Morning Hydration Status and Running Performance in Female Athletes following Nighttime Consumption of Chocolate Milk

Abstract

The nutrient content and fluid retentive capacity of chocolate milk (CM) is beneficial for athletes. However, nighttime consumption of CM has an unknown impact on next morning endurance performance. Preliminary evidence suggests that carbohydrate, casein and whey proteins are efficiently metabolized at night; therefore, nighttime consumption of CM may prime athletes for next morning competition. PURPOSE: To investigate the influence of nighttime consumption of CM on next morning hydration status and running performance in female endurance athletes. METHODS: Eight female runners (age, 31.1 ± 2.7 years; running 30.6 ± 10.8 miles/week; VO2max, 53.3 ± 2.7 ml/kg/min) participated in this randomized, crossover, double-blind study. After one familiarization trial, two experimental trials were completed. Participants consumed either 355 ml of CM or flavor-matched placebo (PLA) 2 hours after the last meal and 30 minutes before sleep. The next morning, participants arrived to the laboratory fasted. Urine output (UO), urine specific gravity (USG), and nude body weight was measured. Following a standard warm up, participants drank 236 ml of water and ran a 10-km treadmill time trial (TT). Participants blindly controlled speed. The only known measure was distance completed. Finish time, UO, USG, and nude body weight were measured post-exercise. Repeated measures analysis of variance and Students T-tests were used to compare results (SPSS, Cary, NC). Significance was set to p<0.05. Values are reported as means ± SD. RESULTS: TT finish time was faster for CM (50.7 ± 5.0 minutes) than PLA (51.9 ± 6.4 minutes), but not significantly (p=0.214). Heart rate, Rating of Perceived Exertion, and average speed during the TT did not vary significantly between CM and PLA. No differences were found for UO (p=0.337), changes in body weight (p=0.210), or USG (p=0.529). First morning USG did not vary between CM and PLA (1.012 ± 0.008 vs. 1.014 ± 0.006; p=0.452). CONSLUSION: Nighttime consumption of CM does not significantly improve next morning 10K performance in female endurance athletes, although finish time was faster with CM than PLA. After consuming a nighttime beverage (355 ml), athletes woke up adequately hydrated (USG of ≤ 1.020). The nighttime beverage did not have a detrimental effect, so it may be a useful solution to help athletes meet nutritional and hydration needs for race mornings.