This is the best beverage to drink to stay hydrated on a hot day

A big glass of milk on a scorching day isn’t as crazy as you’d think. (Getty Images)

We’re about halfway into August the summer’s heat doesn’t look ready to let up any time soon. We all know it’s important to stay hydrated when the weather is steamy, but it’s less clear what’s the best way to do that.

Even our most consistent advice, to drink eight glasses of water a day, is supported by less actual evidence than most of us might realize. A 2012 study published in the British Medical Journal suggested that the best plan of action is not to count out glasses of water but to simply drink when you are thirsty –and when it’s hot, you’ll probably be thirsty more often.

“It really depends on your activity level, age and size, but I think at least six cups of fluids a day would be a safe recommendation,” Toronto registered dietitian Abby Langer tells Yahoo Canada Style. “If you’re active, you’ll likely need eight or more cups.”

Dietitians of Canada says that most people need to take in two or three litres of water daily, with those who are especially active needing more. But those fluids can come from a variety of liquids including coffee and tea, and from foods that are high in fluids.

ALSO SEE: Everything you need to know about heatstroke and heat exhaustion

The uncertainty about how much to drink daily doesn’t mean we don’t have any research to help us figure out which beverages are the best to reach for on hot summer days. Earlier this year a “beverage hydration index” based on a British study published in 2015 was released. The index took information from the study on how long 13 different beverages remain in the body after consumption.

The principle behind the index is a simple one, as the New York Times reported in June: the fluids that stay in the body longer provide more hydration. Water was the standard for the study, and all the other beverages were scored based on how long they stayed in the bodies of the mid-20s men who drank them in comparison to H20.

Water, of course, is a great source of hydration. “Unless you’re exercising strenuously – think training for a marathon – water is the best options for keeping hydrated,” says Langer. “Other fluids, juicy fruits, soup, and even ice cream and popsicles can hydrate as well, but water contains zero sugar and calories, making it my number one pick.”

But the British researchers found that four other liquids did an even better job than water of sticking around in the body. Oral rehydration solutions like Pedialyte did a predictably good job of hydrating, considering that’s why they exist: they’ve been formulated to rehydrate the body after illnesses that bring about fluid loss through vomiting or diarrhea. These solutions have an important role but aren’t a practical or appealing everyday hydration choice.

Perhaps more surprising was that both whole-milk and fat-free milk also beat out water for hydration, according to the study findings. The researchers gave both milk formulas a hydration index score of 1.5, the same as oral rehydration solutions, compared to water’s score of 1.0.

ALSO SEE: 10 non-boring ways to stay hydrated in the heat

Milk is great at rehydrating the body because it contains more than just water, researcher Ronald J. Maughan told the Times. Namely the nutrients and electrolytes in milk slow down the work the kidneys do to produce more urine when we take in extra fluids, keeping it in the body for longer.

As for sports drinks, the British Medical Journal study mentioned above said you’re just as well off with water. Sports drinks do have electrolytes, which may be part of why milk appears to hydrate well. But they also have unneeded sugar and calories and little nutritional benefit, unlike milk.

Another surprising bit of information? Coffee and beer won’t make you lose fluids as quickly as you might think. Caffeine has diuretic properties but those aren’t a significant factor at the concentration contained in most coffee-based drinks, Maughan told the Times. But these drinks do have other downsides, like higher calorie counts than water alone. And of course, the effects of too much caffeine or alcohol add up.

And while you’re thinking about fluids to stay hydrated, don’t forget about food. We get about a fifth of our water intake from solid foods, mostly fruits and vegetables. This means it’s important to eat even if it feels far too hot to do so, and to make an effort to choose high-water foods like berries, watermelon, grapefruit, cucumber and lettuce.

Finally, what about the idea that hot liquids actually hydrate better than cold ones–is that true? “Nope,” says Langer. Once it’s consumed it’s all the same temperature as your body no matter how hot or cold it is going in, she says.

What do you drink to stay cool on a hot day? Let us know by tweeting @YahooStyleCA.