What Runners Should Actually Drink in This Heat, by Distance and Effort

A 53-year-old man died after collapsing during a race in the 20th arrondissement of Paris, and in Lyon a woman died of heat stroke during a separate sporting event hours earlier, deaths that pushed hydration from a performance question to a safety one as record heat settled over Europe. French sports minister Marina Ferrari, who offered condolences and warned that competing in extreme heat “requires absolute vigilance,” suggested a possible link to the conditions, though Le Parisien reported the Paris runner suffered a heart attack and the cause has not been confirmed as heat-related.

The heat has been bending races that went ahead, too. IRONMAN Frankfurt cut its bike leg from 112 miles to about 78 and shortened the run from a full marathon to a half, the Hamburg half-marathon was stopped mid-race, and IRONMAN Nice was postponed outright after French authorities raised concerns about the strain on emergency services. Behind the disruptions is a summer that has broken temperature records across more than a dozen European countries and driven a heat dome over much of the United States.

For runners still training through it, the practical question is simple and the marketing around it is loud: water, electrolytes, or a carbohydrate sports drink. The honest answer is that all three do different jobs, and which you need depends on how long and how hard you are running, with heat shifting the math toward fluid and sodium but rarely toward more carbohydrate.

Start with the mechanism, because it is what the product labels skip. Sweat rates in exercising athletes range roughly from 0.5 to 2.0 liters an hour, and the sodium in that sweat varies about as widely, from lightly to heavily salted, according to a 2017 review of sweat composition in the journal Sports Medicine. Heat widens the gap. As sweat rate climbs, the ducts reabsorb proportionally less sodium before it reaches the skin, so a hard effort in the sun costs a runner both more fluid and saltier fluid than the same effort in the cold. That is the real reason electrolytes matter more in a heat wave, not because a hot day rewrites the rules but because it pushes every runner further along the same scale.

The floor for fluid is set by two failure modes at opposite ends. Losing more than about 2 percent of body weight to sweat begins to degrade performance, which is why the American College of Sports Medicine’s position stand on fluid replacement recommends estimating individual sweat rate by weighing yourself before and after a run rather than following a fixed number. At the other end is overdrinking. Taking on more fluid than you lose dilutes blood sodium and causes exercise-associated hyponatremia, which has killed marathoners, and the third international consensus on the condition concluded that drinking to thirst rather than on a schedule prevents nearly all cases. Heat makes this tension sharper, because it is exactly when nervous runners are most likely to force fluids.

From there the choice tracks the run.

For an easy effort under about 45 minutes to an hour, water is enough, taken to thirst, even in the heat. The sodium and glycogen costs are too small over that span to need replacing mid-run.

For a steady run of roughly one to two and a half hours, fluid to thirst plus sodium is the combination that matters. The ACSM stand suggests including sodium in fluid taken during efforts longer than an hour, and this is the range where a heavy, salty sweater in the heat feels the difference most. Carbohydrate is optional and useful toward the upper end, but sodium and fluid are doing the real work.

For a hard session or anything beyond about two and a half hours, a carbohydrate drink earns its place alongside fluid and sodium. Endurance-nutrition guidelines put carbohydrate needs at 30 to 60 grams an hour for efforts of one to two and a half hours, rising toward 90 grams an hour of mixed glucose and fructose for longer racing, with gut tolerance as the ceiling. Heat does not raise that target. If anything, hot, humid conditions are a reason to be cautious with concentrated drinks and aggressive fueling rather than to pile on more.

Where the marketing gets ahead of the evidence is cramping. A 2005 study of NCAA football players in the International Journal of Sport Nutrition and Exercise Metabolism found that cramp-prone athletes lost about 5.1 grams of sodium over a two-and-a-half-hour session against 2.2 grams for teammates who did not cramp, with roughly double the sweat sodium concentration. But heavy sodium loss appears to drive cramps in some athletes and not others, which is why electrolytes are worth dialing in through your own fluid and electrolyte trial and error rather than treating a salt tab as a guarantee.

None of this changes for a marathon or a half beyond scale, and it takes the body about 10 to 14 days to adapt to hot-weather running, so the runners racing this month are largely working with the sweat systems they arrived with. With the heat forecast to hold over both continents in the coming weeks, the useful takeaway is narrow: the right bottle is the one matched to the day’s run, and on the hottest days that means more fluid and more salt, taken to thirst, not more of everything at once.