue. At the end of each intervention day, they were asked to rate the effort required during the shuttle run using the Borg rating of perceived exertion (RPE) scale (1) and complete a dietary intake questionnaire.
Twenty-six athletes (11 women, 15 men) from the men's and women's soccer teams volunteered to participate. All subjects were healthy and had been cleared to practice by the university athletic training staff through a recent physical examination. Of these, 6 were excluded from the team, 5 withdrew because of injuries or illnesses that occurred during practice, and 2 withdrew because of scheduling conflicts. Thus, 13 athletes (8 women, 5 men; age: 19.5 ± 1.1 years; mass: 70.9 ± 11.5 kg; height: 171.9 ± 9.5 cm; V̇O2max: 41.9 ± 9.7 ml·kg−1·min−1) were able to complete the shuttle run on both days and were included in the final analyses. The research was approved by the Institutional Review Board at Kean University, and written informed consent was obtained from each subject according to the guidelines established by the Institutional Review Board.
The baseline visit occurred at the beginning of the preseason training camp, before the athletes began practicing. Subjects completed a 20-m shuttle run to fatigue (11) from which V̇O2max was estimated. This test involved continuous running between 2 lines separated by 20 m. Running speed was maintained by a series of computer–generated beeps, which began slowly and increased in frequency each minute, indicating when the participants should reach the second line. Fatigue was defined as failure to reach the second line for 2 consecutive ends, and time to fatigue was used to estimate V̇O2max.
During the intervention days, athletes completed their morning practice and received either low fat chocolate milk or an equivalent volume of CE (mean recovery beverage volume of 615 ± 101 ml). Although it was impossible to disguise the distinct flavors of the 2 beverages, to minimize bias, all beverages were served in opaque containers by an individual not associated with the experiment, as has been done previously (7). The study beverages consisted of low fat chocolate milk (Ultimate Chocolate Milk, Low Fat, Tuscan Dairy, Union, NJ, USA) and an isovolumetric amount of a CE (The Gatorade Company, Chicago, IL, USA). The nutrient composition of these beverages is presented in Table 1. In accordance with current recommendations for postexercise carbohydrate replacement (11), the volume of milk given provided 1.0 g carbohydrate per kilogram body weight, and on both days, the beverages were consumed immediately after the first practice and again 2 hours later, as done in previous studies (7,14). Current recommendations state that when rapid postexercise recovery and rehydration are needed, a beverage containing fluid, electrolytes and carbohydrate should be used (3). Given these recommendations and the seriousness of dehydration and depleted carbohydrate and electrolyte stores for athletes exercising in the heat, as was the case in this study, withholding fluid, electrolytes, and carbohydrates could not be justified. Thus, a beverage typically consumed by these athletes during recovery (CE) was used as the comparison trial rather than a water-only or a no-fluid comparison.
The subjects then completed their afternoon practice followed by a 20-m shuttle run to fatigue, modeled after one used in a similar study (9). During this test, subjects ran at speeds alternating between 55 and 95% of their estimated V̇O2max until fatigue, defined by the inability to maintain running speeds (>2 consecutively missed distances of 20 m). On both study days, the same evaluator, who was blinded to the treatments, determined fatigue. Practice sessions, beverage consumption and completion of the shuttle run took place at approximately the same time each day; in addition, practice sessions were similar in duration and intensity and consisted of conditioning activities and soccer-specific drills. The subjects were instructed to maintain normal dietary, hydration, and activity habits throughout the study so that the 2 study days represented “normal” preseason days except for the consumption of the study beverages. They were also instructed to consume the same foods and beverages, aside from study beverages, 2 days before and during both intervention days. At the end of each intervention day, subjects were asked to rate the effort required during the shuttle run using the RPE scale and complete a dietary questionnaire, which was previously validated for use with collegiate athletes (5,17) and modified so that respondents were asked to indicate the number of servings of each food consumed on that day rather than on a typical day in the previous week.
To account for possible gender-related physiologic differences in hormone concentrations (e.g., estrogen) and the structure and capacity of the cardiovascular and respiratory systems (e.g., heart size and blood volume) that may affect fluid retention, gas exchange, ventilatory response, and, ultimately, exercise performance (21), the data were analyzed both for all subjects combined and for men and women separately. Inspection of the run time and RPE data indicated that they were nonparametric; thus, the Wilcoxon paired rank-sign test was used to test for differences in time to fatigue and RPE. Dietary intake data were analyzed using paired samples t-tests. Differences were considered significant at p ≤ 0.05, and all outcome variables were tested for sequence and period effects. Data are presented as mean ± SD unless otherwise noted.
