Shalane Flanagan can’t remember much about finishing the 2016 US Olympic Marathon Trials in Los Angeles. She had led much of the race with teammate Amy Cragg, but the temperature reached 23 degrees on the unshaded course in winter, the hottest Marathon Trials on record. Flanagan, who trains in the cool and rainy Pacific North-West, wasn’t acclimated to that kind of heat.

She staggered through the last few kilometres, feeling dizzy, getting chills, hearing ringing in her ears. Though she hung on to finish third and secure a spot on the US Olympic Marathon Team, she collapsed and was rushed to a medical tent, where she was hooked up to an IV. “I’d been taking my fluids, but I could just tell that they weren’t really absorbing,” she told Runner’s World the day after the race. “It was absolutely an overheating and dehydration problem.”

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On the surface, hydration seems so simple: just drink enough, duh! But if pro runners struggle – and they have a team of experts dialling in their every need – then the hydration calculus is obviously more complex. “You have to factor in heat, humidity, acclimation, altitude, intensity level, and how much you drank before you ran, in addition to your individual sweat rate,” says dietician Lauren Antonucci, owner of Nutrition Energy. “Everyone’s needs are different.”

Hydration Status

Sweat reduces your blood volume, which makes your heart work harder. Becoming dehydrated by more than 2 to 3 per cent may slow you down; a dehydration rate of greater than 4 per cent may land you in a medical tent.

The simplest way to determine whether you’re fully hydrated is with a pee test, says Tiffany Chag, sports performance specialist and sports dietician with the Hospital for Special Surgery in New York City. “Pale yellow is where you want to be,” Chag says. “The only problem is you don’t know until you’re done, so [the test] needs to be done before you run.” Note that certain foods and multivitamins can impact urine colour.

Runners training for half marathons or longer should take an extra step. “If you’re routinely going to be running for more than two hours, calculate your sweat rate,” Chag says. To do this, weigh yourself (without clothes on) before and after an easy one-hour run in which you don’t drink any fluids. Every half-kilo you lose is equivalent to 500ml of liquid you need to replace.

So if you lose a kilogram during an hour-long run, drink a litre to 1.5l of liquids in the two to four hours afterwards, to replenish. (If you drink during your sweat-rate test, simply add that amount to your needs.) For runners who live in areas with changing seasons, repeat the test in the summer – your hydration needs change as your body gets acclimated to heat.

And don’t just rely on thirst, adds Lawrence Armstrong, PhD, associate director of the Human Performance Lab at the University of Connecticut and former president of the American College of Sports Medicine. “Thirst isn’t a good indication of your fluid loss; because as soon as you drink, nerve endings in your tongue and throat send sensory impulses to your brain to reduce your thirst, before your body has absorbed enough water,” he says. A 2017 study published in Medicine & Science in Sports & Exercise showed that athletes who hydrated on a planned schedule performed better in the heat than those who drank to thirst. So here’s exactly how to plan for your next run in the heat.

When: Running up to 60 Minutes
What to Drink: Water

On runs that last up to an hour, Armstrong says plain water will do the trick, especially if it’s an easy effort (keeping intensity below 70 per cent of your max heart rate). If you know your sweat rate, you know how much to take in. Or aim to drink 100 to 200ml of water every 15 to 20 minutes.

But if drinking on a short or medium-distance run isn’t your thing, you can probably skip it altogether; instead, swish water in your mouth and spit, says a 2016 study from Elon University in the US. It compared a group of drinking runners with a group who mouth-rinsed with water during
an 80-minute effort on a 20-degree day, and found that the two groups performed basically the same.

When: Running 60 to 90 Minutes in Hot and Humid Conditions
What to Drink: Water + Electrolytes

When you sweat, you lose sodium and other electrolytes, which speed water absorption and delay fatigue.

And the hotter it gets, the more you’ll sweat. What is ‘hot’? The ideal temperature for many marathoners is 6 degrees, according to a 2012 study; for every 5.5 degrees above that, performance may decline by as much as 10 per cent.

If you’re a salty sweater – your sweat stings your eyes, or you have white stains on your shirt after your run – pay extra attention to electrolytes, particularly sodium. Antonucci recommends starting with 750mg per one litre of fluid. Check the label of your sports drink and gels: most don’t have nearly that much sodium, so you may need to supplement with electrolyte pills or salt packets.

“I taste salt on runs over an hour,” says Antonucci, who carries salt packets with her. “If it tastes great, eat it. If it tastes metallic, your body is saying you don’t need sodium.”

When: Running 90 to 120+ Minutes
What to Drink: Water + Carbs + Electrolytes

At this point you need to start replacing electrolytes and the glucose you’re burning in the form of easily digestible carbs, such as sports drink. For workouts up to 150 minutes (2.5hr), Chag recommends taking in up to 60 grams of carbs per hour. For long runs over 2.5hr, she recommends 60g to 90g of carbs per hour.

To put this in context, 100ml of Powerade’s naartjie-flavoured sports drink contains 8g of carbs and 31mg of sodium. “Electrolytes become even more important for performance in three to four hours of continuous exercise,” says Armstrong.

The most important key to finishing a hot run safely is to have a hydration plan and adapt it to the conditions, Antonucci says. Know exactly how much you will drink, and the carb and electrolyte contents of your beverage.

“If you start a run or race and the temperature climbs to 32, you need to adjust,” she says. “Don’t pretend it’ll be fine, because that’s what’ll get you into trouble.”

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