Abstract: Context: Many active people finish exercise hypohydrated, so effective rehydration after exercise is an important consideration. Objective: To determine the effects of a rehydration solution containing whey protein isolate on fluid balance after exercise-induced dehydration. Design: Randomized controlled clinical trial. Setting: University research laboratory. Patients or Other Participants: Twelve healthy men (age = 21 ± 1 years, height = 1.82 ± 0.08m, mass = 82.71 ± 10.31 kg) participated. In tervention(s): Participants reduced body mass by 1.86% ± 0.07% after intermittent exercise in the heat and rehydrated with a volume of drink in liters equivalent to 1.5 times their body mass loss in kilograms of a solution of either 65 g/L carbohydrate (trial C) or 50 g/L carbohydrate and 15 g/L whey protein isolate (trial CPl. Solutions were matched for energy density and electrolyte content. Urine samples were collected before and after exercise and for 4 hours after rehydration. Main Outcome Measure(s): We measured urine volume, drink retention, net fluid balance, urine osmolality, and subjective responses. Drink retention was calculated as the difference between the volume of drink ingested and urine produced. Net fluid balance was calculated from fluid gained through drink ingestion and fluid lost through sweat and urine production. Results: Total cumulative urine output after rehydration was not different between trial C (1173 ± 481 mL) and trial CP (1180 ± 330 mL) (F1 = 0.002, P = .96), and drink retention during the study also was not different between trial C (50% ± 18%) and trial CP (49% ± 13%) (t11 = -0.159, P =.88). At the end of the study, net fluid balance was negative compared with baseline for trial C (-432 ± 436 mL) (t11 = 3.433, P = .03) and trial CP (-432 ± 302 mL) (t11 = 4.958, P = .003). Conclusions: When matched for energy density and electrolyte content, a solution of carbohydrate and whey protein isolate neither increased nor decreased rehydration compared with a solution of carbohydrate.
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