When workouts leave you drenched, or when it's so hot outside that your clothes cling to your body like Saran wrap, start drinking. If you don't rehydrate, your body and brain can suffer; mild dehydration can tank your mood, concentration and energy levels.
But not all fluids are created equal—and water isn't always the best beverage for the job.
“When we drink, fluids are not available to the body immediately but take some time to be absorbed from the gut into the bloodstream,” says Bridget Benelam, a public health researcher with the British Nutrition Foundation. How fast this happens is determined, in part, by what's in the fluid.
That’s because when you sweat, water isn’t the only thing your body is losing.
Human sweat contains many different metabolites, including lactate, amino acids and fats, as well as sodium. “Drinks containing some carbohydrate in the form of sugars and electrolytes, usually sodium, can be absorbed by the body more quickly than pure water and therefore allow rehydration to happen more rapidly,” Benelam says. Think Gatorade.
On the other hand, even though your body is losing sodium and some other things as you sweat, water alone really will get the job done for the typical sweaty adult, says Lawrence Armstrong, director of the Human Performance Laboratory at the University of Connecticut. “Virtually no studies have shown benefits of sport drinks or carbohydrate-containing beverages unless you’re exercising continuously for more than 50 or 60 minutes,” he says.
Assuming you eat normally and aren't on a super-restrictive cleanse or elimination diet, you’re probably not at risk for any sodium or electrolyte shortages, Armstrong explains. The volume of liquid you consume is the important thing. “During exercise, the average person ought to be drinking about a half a quart of water every 30 minutes, or a full quart in an hour, to replace the fluids they’re losing,” he says. (If you’re worried you’re not drinking enough H20, monitor your urine. If it’s dark yellow, you need to drink more.)
But if you’re the type who does exercise vigorously for long periods, “a complex source of nutrients is likely to have a positive impact on fluid retention,” says Ben Desbrow, associate professor of sports nutrition at Griffith University’s School of Allied Health Sciences in Australia. Desbrow’s research has compared different milk-based beverages to water and sports drinks, with a surprising champ.
“Milk is an ideal recovery beverage,” he says. “It is well retained and is a great source of protein, carbohydrate, vitamins and minerals.”
More research backs him up on this. A 2016 study in the American Journal of Clinical Nutrition found that milk—full-fat, but especially skim—was more hydrating than water, sports drinks, coffee, tea and several other common beverages. Sports drinks do get the job done—especially if you’ve really been pushing yourself and you’re sweating heavily. But milk outperforms them.
You don’t have to pound a half-gallon of it after a workout. Getting some nutrients along with your H2O is the important thing, Desbrow says. You could drink an 8-ounce glass of milk followed by water. Or, if you’re not interested in dairy, drinking water with a little food will help your body absorb more water in a short period of time.
You can take a different route to rehydration, too. Rather than worry about what you’re drinking, just make sure your beverage is accompanied by a bite to eat, like a granola bar or a sandwich. Another recent study found that the type of beverage you reach for to rehydrate doesn’t really matter if you’re drinking it with food.