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In the mid-1960s, a researcher at the University of Florida named Robert Cade went to a bank and borrowed $500 to buy sugar and salt. The homemade drink he’d devised for the university’s football team, dubbed Cade’s Cola—or Gatorade—was attracting widespread attention, and he wanted to mix a big batch of it to sell. The sugar provided crucial fourth-quarter energy for the players, and the salt—well, scientists and athletes are still debating what role the salt in sports drinks plays in athletic performance.
The question lurks in the background of a new study published in the European Journal of Sport Science, by sports nutrition researcher Alan McCubbin of Monash University in Australia. The study uses a mathematical model to determine exactly how much sodium is required during exercise of varying intensities and durations, depending on how much a person sweats, how salty that sweat is, how much they drink, and other factors. The answers depend on the assumptions we make about why athletes need salt. But in the vast majority of real-world situations, McCubbin concludes, we don’t need to worry about it.
There’s no doubt that sodium—the key electrolyte in salt—has a number of crucial responsibilities in the body. It helps muscles contract, conducts nerve signals, and keeps internal fluid levels balanced. It’s also true that we lose sodium through sweat. In the 1930s, after at least 13 workers died of heat exhaustion during the first year of construction on the Hoover Dam, in Nevada, tests conducted by Harvard physiologist D. B. Dill revealed that the workers were sweating out large quantities of sodium. The solution: in the dining hall, alongside a sign that read, DRINK PLENTY OF WATER, Dill’s team added, AND PUT PLENTY OF SALT ON YOUR FOOD.
But taking in salt during exercise is another matter. There are three principal reasons you might want to do this. The most frequently cited is to ward off muscle cramps, but scientific evidence largely contradicts this idea. Studies involving runners and triathletes have found no significant difference in sodium levels between those who experience cramping during exercise and those who don’t, and deliberately lowering those levels appears to have no effect one way or the other. There are many reasons why we cramp, and sodium may be involved in some of them. But when it comes to exercise-related cramping, increasing our salt consumption doesn’t appear to be the solution.
The second reason to boost salt intake while exercising is to avoid hyponatremia (literally, low blood sodium)—a dangerous and occasionally fatal condition. On paper, drinking something salty would seem like a good way to ensure healthy sodium levels. But sports drinks are less salty than blood, so the more you chug, the more diluted your blood becomes. As a result, the main risk factor for hyponatremia is actually taking in too many fluids—be it water or sports drinks—not too little salt. That’s why current guidelines advise drinking when thirsty rather than following an aggressive hydration plan.
The third reason is the one McCubbin considers legit: regulating fluid concentrations. The human body is replete with fluids—in the blood, in the cells, and in the spaces between cells. Your body monitors sodium levels to decide how to allocate fluid stores among these three areas. That means you’ve got a buffer when you start exerting yourself; even though you’re sweating, water from other localities can shift into your blood plasma to maintain sodium concentration. However, if prolonged sweating depletes sodium levels too much and you’re only drinking water, the opposite happens: fluid shifts out of your plasma to keep concentrations from dropping elsewhere, leaving you with lower blood volume to ferry oxygen to muscles and dissipate heat. That, in theory at least, is a problem.
The relevant question, then, isn’t how much sodium you need in order to replace what’s lost to sweat. It’s how much you need to keep your blood concentration from dropping, taking into account that your body is moving fluid around internally. Crucially, the answer doesn’t just depend on how much sodium you sweat out; it also depends on how much fluid you take in.
“There’s a big difference between losing four liters of salty sweat in a marathon and replacing two liters of that with plain water, and losing 20 liters in a 100-miler and replacing it with 18 liters of plain water,” McCubbin explains. In both cases, you’ve lost the same quantity of fluid: two liters, or 4.4 pounds of body weight. But it’s really the turnover that matters. In the latter instance you’ve sweated out far more sodium, and are therefore more likely to exceed your body’s ability to compensate for the loss.
For his study, McCubbin used equations developed by kidney specialists to calculate blood-sodium concentration. Among soccer players and marathoners, he concluded, making a deliberate effort to replace sodium beyond what taste preferences dictate is “unnecessary in all realistic scenarios.” In 100-mile ultramarathons, where longer duration results in far greater salt loss, the picture is more nuanced. For runners whose sweat is saltier than average and who aim to drink aggressively enough to limit fluid losses to 2 percent of their starting weight, drinking water alone would leave them short on sodium. (These ultras are so taxing that runners also lose weight from the carbohydrate and fat reserves they burn, so they may be 3 to 5 percent lighter on the scale by the time their fluid losses hit 2 percent. That’s about what current hydration advice for ultramarathoners recommends.)
The sodium content of your sweat can be roughly inferred from the amount of dried salt left on your clothes and skin after a workout, or it can be determined with greater accuracy through testing offered by companies like Precision Hydration. But even if you’re a salty sweater, mainlining sodium tablets is a risky proposition. Overdoing salt intake can make you thirstier, increasing the chances that you’ll drink too much and, paradoxically, perhaps even putting you at risk of hyponatremia, according to Martin Hoffman, an ultra-endurance researcher at the University of California at Davis. Instead, Hoffman recommends taking in salt with food as dictated by your cravings, rather than following a predetermined salt-intake regimen. “It’s realistic to say don’t worry about it or don’t listen to the so-called experts who have a product to sell as long as one is attuned to one’s body,” he says.
In fact, sodium needs during a 100-miler may be even lower than McCubbin’s calculations suggest, according to Hoffman. There’s some evidence that the body contains additional sodium that gets released into circulation with prolonged sweating, although the idea is controversial among scientists.
Both Hoffman and McCubbin agree that a small minority of people with unusually salty sweat might run into problems in a multi-hour event like an ultra. For those individuals, sweat testing to determine exactly how much salt they’re losing might have value. At a minimum, they’ll benefit from a deliberate plan to restock lost sodium through food, sports drinks, and perhaps even salt tablets. For the rest of us, McCubbin’s advice mirrors the shift in thinking about hydration over the past few decades, from the hard-nosed “Drink to replace what you lose” to the more subjective “Drink when you’re thirsty.” When it comes to salt, McCubbin says, the new rule is: “Season to taste.”