How Lucy Charles-Barclay Trains for the Heat (and the Hydration Mistake That Cost Her Kona)

Lucy Charles-Barclay leads almost every race she enters out of the water, because before she was a triathlete she was an elite pool swimmer who came within a whisker of the 2012 Olympic team. But the most instructive part of her story for a normal athlete is not the swim. It is heat. She won the 2023 Ironman World Championship in Kona, one of the hottest, most humid endurance races on earth, and two years later she lost a commanding lead in the same race to a hydration mistake that had nothing to do with going too hard. Her career is a masterclass in the one variable most amateurs never measure: how much they sweat, and what to put back.

The swimmer who became a triathlete

Charles-Barclay swam competitively from childhood, was a national champion as a teenager, and only narrowly missed selection for London 2012 before turning to triathlon. That background is why she is almost always first out of the water and racing from the front, a lead the bike and run then have to defend. She turned professional in 2016 after winning her age group at the Ironman World Championship as an amateur, and has since won Kona (2023), two Ironman 70.3 world titles (2021 and 2025), and stood on the Kona podium as runner-up four separate times. She is coached by her husband, Reece Barclay.

Being a front-runner in long-course racing means she spends more time exposed, alone, in the full heat of the day than an athlete sitting in a pack. That makes heat management not a detail but a core part of her event.

Training for the heat, not just the distance

Ahead of a Kona campaign, Charles-Barclay and coach Dan Lorang ran a six-week heat-acclimation block, and unusually they did it from home in the UK rather than decamping to a hot climate. She trained indoors in artificially raised heat, around 34 degrees Celsius and high humidity, often in a sauna-style suit, on top of her normal swim volume. She has described treadmill belts getting so soaked with sweat that she had to keep switching machines, once finishing a session's final run leg outdoors because there was no dry treadmill left. She called it one of the hardest training blocks she had ever done.

The principle underneath the theatrics is real physiology. Heat acclimation, repeated training in the heat over one to two weeks or more, expands blood plasma volume, lowers the core temperature at which you start sweating, and makes your sweat more dilute so you hold onto more sodium. It is one of the most reliable performance interventions in hot-weather endurance sport, and it is available to any amateur with a turbo trainer in a warm room. Even a heat-hardened world champion does not simply show up to a hot race and cope. She spends weeks preparing for the temperature specifically.

Sweat rate is the number

Here is the part that transfers directly to you. In the heat, the amount of fluid you lose per hour can be enormous, and it is highly individual. Sports scientists measure endurance athletes losing anywhere from roughly half a litre to two litres of sweat an hour, with an average around 1.3, and the only way to know your own figure is to measure it. Reported numbers put Charles-Barclay's sweat rate at around 1.2 litres an hour, and whether or not that exact figure is precise, it sits squarely in the range the research describes, and it is the number she sizes her race-day drinking around.

Measuring it is simple, and the calculator does the arithmetic:

Weigh yourself before a hard session and again straight after, keep track of any fluid you drank in between, and the calculator turns the change in your body weight into a sweat rate in litres per hour. Do it once in cool conditions and once in the heat, because the two numbers can be dramatically different, and the hot-weather figure is the one that matters when you are racing or training in summer. Performance and safety start to suffer once you have lost roughly two percent of your body weight in fluid, so knowing your rate tells you how much you actually need to drink to stay ahead of that line, rather than guessing with a generic "sip every fifteen minutes."

When even the professionals get it wrong

The most honest lesson in Charles-Barclay's story is that measuring your sweat is only half of it, because the sodium matters too, and it is easy to overcorrect. At the 2025 Kona race she was leading the marathon before withdrawing about nine miles from the finish, not from classic dehydration but from hypernatraemia, too much sodium relative to fluid. In her own words she was "extremely thirsty" and drank heavily but nothing touched it. Afterwards she was blunt about the cause: "It was my own fault in Kona. I overdosed on the salts and then needed excess fluid to try and claw it back, and that wouldn't work." For her comeback win at Ironman Lanzarote in 2026 she halved her electrolyte dosing, taking half a tablet an hour where she used to take a whole one, and executed the plan cleanly.

If a four-time Kona runner-up and world champion can get her salt dose wrong under race stress, the takeaway for an amateur is not "give up on a plan." It is the opposite: build a fluid and sodium plan from your measured sweat rate, rehearse it in training in the heat, and do not improvise new numbers on race day. More salt is not automatically safer than less.

What you can actually steal from it

• Measure your sweat rate, do not guess it. A simple before-and-after weigh-in turns fluid loss into a real number in litres per hour.
• Measure it hot. Your cool-weather rate understates what you lose racing in summer, and the hot number is the one that matters.
• Acclimate before a hot race. Even a world champion runs a multi-week heat block. A warm indoor trainer session counts.
• Match sodium to fluid, and rehearse it. Charles-Barclay lost Kona to too much salt, not too little. Build the plan from your own data and practice it before you need it.
• Front-runners cook. If you race from the front or train alone in the day's heat, you get less shade and less draft, so your hydration margin is smaller, not larger.

Charles-Barclay is the swimmer of this series, but her real gift to a time-pressed amateur is the least glamorous variable in endurance sport. Sweat is measurable, it is personal, and it is the difference between finishing strong in the heat and unravelling in the last hour. Most athletes never once weigh themselves around a workout. She built a world title on taking it seriously, and a painful Kona on the day she got the sodium half of it wrong.

More in this series: Kristian Blummenfelt, Alex Yee, Mark Allen and the fast age-grouper on eight hours a week.

Sources

Charles-Barclay's swimming background, professional record and world titles (2023 Ironman World Champion, 2021 and 2025 Ironman 70.3 World Champion, four Kona runner-up finishes) are documented on her Wikipedia profile. Her six-week UK heat-acclimation block with Dan Lorang is reported by TRI247 and TMR Coaching. Her 2025 Kona hypernatraemia withdrawal, the quotes about it, and her halved electrolyte dosing for the 2026 Lanzarote comeback are reported by TRI247. Her reported ~1.2 L/h sweat rate comes from a single secondary feature and is given as approximate. Sweat-rate measurement methodology and typical ranges are from the Gatorade Sports Science Institute and published normative sweat-rate data; the two-percent body-mass performance threshold reflects American College of Sports Medicine fluid-replacement guidance.

Cover photo by Jürgen Matern, CC BY-SA 4.0, via Wikimedia Commons.