You've heard it since you were a kid, from your parents to your gym teacher: Drink two liters (or eight glasses) of water every day. Lately, though, researchers have been questioning the tried-and-true water rule, and in fact, believe even health-minded individuals could be drinking too much H2O.

“There’s no scientific method behind those numbers,” says exercise physiologist Stacy Sims, Ph.D., a hydration researcher at Stanford University. “And the recommendation doesn’t take into account gender, environment, altitude, fitness level--factors that could affect fluid intake needs.”

In fact, there’s a lot of marketing behind popular hydration recommendations, Sims says--with potentially dangerous consequences. “Drinking too much fluid can lead to hyponatremia, which is when sodium in blood becomes too diluted,” Sims says. (For more ways to stay hydrated while you’re working out, check out this guide to drinking up properly.)

Check Out This Guide to Drinking Up Properly - 

Symptoms include confusion, headaches, nausea and bloating--stuff that’s easily confused with dehydration. In severe cases, hyponatremia can lead to seizures, organ failure and even death.

Typically, though, guzzling down too much fluid--especially from certain "rehydration" drinks--can ironically cause dehydration. “When the drink has a high level of sugar and additives, for example, and thus more solutes than you naturally have in your blood, the body has to take its own fluid to dilute it, so it can be absorbed,” Sims says. “And if you’re simply drinking too much water at once, you may end up peeing too much, and not absorbing any fluid. The body has a natural volume response that causes you to pee.”

So how do you know how much water you need? For starters, don’t rely solely on thirst, says Sims. “As soon as you put water on your tongue, you kill your thirst mechanism.” Instead, Sims suggests the following:

1. Weigh yourself daily for a week, to check hydration. “Your body weight shouldn’t fluctuate too much,” Sims says.

2. Notice how much you pee--and its color--in the morning. “It should be a copious amount and pale or clear,” Sims says. (Your pee is an excellent predictor of your health. Look out for the six signs on What Your Urine Says About Your Health.)

What Your Urine Says About Your Health -

3. Aim to wake feeling hydrated. If you’re thirsty when you get out of bed in the morning, you may not be consuming enough fluids.

4. When choosing sports drinks, search for labels with low sugar. Sims recommends about 5 grams per 8-ounce serving. “Even natural drinks like coconut water have too much sugar and potassium to hydrate.” (Learn how consuming sports drinks as a regular beverage is associated with other not-so-healthy behaviors.)

Consuming Sports Drinks as a Regular Beverage - 

5. Coffee, tea, and watery fruits and vegetables count toward fluid intake. “Also, caffeine is not a diuretic,” Sims says. “It’s about volume, so if you drink five cups, you’ll pee more.”

6. Start slowly. “Sleep is a six- to eight-hour fast, so if you drink three cups of juice or water right away, you’ll trigger the volume response. Sip instead.”

7. Drink one thing a day that’s not water. “A low-carb electrolyte drink in the afternoon can pop you up, as can hot tea with a pinch of salt and lemon,” Sims says. “It will increase your core temperature, and the tiny amount of salt can help you absorb fluid. (Try these 10 Suprising Hydrators and never settle for drinking  plain ol' H2O again.)

Try these 10 Suprising Hydrators -