When both men and women were considered as a group, time to fatigue did not differ significantly with chocolate milk (6.11 ± 5.12 minutes; median 4.21 minutes) compared with CE (5.03 ± 3.41 minutes; median 4.07 minutes; p > 0.05). For the men only, there was a trend of increased time to fatigue with chocolate milk (8.31 ± 6.53 minutes; median 4.34 minutes) compared with CE (6.24 ± 5.03 minutes; median 4.06 minutes; exact p = 0.03; effect size 0.2; Spearman correlation coefficient 0.90) (Figure 1). The RPE did not significantly differ with chocolate milk (15.8 ± 3.0; median 15.5) compared with CE (14.5 ± 2.1; median 14.5; p > 0.05). For the women, time to fatigue did not significantly differ between the 2 beverages, and RPE was not significantly different with chocolate milk (12.6 ± 0.96; median 13) compared with CE (13.3 ± 0.5; median 13; p > 0.05). To further evaluate time to fatigue with the 2 beverages, a difference score was calculated as run time with milk minus run time with CE. For the men, the average difference in run time between the 2 beverages was 1.67 ± 1.95 minutes, and for the women, it was 0.40 ± 2.81 minutes. No sequence or period effects were found for any of the data, and no differences in intake between the chocolate milk and CE days were reported for any food groups besides dairy (p < 0.00).
The results of this study suggest that consuming low fat chocolate milk between morning and afternoon practices may be as good as an isovolumetric amount of CE at promoting recovery, measured by shuttle run time to fatigue, among Division III soccer players. These findings are consistent with those of previous reports (7,10,14): They support the recommendation of low fat chocolate milk as an effective recovery beverage (3), and importantly extend earlier findings to female athletes and practical, field settings (e.g., preseason).
Although the mechanisms explaining the effectiveness of chocolate milk are not completely clear, the combination of carbohydrate, milk protein, fluid, and electrolytes may promote muscle glycogen resynthesis and rehydration to speed recovery between exercise sessions (12). During low- to moderate-intensity exercise (below the lactate threshold or at ca. 70% V̇O2max), the greater fat content of milk compared with that of CE may also increase blood levels of free fatty acids, delaying glycogen depletion (7,14). Future studies should investigate whether low fat chocolate milk is effective for recovery from high-intensity exercise, as well, given the greater reliance on carbohydrate as a fuel during this type of activity, and to clarify these mechanisms using biochemical measures (e.g., muscle glycogen content and blood lactate). Continued research with larger samples that include both men and women will also help clarify whether gender-specific effects of milk on recovery exist in a field setting.
Several previous studies have also examined the use of isocaloric or isocarbohydrate amounts of milk compared with a high carbohydrate replacement drink (7,10,14). To maximize practical applications to collegiate athletes, participants in this study consumed a carbohydrate electrolyte beverage (CE) typically used by team sport athletes during recovery and an isovolumetric amount of chocolate milk. Thus, a limitation of this study is that the caloric and carbohydrate content of the recovery beverage was less than that of the chocolate milk. To consume isocaloric or isocarbohydrate amounts of these beverages, participants would have needed to drink nearly twice the amount of CE as chocolate milk, or on average, 4–6 cups per feeding. It is unlikely that this amount could be comfortably consumed, and it would not have practical significance, because it is more than is usually consumed per feeding and more than was given in any of the other studies (7,10,14). In addition, providing a different volume of fluid each day could affect hydration status and feelings of gastrointestinal comfort, possibly influencing the results in other ways. However, research comparing the consumption of a high carbohydrate replacement drink with an isocaloric amount of chocolate milk reported that participants cycled longer on consuming the milk (14), even though both beverages provided equivalent energy, suggesting that differences in the carbohydrate type or fat content might alter the effects of the beverages on recovery. Further, comparison of isocarbohydrate (7,10) amounts of milk and a high carbohydrate replacement drink reported that time to exhaustion was no different (10) or longer with the milk (7), again suggesting that other factors, beyond the energy and amount of carbohydrate provided by the milk, may account for its effectiveness. Further work is needed to clarify these findings.
This study suggests that low fat chocolate milk may be as effective as an isovolumetric amount of CE at promoting recovery between preseason practice sessions. Practitioners can suggest drinking low fat chocolate milk immediately after exercise and throughout the recovery period in a volume that will help athletes obtain about 1 g carbohydrate per kilogram body weight per hour. They can encourage their athletes to substitute low fat chocolate milk for full fat dairy products and suggest foods such as fruit smoothies or cereal made with chocolate milk to improve diet quality while promoting recovery.
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