Triathlete crawling on the road at the Kona Ironman, with the word “Salt?”

Running in the heat: why the body gives out, and what the studies say

Energy & Performance Cyril Certain
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A long run in July, 30°C in the shade, not a breath of wind, and your legs respond a little less with every kilometre. You can still run in the heat, and most runners get away with nothing worse than a tough session. When the body really gives out, several things can combine: rising temperature, the brain easing off the throttle, sugar or sodium dropping. A lack of salt, the explanation you hear everywhere, is rarely the main cause.

To the naked eye, these mechanisms look alike. But you don’t prepare for them the same way, and they don’t call for the same response at the roadside. Here’s what the studies show, what they leave open, and what you can take away for your summer running.

The short answer

To run in the heat, slow down, acclimatise over one to two weeks, eat and drink what you’ve tested in training, and cool yourself without relying only on how you feel. When a runner collapses, four causes are on the table: heat, low sodium, low blood sugar and a drop in blood pressure. Only measurements can settle it. Anyone who is confused or staggering should stop, and emergency services should be called.

The case that reignited the debate

On 10 October 2026, at the Ironman World Championship in Kona, Hawaii, defending champion Solveig Løvseth was leading just over a kilometre from the finish when she staggered and fell several times. She lay down for a few minutes, then walked to the line in eighth place. She’s fine. That day, 22 of the 77 professionals dropped out. [36]

She talked about a probable form of overheating, and her team about a shortage of food and salts. [37] [35] No temperature, sodium level or blood sugar reading has been published. Without those three numbers, nobody knows what happened, and this article won’t try to guess. What follows is about runners in general, and what sports doctors check when one of them goes down.

One detail concerns you directly: this wasn’t extreme weather. The Kona airport station recorded 30.6°C and 63% humidity at 2.53pm, a feels-like temperature of 34.7°C. [38] The 85% humidity quoted in the press was the morning figure. The airport is by the sea and the road was probably a little hotter, but these are conditions you could meet at a summer race in southern Europe, or during a British heatwave.

Cyril Certain’s video, published on 29 October 2026. Length: 16 min 55.

What does heat do to your body when you run?

Your muscles produce heat with every stride, and more of it when you speed up. To get rid of it, the body sends blood to the skin and sweats. As long as that exchange keeps up, core temperature stabilises. Triathletes fitted with thermometer capsules hovered around 38.1°C during an Ironman in Australia, in about 23°C. [2] On the Kona marathon, 15 amateurs averaged 38.55°C at around 16 km. [3]

At around 40°C, the muscle isn’t the first thing to give in. A Danish team had 14 men cycle up to that temperature, then asked for a two-minute maximal contraction, adding electrical stimulation to measure what the muscle could still deliver. The available strength hadn’t changed. What had dropped was the part controlled by the brain. [4] The engine is still running, it’s the driver who eases off.

🧠
At 40°C, the muscle still responds, but the brain's command drops54% versus 82% voluntary activationVoluntary activation (%) 14 men, 40°C versus 18°C in the room, internal temperature of 40.0°C versus 38.0°C, then 2 minutes of maximum thigh contraction. The total muscle strength, measured with electrical stimulation, did not differ between the two trials. Laboratory study, on a strength test and not on a race. Source: Nybo and Nielsen, 2001, DOI 10.1152/jappl.2001.91.3.1055. In the cool, around 38°C internal temperatureIn the heat, around 40°C internal temperature82% (± 6) of the available strength was commanded by the brain after one hour of cycling at 18°C.54% (± 7) only after an effort carried out to exhaustion at 40°C.

That threshold isn’t a wall. During a sprint triathlon in Hong Kong held in a heatwave, 17 triathletes wearing sensors went past 40°C, two of them beyond 41°C, and none needed medical attention. [5] The race lasted about an hour, not eight, but the message stands: temperature alone doesn’t tell you who will collapse.

Four possible causes of a collapse during exercise

Sports doctors work by elimination, and the American consensus on heat illness insists on one point: what you can see isn’t enough to conclude. [1] A staggering gait or confusion can fit several scenarios. [7]

Four possible causes of a collapse during exercise

Choose a cause to see what characterises it, and what should be measured to confirm it.

