Most Americans have heard that they should drink eight glasses of water a day to stay hydrated, but there is surprisingly little data to support this advice. 

 

大多数美国人都曾听说日饮八杯水对补水的重要性。出人意料是，这个建议基本没有任何数据支持。

 

But now, a new "beverage¹ hydration index" provides evidence-based suggestions for how to most efficiently² hydrate. The index was developed from a British study published in December that tracked how long 13 common beverages³ remain in the body after being consumed.

 

The hydration index is modeled after the well-known glycemic index, which measures how the body responds to the carbohydrate⁴ content of different foods. (The glycemic index is used to help individuals keep their glucose-insulin response under control.) The guiding principle behind the new hydration index is that some fluids last longer in your body than others, providing more hydration. After all, if you drink a cup of water and then immediately excrete half that amount in your urine, you haven't added eight ounces to your water supply, but only four. 

 

The British study determined⁵ the hydration index of 13 common beverages by having the participants, 72 males in their mid-20s, drink a liter of water as the standard beverage. The amount of water still remaining in subjects' bodies two hours later -- that is, not voided in urine -- was assigned a score of 1.0. All other beverages were evaluated in a similar manner, and then scored in comparison to water. A score higher than 1.0 indicated that more of the beverage remained in the body as compared to water, while a score lower than 1.0 indicated a higher excretion rate than water. 

 

The results showed that four beverages -- oral rehydration solution, like Pedialyte; fat-free milk; whole milk and orange juice -- had a significantly higher hydration index than water. The first three had hydration index scores around 1.5, with orange juice doing slightly better than water at 1.1. Oral rehydration solutions are specifically formulated⁶ to combat serious dehydration⁷ such as that resulting from chronic⁸ diarrhea.

 

Why is milk so efficient at rehydration? "Normally when you drink, it signals the kidneys to get rid of the extra water by producing more urine," Dr. Maughan said. "However, when beverages contain nutrients⁹ and electrolytes like sodium¹⁰ and potassium, as milk does, the stomach empties more slowly with a less dramatic effect on the kidneys."

 

Perhaps surprisingly, drinks containing moderate amounts of caffeine and alcohol or high levels of sugar had hydration indexes no different from water. In other words, coffee and beer are not dehydrating, despite common beliefs to the contrary, and regular soda¹¹ can hydrate you just as well as water.

 

The hydration index could prove useful when making decisions about what beverages to consume and when. For example, if you're going on a long drive and won't have access to fluids (or to bathrooms), you'd be smarter to drink milk with its high hydration index rather than water or iced coffee. But don't forget that milk has many more calories than water, so don't overdo¹² it, either. 

 

While severe dehydration is rare except in heavy exercise, extreme environments and disease, studies have shown that heat and dehydration can contribute to increased mortality rates during hot weather. "Mortality increases sharply during heat waves, mostly because people don't drink enough to compensate¹³ for their increased fluid losses," Dr. Maughan said.

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