The ingestion of low-fat milk has been shown to be more effective at restoring fluid balance after exercise-induced dehydration than the ingestion of a commercially available carbohydrate–electrolyte sports drink(1). More recently, it has been shown that after exercise-induced dehydration, the inclusion of 25 g/l milk protein in a carbohydrate–electrolyte rehydration solution increased drink retention in comparison with an isoenergetic, electrolyte content matched carbohydrate solution(2). This suggests that the protein present in milk (~36 g/l) accounts for at least some of the increased drink retention previously reported. It is currently unknown whether there is a dose-response effect of milk protein on drink retention after exercise-induced dehydration. The aim of the present study was to investigate this.
Eight males [mean (sd): age 22 (sd 2) years, height 1.77(sd 0.08) m, body mass 76.96(sd 8.73) kg] completed intermittent exercise in a hot environment [35.0(sd 0.1)°C, 51.8(sd 5.9) relative humidity] until they lost 1.83(sd 0.10)% of their initial body mass. Subjects then ingested a volume of drink in litres equivalent to 150% of their body mass loss in kg. This drink was provided in four aliquots of equal volume at 15 min intervals (0, 15, 30 and 45 min) over a 1 h rehydration period. Subjects then remained in the laboratory for a further 4 h. During each trial, subjects consumed one of the three drinks: a 60 g/l carbohydrate solution (C); a 40 g/l carbohydrate, 20 g/l milk-protein solution (CP20); or a 20 g/l carbohydrate, 40 g/l milk-protein solution (CP40). Drinks were matched in terms of energy density, as well as Na (~20 mmol/l) and K (~5 mmol/l) content. Urine samples were collected before and after exercise, after rehydration and every hour during the 4 h recovery period. Urine samples were measured for volume, osmolality and Na and K concentration. Trials were administered in a double blind, randomised crossover design.
Total cumulative urine output after rehydration was greater for trial C [1150(sd 245) ml] than for trial CP20 [857(sd 270) ml] (P=0.007) and CP40 [769(sd 129) ml] (P=0.006), with no difference between CP20 and CP40 (P=1.000). As a result, total drink retention was greater for CP20 [58(sd 9)%] (P=0.002) and CP40 [64(sd 7)%] (P<0.001) than C [43(sd 7%] (P=0.008), but there was no difference between CP20 and CP40 (P=1.000). At the end of the study period, whole-body net-fluid balance (estimated from fluid lost through sweat and urine production and fluid gained through drink ingestion) was less negative for trials CP20 [−203(sd 315) ml] (P=0.029) and CP40 [−97(sd 146) ]l) (P=0.001) than for trial C [−487(sd 149) ml], but there was no difference between CP20 and CP40 (P=1.000). Although the mean net-fluid balance was negative for all trials at the end of the study, it was only significantly negative after ingestion of drink C (P=0.002).
This study further demonstrates that after exercise-induced dehydration, a carbohydrate–milk protein solution is better retained than a carbohydrate solution, when solutions are matched in terms of energy density, as well as Na and K content. The results also suggest that there is no dose-response relationship between milk-protein ingestion and drink retention after exercise-induced dehydration, at least in the concentrations of milk protein used in this study.