Why Everything You Thought You Knew About Hydration Is Probably Wrong

Written by Will Clower, PhD

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We've all been told that to keep hydrated, the best drink is always water, and we should consume at least 64 ounces per day. Conversely, to keep from dehydrating, the standard advice is to avoid caffeinated beverages because they're diuretics that cause you to lose water. Ditto for alcohol.

Unfortunately, most of the things we thought we knew about healthful hydration no longer seem to be certain.

For example, the advice to drink at least eight 8-ounce glasses of water per day turned out to have very little evidence supporting it. The idea that coffee and other caffeinated beverages dehydrate you also seems to be incorrect, given the conclusions of this meta-analysis that any concerns "regarding unwanted fluid loss associated with caffeine consumption are unwarranted."

So what should you really be drinking to stay hydrated?

The best drinks to keep you hydrated:

Given this confusion, a recent study by researchers at the School of Sport, Exercise and Health Sciences at Loughborough University in the UK is very important. They did a straightforward study by creating a "hydration index." In other words, they measured many drinks to show how well your body is hydrated by each of them.

Their test drinks included still and sparkling water, soda (Coke and Diet Coke), sports drink (Powerade), Orange Juice, lager beer, black coffee, black tea (hot and cold), milk (skim and normal), and an oral rehydration drink designed for children with prolonged diarrhea.

It's important to note that the study was partly funded by the European Hydration Institute, which is funded by Coca-Cola (hence the study's choice of test sodas and sports drink). But if the soda giant was looking to juice a study to get some kind of marketing boost, the results didn't fall their way. In fact, the researchers found that the best drinks for hydration were actually milk (both versions) and the oral rehydration drink.

Their sports drink, which is manufactured and marketed specifically for rehydration, was actually worse than water for this purpose. In this case, the source of funding was inversely associated with a positive outcome.

Other interesting findings were that the drinks normally thought to be diuretics (in other words, drinks that would make you urinate more, losing more water) did not have that effect. Alcohol and caffeine have typically been viewed as diuretics, but the lager beer, despite its alcohol, was not a net diuretic. Neither was caffeinated coffee or tea.

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What explains these findings?

Alcohol and caffeine have been found to be diuretics in prior studies. But in this one, the lack of diuresis may come down to the simple, common-sense rule that is applied to almost everything: A little may be good, but a lot is not.

The authors point out that the acute doses of caffeine are unlikely to have a diuretic effect until they exceed ~300 mg. In their study, the amount of caffeine given to the subjects ranged between 96 and 212 mg.

As for alcohol, this study has subjects drink 2 pints of 4 percent alcohol lager, which did not increase urine output compared to the other drinks. In other words, the water in the beer was offset by any diuretic effect of the alcohol. This was also true for athletes who had a beer after training. But as in the case of caffeinated beverages, as the alcohol concentration increased above 4 percent, the diuretic effect increased as well.

Surprisingly, milk was more likely to sustain hydration than any other drink—and this wasn't the first study to show this. The authors point out that this may be because of the higher energy content in the drink. Any food or drink containing fat, fiber, or protein will slow the rate that the stomach empties into the small intestine, effectively slowing the rate of absorption.

The authors' suggestion that milk proteins may contribute to the reduced diuretic effect is also supported by other research concluding that "milk protein has been shown to be more effective for post-exercise rehydration than an isoenergetic amount of carbohydrate."

The bottom line:

Even though this is a small study of 72 subjects assessing a small number of drinks—and it definitely needs to be replicated—it's a really clarifying start to create an index for hydration. This opens the door to follow-up research that can do at least two things:

  1. Study more kinds of drinks so consumers can get a better picture of those drinks that help hydration and those that don't.
  2. Study different concentrations of alcoholic and caffeinated drinks to show the inflection point where they turn from hydrating to dehydrating.

In the meantime, while we're waiting on new studies, at least we know this: Milk seems to be the most hydrating drink of the current hydration index, and beer and coffee probably aren't dehydrating until you overconsume them.

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