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 pick 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.
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 the 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.