Hydration for Runners Isn't What You Think

From electrolyte drinks to cramp prevention, one of the world's leading researchers explains why most hydration advice is far more nuanced than social media would have you believe.

Photo Credit: Graham Parkinson

Everyone has an opinion about hydration. But effective hydration for runners isn't simply about drinking more water. It depends on your training, the environment, your sweat rate and the duration of your run. Drink before you're thirsty. Never let yourself get dehydrated. Add electrolytes. Eat salt. Avoid cramps. Drink more. Drink less. Spend five minutes scrolling running Instagram and you'll find advice that often contradicts itself. The problem isn't that people are trying to help. It's that hydration is surprisingly complicated.

To separate fact from fiction, we sat down with Professor Robert Kenefick, Professor of Biomedical and Nutritional Sciences at the University of Massachusetts Lowell, former Deputy Chief of Thermal and Mountain Medicine at the US Army Research Institute of Environmental Medicine, and one of the world's leading researchers in hydration, heat and human performance. The conversation came as OneTrack CTO Ben Burch prepared to tackle the Badwater Ultramarathon, a 135-mile race through Death Valley where temperatures regularly exceed 50°C.

What followed challenged some of the biggest assumptions in endurance sport.

There is no such thing as "perfect hydration"

One of the biggest misconceptions is that hydration is a fixed target. According to Kenefick, the body is constantly moving through small fluctuations in fluid balance. Scientists refer to this as euhydration—your normal state—not being permanently "fully hydrated." Rather than chasing perfection, the goal is understanding how your body responds throughout the day and during exercise. "People often talk about dehydration as a fixed state," he explained. "But technically dehydration is the process of becoming hypohydrated." Already we've learned something.

Drinking too much can become a problem

Social media often frames hydration as "more is better." The science says otherwise.

One of the biggest risks during very long endurance events is hyponatraemia—a dangerous dilution of sodium caused by consuming large amounts of plain water over many hours. That doesn't mean runners should fear drinking. It means hydration and electrolyte replacement need to work together. Importantly, Kenefick points out that food often gets forgotten in these conversations. "People focus entirely on electrolyte drinks," he says. "But if you're eating salty foods throughout a long event, that also contributes."

Most runners probably don't need electrolyte supplements

Perhaps the biggest surprise? For everyday runners, electrolyte products are often unnecessary. "If you have a fairly normal diet," Kenefick explains, "you're likely getting plenty of sodium already." Where electrolytes become genuinely useful is during exceptionally long events, prolonged exposure to heat or situations involving unusually high sweat losses. For the average 5k or Sunday long run, the evidence is much less compelling than marketing might suggest.

Why Heat Changes Your Hydration Strategy

Badwater is different. Ben's recent heat-acclimation block involved ten consecutive days running inside a purpose-built heat chamber reaching around 40°C. The transformation was dramatic. Heart rate fell. Running felt easier. Core temperature stabilised more quickly.

Kenefick wasn't surprised. What Ben experienced was classic heat acclimation. "You had the classic responses," he explained. "Lower heart rate, lower perceived effort, increased plasma volume and more efficient sweating." These are exactly the adaptations sports scientists—and military researchers—have been studying for decades.

Your Running Hydration Strategy Should Never Start on Race Day

Trying something new on race day. If there was one piece of advice Kenefick returned to repeatedly, it was this. Train your hydration strategy exactly as you train your legs. New electrolyte mixes. Different gels. Salt tablets. Shoes. Race day isn't the place to experiment.

Instead, every long training run should become an opportunity to refine what works for you.

So... do electrolytes stop cramp?

This might be the biggest myth of all. Despite how commonly it's repeated, Kenefick says there is remarkably little evidence that electrolyte loss directly causes muscle cramps. Current evidence points much more strongly towards fatigue and neurological factors. "No," he said simply when asked whether electrolyte loss causes cramp. That doesn't mean electrolyte drinks are useless. It simply means people may be attributing the wrong mechanism to why they feel better. If stopping, recovering and eating coincides with the cramp disappearing, the electrolyte drink often gets the credit. Science isn't so convinced.

The most important part of an ultra isn't your body

Towards the end of the conversation, the discussion shifted away from physiology entirely. Because eventually every ultra becomes psychological. Kenefick recalled working with US Marines who insisted the physiological data didn't apply to them. His response has stayed with us. "Your physiology is the same," he told them. "Your mentality is vastly different." That idea feels true well beyond military training. The body matters. The science matters. But eventually every endurance event becomes a conversation between your legs and your mind.

Photo Credit: Graham Parkinson

One of the things about conversations like this is discovering that science rarely deals in absolutes. Almost every answer began with the same phrase: "It depends." That's frustrating if you're looking for a simple rule. But it's also reassuring. Because understanding your own body—how much you sweat, what you tolerate, how you respond to heat, what fuels you best—is ultimately more valuable than following someone else's checklist from Instagram. Hydration, it turns out, isn't about finding the perfect electrolyte drink. It's about understanding yourself.

This article is adapted from a conversation on the OneTrack On The Run podcast with Professor Robert Kenefick and OneTrack CEO Anthony Fletcher & CTO Ben Burch as they discuss the science behind hydration, heat acclimation and preparing for the Badwater Ultramarathon.

Want to train smarter? At OneTrack, our coaches use evidence-based training, performance testing and personalised coaching to help runners understand what works for their body—not someone else's.