Milk has been shown to be an effective post-exercise rehydration solution, but the specific effects of milk protein on rehydration after exercise are unknown. This study examined the effects of milk protein on rehydration after exercise in the heat, via the comparison of carbohydrate and carbohydrate-milk protein solutions matched in terms of energy density and electrolyte content. Eight male subjects (mean±SD: age 21±3 years, height 1.78±0.08 m, body mass 75.7±11.6 kg) lost 1.9±0.2% of their initial body mass via intermittent exercise in a warm environment (35±0.1°C, 50.9±2.1% RH). Subjects then rehydrated with 150% of their body mass loss over 1 h, with either a 65 g/l carbohydrate solution (C) or a 40 g/l carbohydrate, 25 g/l milk-protein solution (CP). Urine samples were collected before and after exercise and for 4 h after rehydration. After rehydration, total cumulative urine output was greater for trial C (1212±310 ml) than trial CP (931±254 ml) (p<0.05), while total drink retention was greater after ingestion of drink CP (55±12%) than drink C (43±15%; p<0.05). At the end of the study period, whole body net fluid balance (estimated from fluid lost through sweat and urine production and gained through drink ingestion) was less negative for trial CP (−0.26±0.27 l) than trial C (−0.52±0.30 l), and although net negative for both trials, was significantly negative only after ingestion of drink C (p<0.05). These results suggest that after exercise in a warm environment, a carbohydrate-milk protein solution is better retained than a carbohydrate solution, when solutions are matched for energy density and electrolyte content. Some of the beneficial post-exercise rehydration effects of milk might be attributable to the milk protein.
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