Abstract
Background: Beginning an exercise session euhydrated is important for performance and health. Rapid and adequate rehydration is important for many athletes, especially those partaking multiple sessions of exercise each day, or those involved in weight category sports. The macronutrient and electrolyte concentration of the fluid ingested following exercise can affect the amount retained within the body and so can influence hydration status. However, the optimal rehydration beverage composition is currently unknown. Electrolytes and carbohydrates have been thoroughly studied, however the role of protein in rehydration is yet to be determined.
Objective: To compare the effect of a commercially available whey protein beverage against trim milk, in terms of rehydration after exercise induced dehydration.
Design: Ten healthy active males aged 23.1 (1.5) years provided written informed consent prior to participating in the study. All trials commenced between 17:30 and 18:00 hours and were separated by at least one week. For the two trials, participants cycled in the heat (35oC and 65% RH) until 1.89 ± 0.36% of their body mass was lost. They then consumed either whey protein or trim milk in a randomised order replacing 150% of body mass losses in the hour post-exercise. Urine samples were collected pre-exercise, 1 hour post, 2 hours post, and first void of the following morning.
Results: Urine specific gravity values the following morning were not different between the whey trial (1.020 ± 0.004) and the milk trial (1.021 ± 0.005) (p=0.684). Total urine output was also not different between the whey trial (1498.0 ± 245.6mL) and the milk trial (1325.5 ± 426.4mL) (p=0.150). At the end of the study, compared to baseline, net fluid balance was negative for the whey trial (-733 ± 223mL) (p<0.001), and the milk trial (-544 ± 362mL) (p<0.001), and between the two drink trials, final net fluid balance was not different (p=0.088).
Conclusion: The main finding of this study is that a whey protein beverage is no better or worse at rehydrating than a trim milk beverage. Uniquely, the present study shows that athletes who exercise in the evening and follow the current rehydration recommendations of consuming 1.5L for every 1kg body mass lost during exercise, were likely to wake up the next day in a hypohydrated state. This would mean that more fluid would need to be ingested before beginning another bout of exercise. Previously such an overnight protocol has not been utilised in rehydration studies.