Electrolyte-free milk protein solution influences sodium and fluid retention in rats

Milk is an effective post-exercise rehydration drink that maintains the net positive fluid balance. However, it is unclear which components are responsible for this effect. We assessed the effect of milk protein solution (MPS) obtained by dialysis on body fluid retention. Milk, MPS, milk electrolyte solution (MES), sports drink and water were administered to male Wistar rats at a dose of 6 ml/rat after treadmill exercise. Total body fluid retention was assessed by urine volume 4 h after administration of hydrating liquids. The rate of gastric emptying was evaluated by a tracer method using (13)C-labelled acetate. Plasma osmolality, Na and K levels, and urinary Na and K were measured by HPLC and osmometry, respectively. The gastric emptying rate was not delayed by MPS. During 4 h of rehydration, cumulative urine volumes differed significantly between treatment groups (P < 0·05) with 4·9, 2·2 and 3·4 ml from water-, milk- and MPS-fed rats, respectively. Thus, MPS elicited 50 % of the total body fluid retention of milk. Plasma aldosterone levels were significantly higher in MPS- and milk-fed rats compared with water-fed rats. Plasma osmolality was maintained at higher levels in MPS-fed rats than in water- and MES-fed rats (P < 0·05). Cumulative urine Na excretion was also suppressed in the milk- and MPS-fed groups compared with the MES-fed group. Our results demonstrate that MPS obtained by dialysis clearly affects net body water balance without affecting gastric emptying after exercise. This effect was attributed to retention of Na and water, and maintenance of plasma osmolality.