Robert Sallis has seen it all. As a medical director for the Ironman World Championships in Kona, Hawaii, he’s spent 20 years watching athletes in every manner of distress get wheeled into the medical tent. He’s seen hyponatremia, or overhydration, a handful of times. He’s seen hundreds, maybe thousands, of dehydration cases. Sallis has even seen athletes show symptoms of both at the same time: they’ve dropped weight over the course of the race, signaling dehydration, but their blood sodium levels are dangerously low, a sign of hyponatremia.

What most surprises Sallis, a former president of the American College of Sports Medicine, is the heap of misinformation on hydration that he hears swirling among athletes. He partially blames the media that picks up stories like this Cycling News article from December 2016, in which Roger Palfreeman, Team Sky’s top doctor, touted “functional dehydration” as a strategy for making his athletes lighter and thus faster. “It’s stupid,” says Sallis, adding that mental and physical performance plunges when you’re 2 percent dehydrated—any advantage from a reduction in weight would likely be offset by a reduction in power and mental resolve.

“Thirst is a very poor indicator of dehydration, especially if you’re up at altitude or training somewhere dry.”

Advocates of functional dehydration tend to cite two athletes in particular to support their position: Tour de France rider Tommy Simpson and marathoner Jim Peters, who was a lifelong proponent of abstaining from food and water during races. “There is no need to take any solid food at all and every effort should also be made to do without liquid, as the moment of food or drink is taken, the body has to start dealing with its digestion,” Peters said in 1957. According to Sallis, both athletes experienced extreme, career-ending cases of heat stroke because of this strategy. Simpson, known for trying to ride long stages with just a few bottles, died in 1967, likely from a combination of heat stroke and amphetamines, after collapsing during a scorching climb up Mont Ventoux.

But there is a lot of new, thorough research on the rules of hydration, and these rules could save your life. Learn them, and then practice them. Here are the basics.

Thirst Is a Poor Indicator of Dehydration

For easy workouts in cool weather lasting an hour or less, drinking only when you’re thirsty is fine. But if it’s at all hot or humid, or you’re going out for a long time, that won’t be adequate.

“Thirst is a very poor indicator of dehydration, especially if you’re up at altitude or training somewhere dry,” says Eric Sternlicht, an associate professor of kinesiology at Chapman State University. A 2016 study published in the International Journal of Sports Nutrition and Exercise Metabolism found that, when allowed to drink freely, 32 percent of collegiate athletes started exercise in a dehydrated state and more than 40 percent finished dehydrated. And these were young people. Sternlicht says our thirst mechanism gets even more faulty as we age.

Drinking to thirst doesn’t totally rule out the threat of hyponatremia, either, says Nanci Guest, a sports dietitian and PhD candidate in nutritional sciences at the University of Toronto who has worked with some of Canada’s top endurance athletes. After all, thirst is a subjective measure. Some may drink only enough to take the edge off their thirst, while others may drink until they are fully satisfied. Sallis has seen these cases at races like Ironman Kona. “They’ll come in and they’ll say, “I drank to thirst. I thought I was thirsty.’” Additionally, the symptoms of hyponatremia are similar to dehyration—lethargy, headache, nausea, vomiting, and swollen hands—which makes it tricky for a racer to recognize mid-competition.

There’s an easy method to figure out exactly how much fluid you need: weigh yourself before you go out for an hour of exercise, and then weigh yourself again when you get home. That’s the weight of fluid you should be taking in per hour. As the saying goes: a pint’s a pound the world around. So plan to drink a pint, or two cups, of liquid for every pound you lose during exercise. For example, if you come back half a pound lighter, you should drink one cup of water per hour of exercise. Do this in a range of temps and intensities, says Sternlicht, and you’ll have guidelines to follow in every possible set of conditions. Just don’t take in substantially more water than you lose—that’s a recipe for hyponatremia.

Plain Water Doesn’t Cut It for Long Events

It’s common for Sallis to hear hyponatremic athletes say, “My stomach was feeling weird, so I switched to plain water.” Sometimes his athletes ditch sports drinks to cut calories. Over the span of a four- to five-hour competition, this is a recipe for disaster. “Taking in water without sodium is how you dilute yourself,” says Guest.