Verified by: internal temperature

Heatstroke or heat exhaustion

Heatstroke combines an affected brain, with disorientation and staggering gait, with an internal temperature generally above 40°C. It is an emergency, and it does not disappear in a few minutes without active cooling. Heat exhaustion, less severe, recedes when one stops.

A temporary improvement does not exclude heatstroke. Consensus of the American College of Sports Medicine, 2021.

Verified by: sodium in the blood

Exercise-associated hyponatraemia

Sodium drops too low, with confusion, nausea and possible falls. The main cause is too much water: drinking more than you lose. Lost salt weighs more on very long and very hot events.

Field indicator: finishing heavier than at the start. International consensus, 2015.

Verified by: blood sugar

Hypoglycaemia

The brain lacks sugar: confusion, loss of coordination. It is the one that recovers fastest with food, and heat contributes by burning more glycogen, the muscles' sugar reserve.

In intense heat, many athletes stop with normal blood sugar levels.

Identified by: the moment of the fall

Finish line collapse

You stop suddenly, blood stays in your legs, your brain runs short, and you fall. It is the most common collapse at a medical tent at the finish line.

It occurs after the line, once stopped. A fall during the race should prompt a search for a more serious cause.

One trap makes this harder to read. Stopping cuts off the heat produced by the muscles, and lying down sends blood back to the brain. If a runner feels better after stopping, lying down and having something to eat, it’s impossible to say which of those made the difference.

Salt: a very convenient culprit

Exercise-associated hyponatraemia (hyponatremia in US spelling) means the sodium level in your blood is too low. The 2015 international consensus mainly blames drinking more than you lose, while antidiuretic hormone, which tells the kidneys to hold on to water, fails to switch off. [8] The sodium gets diluted, like squash with too much water. In an analysis of 2,135 athletes weighed before and after exercise, sodium fell as body weight rose. Among the most marked cases, 81% had gained weight and none was dehydrated. [9]

Salt capsules don’t change this picture much. At an Ironman in South Africa in 2001, 413 triathletes were randomly given salt, a placebo or nothing, in an average of 17°C. Sodium at the finish and race times were the same in all three groups. The only serious case in the trial was in the placebo group: an athlete who had gained 3.8 kg from drinking too much. [10] The prevention that worked targeted water: at the New Zealand Ironman, after an information campaign and fewer aid stations, treated hyponatraemia fell from 3.8% to 0.6% of starters from one year to the next. [11]

Salt still has its place when sodium is low and has been measured: in Boston, 16 runners with hyponatraemia recovered in under 30 minutes with a very salty solution drunk on the spot. [12] And salt loss isn’t a myth. In a database of 1,303 athletes, endurance athletes lost an average of 1.28 litres of sweat an hour, with large differences in concentration from one person to the next. [13] Over 161 km in California, 35.6% of runners with hyponatraemia were dehydrated and hadn’t drunk too much. [14] At Kona, between 1989 and 2019, 29.7% of women and 21.2% of men tested in the medical tent had low sodium, and the authors also describe this loss-related form. [15] Bear in mind these were athletes who ended up in the tent, not all finishers.

Remember the nuance. A little salt helps you hold on to sodium when you’re drinking and sweating a lot, but it doesn’t protect you from too much water.

Should you worry about dehydration?

Less than you might think on a long race, as long as you drink. In the Australian Ironman mentioned above, triathletes lost an average of 3% of their body weight with no sign of temperature control failing. [2] In South Africa, significant weight loss wasn’t linked to more medical problems or a higher temperature. [16] Both races took place in moderate weather, so these results don’t transfer directly to a heatwave. Losing a little weight is normal, and the fastest athletes even tend to lose more. [14] The warning sign is finishing heavier than you started.

