Milk held on to fluid as well as a rehydration solution
Given a litre each of 13 drinks, 72 men retained as much fluid over four hours from full-fat and skimmed milk as from an oral rehydration solution. Both beat still water.
What the study found
Milk matched a rehydration solution for holding on to fluid
The best hydration drink in the test was already in the fridge. Given a litre each of 13 beverages, 72 men retained as much fluid over four hours from full-fat and skimmed milk as from an oral rehydration solution, and all three beat still water. Nothing else tested did.
The caveat · Milk's protein and fat may do it
Fully hydrated, fasted men, all 72 of them male, drinking a litre in half an hour. Retention was measured as urine volume over four hours, not as anything a person would notice. And milk brings protein, fat and lactose along with its sodium and potassium, so what did the holding is not separated out here.