Effect of milk protein addition to a carbohydrate-electrolyte rehydration solution ingested after exercise in the heat

Journal Title (Medline/Pubmed accepted abbreviation): Br. J. Nutr.
Year: 2011
Volume: 105
Page numbers: 393-399
doi (if applicable): 10.1017/S0007114510003545

Summary of Background and Research Design

Background:Insulin is a hormone that stimulates body cells, including muscle, to take up glucose from the blood stream. Patients with type 1 diabetes do not produce insulin and therefore must administer it themselves in conjunction with a meal. During exercise, muscle takes up glucose at a faster rate than at rest. In healthy subjects, the liver replenishes blood glucose via tapping into glycogen stores or gluconeogenesis. However, patients with type 1 diabetes often do not replenish glucose at an adequate rate and they experience hypoglycemia. To circumvent hypoglycemia, lower injections of insulin at the pre-workout meal have been proposed. However, this necessitates the anticipation of exercise. Ingesting additional carbohydrate is another possible solution. The patient’s physical fitness, exercise intensity, exercise duration, current insulin levels (effected by time of day), and current glucose levels are all factors in the calculation of how to control metabolism, and it often requires several attempts for the individual to learn what his or her body requires to prevent hypoglycemia. This can be very discouraging and/or dangerous.

Hypothesis: A beverage composed of carbohydrates and protein will yield better water retention than an isocaloric beverage with just carbohydrates.

Research design: Randomized, counterbalanced, cross-over design. Subjects were blinded to the treatment.

Subjects:8 healthy males, age 21 ± 3.

Treatments:The carbohydrate beverage (Trial C) contained 65 g/L carbohydrates (35 g/L glucose and 30 g/L maltodextrin), while the carbohydrate + protein beverage (Trial CP) contained 40 g carbohydrate/L (33.5 g/L glucose, 5 g/L maltodextrin, 1.5 g/L lactose) and 25 g/L protein (from milk).

Experimental protocol: Subjects were first familiarized with the training protocol and then completed it two times, once with each recovery beverage. On test days, participants arrived at the laboratory after an overnight fast yet euhydrated. Subjects cycled on a cycle ergometer at 35 ± 0.1ºC and 50.9 ± 2.1 % relative humidity until they lost approximately 1.6% of their pre-exercise body mass. This took 57 ± 7 min. Fifteen min after exercise they provided a urine sample and were subsequently weighed for their post-exercise mass. Subjects then consumed a 16.6 ± 6 ºC beverage with a volume (in L) equivalent to 150% of their loss of body mass (in kg) in 4 doses over 45 min. Fifteen min after completion of the beverage, urine samples were collected every hour for 4 hrs. Body mass was measured after the final urine sample. Questionnaires were administered at 6 points during the experiment regarding feelings (thirst, bloatedness, energy, etc.).

Summary of research findings:
  • During exercise, subjects lost 1.43 ± 0.23 kg (similar for trials C and CP), representing 1.9 ± 0.2 % of their initial body mass.
  • Total urine output for the 4 hours after rehydrating with the beverages was greater during trial C than trial CP (1212 ± 310 mL for trial C and 931 ± 254 mL for trial CP).
  • Retention of water was greater during trial CP than trial C. After 4 hrs, 43 ± 15% of the carbohydrate beverage was retained and 55 ± 12% of the carbohydrate-protein beverage was retained.
  • Urine osmolality was highest 15 min after exercise and decreased after rehydration. Osmolality was lower at 2 and 3 hrs after rehydration during trial C compared to trial CP, indicating that their urine was less concentrated and they were voiding more water.
  • There were no differences between beverages for the subjective variables.

Interpretation of findings/Key practice applications:

An energy density-matched rehydration beverage that contained milk protein in addition to carbohydrate was retained better after an endurance workout in a hot environment. The authors speculate that reduced rate of gastric emptying could explain this


The article did not state the normal exercise habits of the subjects. Someone who is used to outdoor endurance activities may respond differently to rehydration beverages than a sedentary individual. Also, the exercise was performed in a fasted state, maybe causing gastric emptying to be faster than normal. It would be interesting if they actually measured gastric emptying rate (by adding a compound to the beverage and then analyzing it in blood or urine) to corroborate their hypothesis.
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