Female drinking a bottle of water silhouette.

Weekend Reads | The Best Beverage for Staying Hydrated

by Kevin Schofield

This weekend’s read is particularly topical now that warmer weather has arrived in Seattle. It asks (and largely answers) the question: What’s the best beverage to drink in order to stay hydrated?

Staying hydrated is important, especially if you are engaging in strenuous physical activity. But even in our everyday lives, many of us don’t consume enough liquid; that can affect our physical and mental performance, as well as put us at a higher risk for a range of urologic, gastrointestinal, circulatory, and neurological diseases. But faced with a dizzying array of beverages, it helps us to know if some are better at keeping us hydrated (or if the liquid just passes straight through and doesn’t stick around). 

Many of us have heard lectures on the conventional wisdom around beverage choices, particularly that soda, caffeine, and alcohol are diuretics that cause our bodies to shed more liquid — thus making us even less hydrated the more we drink them. On the flip side, sports drinks and so-called “oral rehydration solutions,” such as Pedialyte, are supposedly designed to rehydrate us and replace electrolytes and other essential nutrients our bodies need by helping us to retain fluids or slowing the rate at which they are emptied from our stomach and absorbed through our small intestine. But marketing and conventional wisdom aside, what really works? A group of academic researchers in the United Kingdom set out to get some definitive answers, and their report provides some interesting and surprising insights and also points out how difficult it is to do this kind of study well.

The first question the researchers had was, “How do you measure the effect on body hydration that drinking a particular liquid has?” To answer that, they defined a “body hydration index” (BHI), essentially a measure of how much of a liquid that is consumed remains in the body two hours later. Of course, one of the tricks to conducting a study like this is to make it an “apples to apples” comparison. They recruited 75 willing test subjects and subjected them to the same rigorous schedule: fast overnight, pee first thing in the morning, get into a “fully hydrated” state by drinking half a liter of water, wait half an hour, pee again, drink 1 liter of a test beverage over 30 minutes, and then collect and measure the amount of urine excreted over the next two hours.

But wait, there’s one more catch: The test beverages ranged from 88% water to 100% water, so the results had to be re-scaled to account for the proportion of water actually consumed by each test subject.

The results were interesting and, in some cases, surprising. The drinks with the highest fluid retention were skim milk, whole milk, “oral rehydration solution,” and orange juice. The worst was coffee, followed by water. But coffee was barely worse than water, and most of the other beverages tested were only slightly better than water. Among the surprises were that moderate amounts of coffee and alcohol made little to no difference, and none of the beverages lived up to the hype of being a powerful diuretic that would sap our bodies of its precious water. 

What seemed to provide the biggest benefit were the drinks with substantial amounts of sodium, potassium, or electrolytes. In fact, the researchers suggest that the reason cola and beer aren’t that bad for hydration is because the fluid-removing caffeine and alcohol were balanced out by the electrolytes, sodium, and potassium they also contain. Coffee, on the other hand, has a lot of caffeine but none of the positives; thus, it shouldn’t be too big of a surprise that it performed the worst.

The researchers are refreshingly transparent on the many limits of their study. First and foremost, all the test subjects were healthy, physically active men between the ages of 18 and 35 and without health conditions that might affect their hydration level (e.g., kidney disease), so we can’t extrapolate these results to women, children, older populations, or people who might have a more urgent need to monitor and control their hydration. Also, the researchers chose not to vary the amount of liquid consumed based upon the test subject’s body mass, so a 120-pound man and a 220-pound man would drink the same amount even though their bodies have vastly different capacities for retaining fluid. Third, they started everyone fully hydrated, and didn’t gather data on what is perhaps a more important scenario: how these drinks affect the hydration level of someone who is currently dehydrated and needs more fluid quickly. Finally, they weren’t able to precisely identify which ingredients in the drinks made the most difference in helping someone retain liquid for a longer period of time.

That said — and with a nod to the good advice that we should all consult medical professionals about our own personal situations — it turns out that a glass of skim milk or orange juice does a pretty good job of hydrating us and keeping us hydrated for a longer period of time and compares well with expensive “oral hydration solution.” And while we might want to lay off the coffee, most of the other drinks, including beer, seem to be no better or worse than water itself when it comes to hydration.

A randomized trial to assess the potential of different beverages to affect hydration status: development of a beverage hydration index

Kevin Schofield is a freelance writer and publishes Seattle Paper Trail. Previously he worked for Microsoft, published Seattle City Council Insight, co-hosted the “Seattle News, Views and Brews” podcast, and raised two daughters as a single dad. He serves on the Board of Directors of Woodland Park Zoo, where he also volunteers.

📸 Featured image by FocusStocker/Shutterstock.com.

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