Plain water isn't the most hydrating drink around, according to a new study from health researchers at the UK's Loughborough University. Better are beverages with some fat, protein, or sugar. But not TOO much sugar. From CNN:
For example, milk was found to be even more hydrating than plain water because it contains the sugar lactose, some protein and some fat, all of which help to slow the emptying of fluid from the stomach and keep hydration happening over a longer period of time.
Milk also has sodium, which acts like a sponge and holds onto water in the body and results in less urine produced.
The same can be said for oral rehydration solutions that are used to treat diarrhea. Those contain small amounts of sugar, as well as sodium and potassium, which can also help promote water retention in the body.
"A randomized trial to assess the potential of different beverages to affect hydration status: development of a beverage hydration index" (American Journal of Clinical Nutrition)