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A fluid-retention effect is required for beverages that are designed to prevent dehydration. That is, fluid absorbed from the intestines should not be excreted quickly; long-term retention is desirable. Here, we focused on the effect of milk protein on fluid retention, and propose a new effective oral rehydration method that can be used daily for preventing dehydration. We first evaluated the effects of different concentrations of milk protein on fluid retention by measuring the urinary volumes of rats fed fluid containing milk protein at concentrations of 1, 5, and 10%. We next compared the fluid-retention effect of milk protein-enriched drink (MPD) with those of distilled water (DW) and a sports drink (SD) by the same method. Third, to investigate the mechanism of fluid retention, we measured plasma insulin changes in rats after ingesting these three drinks. We found that the addition of milk protein at 5 or 10% reduced urinary volume in a dose-dependent manner. Ingestion of the MPD containing 4.6% milk protein resulted in lower urinary volumes than DW and SD. MPD also showed a higher water reabsorption rate in the kidneys and higher concentrations of plasma insulin than DW and SD. These results suggest that increasing milk protein concentration in a beverage enhances fluid retention, which may allow the possibility to develop rehydration beverages that are more effective than SDs. In addition, insulin-modifying renal water reabsorption may contribute to the fluid-retention effect of MPD.

Inflammation has been implicated in development of the insulin resistance that leads to elevated blood sugar levels associated with type 2 diabetes. It is reported that salsalate, a common non-steroidal anti-inflammatory drug, has been shown to decrease blood glucose concentration in some clinical study. Recently, we found that bovine milk-derived α-lactalbumin had anti-inflammatory activity caused by inhibiting cyclooxygenase-2. In this study, we investigated the effects of chronic administration of α-lactalbumin on glucose tolerance in Goto-Kakizaki (GK) rats, a model of type 2 diabetes. After 10 weeks administration of the α-lactalbumin (300 mg/kg, twice a day), oral glucose tolerance tests revealed significant decrements of blood glucose levels after glucose loading. However, significant differences of insulin levels were not observed among three GK rats groups after glucose loading. α-Lactalbumin treatment enhanced high molecular weight form of adiponectin and suppressed prostaglandin E2 levels in plasma. These results suggest that α-lactalbumin effectively decreased blood glucose levels after glucose loading in GK rat, and the decrements may be due to enhancement of adiponectin.