Effect of milk protein addition to a carbohydrate-electrolyte rehydration solution ingested after exercise in the heat
Journal Title (Medline/Pubmed accepted abbreviation): Brit J Nutr
Year: 2010
Page numbers:
doi: 10.1017/S0007114510003545

Summary of Background and Research Design

Background:During exercise, sweat losses exceeding fluid intake result in a hypohydrated state. Restoration of fluid balance after exercise requires ingestion of fluid volumes greater than the volume of sweat lost. However, to maintain fluid balance the composition of the ingested solution must allow it to be effectively retained. Solutions containing protein may have advantages over protein-free solutions in maintaining fluid retention; however, it is currently unclear how the addition of protein to a rehydration solution affects fluid retention when it is consumed after exercise-induced dehydration.

Hypothesis:The authors hypothesized that replacement of exercise-induced fluid losses (150% of body mass loss) with a carbohydrate- and protein-containing drink would lead to better fluid retention than a calorically and electrolyte-matched carbohydrate drink. 

Subjects:Healthy male subjects (N = 8) with mean ± standard deviation (SD) age = 21 ± 3 years, height = 1.78 ± 0.08 m, and body mass = 75.7 ± 11.6 kg who provided written consent and completed a medical screening questionnaire before participation were enrolled. 

Experimental design:Random, single-blind, counterbalanced, crossover  

Treatments and protocol: Body mass of subjects was measured to the nearest 50 g at the beginning of the study. Subjects then exercised on a friction-brake cycle ergometer until they had lost 1.6% of their pre-exercise body mass. Upon completion of the exercise, subjects were allowed to shower for 15 minutes, after which body mass was again measured. Additional weight loss from sweating occurred, bringing total weight loss to 1.9% of body weight. Subjects then consumed a random, blinded, rehydration drink equal to 150% of their body mass loss in 4 equal-volume aliquots every 15 minutes over a 1-hour period. Drinks contained either 35 g/L glucose plus 30 g/L maltodextrin (the carbohydrate [C] drink) or 33.5 g/L glucose, 5 g/L maltodextrin, 1.5 g/L lactose, and 25 g/L milk protein (the carbohydrate-protein [CP] drink). Subjects then provided urine samples for osmolarity analysis every hour for 4 hours, after which their body mass was again measured. Subjects also completed questionnaires of subjective feelings (eg, fullness, thirst, energy, and drink taste) before providing each urine sample. Data were analyzed using Shapiro-Wilk test, 2-way analysis of variance (ANOVA), Bonferroni-adjusted paired t tests, or Wilcoxon signed-rank test, as appropriate. Differences were considered significant for P < .05. Normally distributed data are presented as means ± SD, and non-normally distributed data are presented as medians (range).

Summary of research findings:
  • Pre-exercise body mass and urine osmolarity were not significantly different for subjects who consumed the C drink (C group) vs subjects who consumed the CP drink (CP group)
    • Body mass: 75.66 ± 11.86 kg vs 75.91 ± 11.50 kg; P = .284
    • Urine osmolarity: 538 ± 368 mosmol/kg vs 417 ± 337 mosmol/kg; P = .314
  • Mean body mass loss after exercise across the trials was 1.43 ± .23 kg, which was equivalent to 1.9% ± 0.2% of pre-exercise body mass
  • Body mass loss after exercise-induced dehydration was similar for C and CP groups (1.41 ± 0.24 kg vs 1.44 ± 0.22 kg; P = .25)
  • Fluid intake during the 1-hour rehydration period was similar for C and CP groups (2.16 ± 0.33 L vs 2.12 ± 0.36 L; P = .334)
  • Total cumulative urine output after rehydration was significantly greater for the C group vs the CP group (1,212 ± 310 mL vs 931 ± 254 mL; P < .05)
  • Total fluid retention was significantly greater after ingestion of the CP drink vs the C drink (55% ± 12% vs 43% ± 15%; P < .05)
  • Whole body net fluid balance was less negative for CP group vs C group (–0.26 ± 0.27 L vs –0.52 ± 0.30 L; the net negative change in body fluid balance was statistically significance for the C group only (P < .05)

Interpretation of findings/Key practice applications:

After an exercise-induced 1.9% reduction in body mass, the CP drink was retained better than the C drink. Whole body net fluid balance was negative for both drinks, but was significantly less negative than pre-exercise fluid balance for the C drink only. These findings emphasize the importance of drink composition to the recovery of pre-exercise fluid balance after exercise-induced dehydration. Drinking a sufficient volume of fluid to replace what was lost through sweating is essential to rehydration, but the composition of the drink influences how much fluid is retained. This study suggests that protein-based rehydration drinks may be more effective at augmenting fluid retention than carbohydrate-based rehydration drinks. One limitation of the study is that there were no measurements of gastric emptying rate of the 2 beverages. It also should be noted that the rehydration was aggressive, such that subjects had to consume over 2 L of fluid in the first hour post-exercise. It is not clear if the observed differences between the solutions would have been present in a less aggressive rehydration protocol. Finally, despite a statistically significant loss in whole body net fluid balance in the C group, the authors noted that the absolute difference between the C and CP groups (0.26 ± 0.29 L, or 0.3% body mass) was small and would be unlikely to result in differences in exercise performance between groups. Further studies are needed to determine if reduced gastric emptying plays a role in overall fluid uptake and maintaining fluid balance.  
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