Sodium helps your body regulate how much water a cell can hold. When your body’s sodium content drops to critically low levels, your cells take on too much water and swell. In the most extreme case, this can lead to cerebral and pulmonary edema (swelling around the brain and heart), which can be fatal.

While taking in sodium during sporting events won’t entirely eliminate your risk for hyponatremia, Guest and Sallis both say it’s a useful tool for helping to mitigate risk. If you don’t like the taste of sports drinks, try electrolyte tablets.

You Can Speed Up Absorption Rates

When you drink, fluids must pass through your stomach and into your small intestines before being absorbed into your bloodstream. But not all fluids move at the same rate. You can speed gastric emptying—or the rate at which fluids move through the stomach and into the small intestine—by tweaking how and what you drink. This translates to faster, more efficient delivery of calories, sodium, and fluids to your cells.

First, volume matters—to a degree. A review of studies on gastric emptying published in Nutrition Reviews in 2015 found that when people consumed 20 ounces of fluid at a time, the fluid moved faster through their stomachs than when they ingested 13.5 ounces. And 13.5 ounces moved faster than seven ounces. The increased pressure in the stomach from more liquid signals to the body that it’s time to get digestion moving. So what does this mean? If your gut can manage it, it’s smarter to take a few long pulls off your bottle than tiny sips every ten minutes. You still need to keep total quantity in mind, but if you know you need to take in, say, two cups per hour, it’s smarter to do that in two one-cup sessions rather than small sips. (There is a limit to this principle: researchers have found that ingesting around 30 ounces at one time may actually slow absorption, although they’re not yet clear on why.)

The amount of carbs in your sports drink also matters. Anything higher than 7 percent carbohydrate (like juice or soda) will languish in your stomach for a while. A lower-carb drink mix—like Clif Bar’s Hydration Electrolyte or Skratch Labs Exercise Hydration, both of which are made up of 4 percent carbohydrate—will move through your body faster.

Sports Drinks Aren’t as Bad as You Might Think

If you’re not exercising for more than 90 minutes, sipping on one of these is the caloric and sugar equivalent of sipping a soda. But for all-day endurance efforts? “Sports drinks work,” says Guest. “We know the science is there and that there’s an abundance of research supporting it.” Lots of studies—including those found in the Journal of Sports Science, Sports Medicine, Indian Journal of Medical Research, and International Journal of Sports Medicine—show increased endurance performance while using sports drinks, mainly because of their carbohydrate load. Sternlicht agrees, saying that almost all commercial sports drinks formulas hit the mark when it comes to balancing carbs and electrolytes. Yes, they contain lots of sugar. But if you’re doing any sort of prolonged exercise or bursts of higher-intensity work, that sugar provides crucial fuel.

…But the One You Make at Home Is Even Better

Sports drinks are a pretty simple mix of water, carbs, and electrolytes. It’s easy to DIY your own performance mix. Guest likes to blend cold watermelon juice with water, ice, and salt. You can dilute just about any juice in a one-to-one ratio (one part water, one part juice) and reach a nice 6 to 7 percent carbohydrate blend.

To Get Really Scientific, You Need to Check Your Pee

Many studies have found that there’s a huge range of data when you track fluid intake rates, dehydration, and hyponatremia among athletes racing the same course on the same day. In other words, platitudes on taking in X number of ounces of liquid for Y number of minutes of exercise aren’t helpful. You need to tailor your program to your body.

Want to get super specific? Use a pee stick, or reagent urine test strip, that can provide you with stats like urine pH, ketones, and protein and glucose levels. The measurement you should pay most attention to is specific gravity—or how concentrated your urine is. A measurement of 1.005 to 1.015 (compared to water’s measurement of 1) means you’re hydrated. Anything higher than that (Guest says 1.025 or above) means you’re dehydrated.

Guest adds that she used pee sticks, which you can get a most pharmacies, to monitor her athletes during the Pan Am Games. Occasionally, the light color of their urine would make it appear that her athletes were hydrated, but the pee sticks indicated otherwise. If you’re training hard and are concerned about proper hydration, this is the way to get the most accurate, current information.