Drink eight glasses of water a day. Coffee will make you dehydrated. Drinking extra water can help you lose weight.

You've probably heard these claims about water and hydration before. But are they true?

To set the record straight, Life Kit talks to Tamara Hew-Butler, associate professor of exercise and sports science at Wayne State University; Mindy Millard-Stafford, director of the Exercise Physiology Laboratory at Georgia Tech; and Yuki Oka, a professor of biology at Caltech who specializes in thirst.

They explain the science of hydration and bust 5 common myths about water.

Myth #1: You need to drink at least eight glasses of water a day.

Is the advice of drinking eight, 8-ounce glasses of water a day to stay hydrated true? Researchers in 2002 tried to pin down studies that might support the claim by looking through multiple scientific databases — but were unable to find rigorous evidence behind it.

What we do know, says Hew-Butler, is that water is essential for our bodies. It makes up a majority of our cells and blood, flushes out waste through our urine and helps cool our bodies through sweat. Too little water, and our cells shrivel up from dehydration. Too much water, and our cells swell up from hyponatremia.

So how much water should we be drinking on a daily basis? It depends, says Hew-Butler, on your body size, your activity level, the temperature and how much you're sweating.

Because of these factors, there's no hard and fast rule for how much water you should consume. "The best advice is to listen to your body," she says. "If you get thirsty, drink water. If you're not thirsty, you don't need to drink water."

"This will protect you against the dangers of both drinking too much and drinking too little," she adds. "And this recommendation applies to [people of] all shapes and sizes in all temperature conditions."

Hew-Butler says hydration is also about the balance of water to salt. Sodium is necessary for our nerves and muscles to function. And it's what our body uses to regulate the amount of fluid it needs to stay hydrated.

Thirst plays a central role in fine-tuning that balance, she explains. "There are sensors located in your brain and they are constantly tasting your blood to see if [there's] just the right [amount of] salt. If it's too salty, then [those sensors are] like, 'Oh my God, I need more water.' When that happens, it makes you thirsty."

Then, if you drink too much water and the sensors in your brain detect that your blood is too watery, they signal a hormone that tells your kidneys to pee out the extra water, she says.

In short: you don't need an app to tell you how much water to drink or guzzle a gallon of water a day – just trust your body to let you know when to drink water, says Hew-Butler.

There are, however, a few exceptions. Some research suggests that older people may have a reduced sensitivity to thirst and a decreased amount of water in their bodies — and are therefore at higher risk of dehydration. So they may need to be more intentional about their water intake. And other research has demonstrated that drinking more water can help with certain medical conditions, including kidney disease, kidney stones and urinary tract infections.

Myth #2: Caffeine makes you dehydrated.

Another persistent myth about hydration states that caffeine is a diuretic that makes you pee, and therefore caffeinated drinks like coffee and tea don't hydrate your body. The idea is based on the findings of a study from 1928 that looked at three people. Not only is that sample incredibly small by today's standards, but the finding has not held up to more recent experiments. So consider this myth busted.

According to multiple studies, ranging from a 2003 review of research dating back to 1966 to a 2014 clinical trial that compared coffee to water ingestion in 50 men, caffeine can be a mild diuretic in large amounts for people who aren't accustomed to it. But caffeinated drinks consumed in moderation provide the same hydration as non-caffeinated drinks.

"Those studies have shown that drinking caffeinated and some low alcohol-content beverages [such as beer] are not much different than drinking water," says Millard-Stafford of Georgia Tech.

Essentially, with the exception of higher alcohol-content beverages like hard liquor, all liquids count towards hydration. As does food. The experts we spoke to say about 20% of your fluid intake comes from the food you eat, from fruits and vegetables to pasta.

Myth #3: We need sports drinks to replace salt and other electrolytes.

You might hear that you need sports drinks to replace salt and other minerals known as electrolytes (like potassium and chloride, which are also essential for our bodies) when you're active.

If you're exercising for more than an hour or so, it's likely you will need to replace the salt you're sweating out along with water, say the experts. But you don't have to do that by drinking sports drinks like Gatorade. While they can be one effective way to replace the body's salt, you can get that salt from other foods and drinks. And like thirst, you can trust your body to tell you how much you need.

Researchers have found that along with a thirst for water, humans have evolved a thirst for salt and other minerals too. "The brain monitors [how much you lose], then triggers a precise appetite" for something salty, says Oka, the professor of biology at Caltech. That might be sports drinks — or a salty snack like peanuts.

Hew-Butler and a team of colleagues conducted a study to find out just how well the body's thirst mechanism for salt works. They analyzed five years of research on ultra-marathon runners in northern California. Organizers at the races set out tables with salty snacks such as peanuts, pickles, salted watermelon and even salt packets in addition to water, soda and sports drinks and encouraged the runners to consume only what they craved. The researchers found that the runners were able to keep their salt-balance levels in check just by following their thirst and appetite.

Bottom line? Your body will tell you when it's got a hankering for salt — so let your cravings be your guide.

Myth #4: Drinking water can help you lose weight.

Some small studies have found that drinking water before meals can help certain groups of people lose weight. The idea is that water makes your stomach feel full, and therefore, you eat less.

However, there are many conflicting studies on this topic. For example, one paper found that drinking up to 500 mL of water 30 minutes before a meal led to weight loss in a group of young men, but another paper found that the tactic did not work for younger people in the study — only the older ones.

And when scientists looked at papers on this subject in a systematic review, they concluded that there's just not enough evidence for the general public. In a 2013 study published in the American Journal of Clinical Nutrition, researchers surveyed four electronic databases and found that only three studies suggested that increased water consumption could lead to weight loss if it's part of a diet program. But the results were inconsistent for people who were not dieting. Ultimately, the researchers concluded, "The evidence for this association is still low, mostly because of the lack of good-quality studies."

Studies have shown that drinking water can help with weight loss if it's replacing sugary beverages like soda, sweet juices and sports drinks. In a study published in the American Journal of Clinical Nutrition, researchers asked a group of more than 300 overweight and obese individuals to replace such beverages with water for 6 months and found it helped reduce the subjects' weight by an average of 2 to 2.5%.

Myth #5: Dark-colored pee means you're dehydrated.

Scientists commonly measure dehydration by looking at the concentration of sodium and other solids in urine, which is what makes pee darker in color. But that isn't the most precise way to tell whether someone needs more water, says Hew-Butler.

In 2017, she conducted a study published in the journal BMJ Open Sport & Exercise Medicine to see if measuring the salt concentration of urine was an accurate reflection of the salt concentration in blood. She asked 318 athletes to "pee in a cup, then we drew their blood," she says. More than half of the athletes showed up as dehydrated when she measured their urine — but when she looked at their blood, none of them showed up as dehydrated.

Just because your urine is dark gold, says Hew-Butler, it doesn't mean your body is dehydrated. It just means your kidneys aren't releasing as much water in order to keep your blood's water-sodium level balanced. It would be more accurate to look at the concentration of sodium in our blood, she says, because our brain's sensors use that to decide how much water our bodies need.

That said, if you're not great at paying attention to your thirst, some hydration experts recommend drinking enough water to keep your urine a light, straw-yellow color — a simple way to assess hydration.

Hydration, like so many things, comes down to balance.

"It's a happy medium, right?" says Millard-Stafford. "Not too much. Not too little. Just right – the Goldilocks sort of approach."

The audio portion of this episode was produced by Summer Thomad, with help from Sylvie Douglis. The digital story was edited by Malaka Gharib. We'd love to hear from you. Leave us a voicemail at 202-216-9823, or email us at [email protected].