Researchers recruited 72 healthy, physically active men ages 18-35 and divided them into three groups. During the study, each participant drank still water (Highland Spring) and three of the following beverages that were randomly picked for them: cola (Coca-Cola), diet cola (Diet Coke), sports drink (Powerade), oral rehydration solution (Dioralyte), orange juice (Tesco Everyday Value), Lager beer (Carling), hot black coffee (Nescafe Original), hot black tea (PG tips), cold black tea (PG tips), full fat milk (3.6% fa) or skim milk (0.1% fat).
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By tracking the men’s urine output after each beverage, the scientists could assign a beverage hydration index, or BHI, to each drink.
The St. Andrews team found that beverages with a little sugar, fat or protein did a better job than water of keeping the men hydrated.
Skim milk — which has a little fat, some protein, the sugar lactose and some sodium— did the best job of hydrating the participants. The sodium in milk "acts like a sponge and holds onto water in the body and results in less urine produced," CNN wrote.
Fruit juices and colas had higher hydration in first four hours, however, don’t hydrate as well-long term because of their high concentrations of sugar, the researchers found. When that sugar enters the small intestine, water is pulled from the body to dilute it. That results in dehydration.
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The researchers ranked the 13 beverages they tested, from most hydrating to least over a four hour period:
- Skim milk
- Oral rehydration solutions (like Pedialyte or Liquid I.V.)
- Full fat milk
- Orange juice
- Diet Cola
- Cold tea
- Sports drink
- Still water
- Sparkling water
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