The International Olympic Committee offers a simple guide: weight, urine colour and thirst. It also points out that a drinking plan should be tested in training, at a similar intensity and heat to the race. [26]

Heat also empties the tank

Your brain runs mainly on sugar. When it runs short, you get confused, lose coordination, and recover quickly once you eat. Now, a 2025 meta-analysis of 51 studies shows that, for the same effort, muscles burn more glycogen, their sugar store, when it’s hot (standardised difference of 0.78, with a wide margin of uncertainty). [17] In a trial at 35°C, glycogen use rose by 25% and swallowed sugar was used about 10% less efficiently. [18]

The gut struggles too, because blood is diverted to the skin. During two hours of running at 35°C, 9 out of 10 runners had digestive problems, compared with 7 out of 10 at 22°C. [19] Just when you need food most, it’s the last thing you feel like. At the Hawaii Ironman, athletes surveyed took in an average of 62 g of carbs an hour, [20] while recommendations go up to 90 g at these distances. [21] Across all the events in that study, intakes ranged from 6 to 136 g an hour.

Eating is something you can train. Two weeks of carbs during sessions cut digestive problems by about 60%, tested in cool conditions. [22] At 32°C, a glucose and fructose mix was used about a third better than glucose alone. [23] This is the two-transporter principle, explained in our article on lactic acid and lactate gels.

One important limit: in the lab, in full heat, many athletes stop with normal blood sugar, simply because their temperature has reached 40°C. [6] A sugar crash is a credible explanation, never an obvious one.

How to run in the heat without overdoing it

Four pointers for running in the heat

These are the best-documented levers, details follow just below.

🗓️
Acclimatise
one to two weeks of sessions in the heat, progressing
🐢
Keep something in reserve
slow down, and beware of a late acceleration
🍌
Eat and drink
what you have tested in training, without ending up heavier
🧊
Cool down
ice and cold water, without relying solely on feeling

General guidelines drawn from studies on trained athletes. They guarantee nothing on any given day.

Give your body time to adapt

In a trained athlete, 75 to 80% of the adaptations happen in 4 to 7 days: you sweat earlier, more, and with less salty sweat. [24] Performance keeps improving after that, though. In Qatar, cyclists were still gaining 5% power between day 7 and day 14. [25] The International Olympic Committee consensus recommends 60 to 90 minutes a day for at least two weeks, while noting that a shorter block is still useful. [26]

No trip to the sun planned? A Norwegian study got the same gain, about 9.5% more power in the heat over 30 minutes, from 10 days of cycling in a hot room or simply wearing lots of layers. [27] These adaptations fade quickly, within about two weeks of the end of the block. [28] If you try it, build up gradually and stop at the slightest unusual sign.

Keep something in reserve for the end

Speeding up late in a long effort means producing a spike of heat when your body has the least margin left. Normally, the brain slows you down before the red zone. In the lab at 30°C, nine cyclists on a drug acting on dopamine and noradrenaline finished faster and hotter, at 40.0°C against 39.7°C on placebo. [29] Can the motivation of a race play the same role? Nobody has measured it. In serious heat, easing off early is better than finding out at the end.

Cool down, but don’t trust the feeling

Cooling the body before or during exercise improves performance in the heat by about 6.7% in the lab, in studies of men and efforts under two and a half hours. [30] Ice, cold drinks and sponges aren’t magic, because cold sometimes soothes the feeling more than the temperature. In one study, crushed ice taken before a run let people last longer than cold water, 50.2 against 40.7 minutes, and they finished hotter, 39.4°C against 39.1°C. [31] In humid, windless weather, a heat balance calculation suggests that an ice drink really does cool you, because sweat evaporates poorly. [32]

The field benchmark is still the thermometer capsule, with an average gap of under 0.1°C compared with an oesophageal probe, and up to 0.4°C from one reading to the next. [33] It’s hardly an everyday item. Remember this instead: in the heat, your feelings are not a thermometer.

If a runner collapses in front of you

The signs that aren't up for debate

Confused speech, staggering, incoherent answers or loss of consciousness: the race stops there, even if the person wants to carry on. Move them into the shade and call 999 in the UK, 112 in Europe, or your local emergency number.

At an organised race, the doctors’ rule fits in four words: cool first, transport later. [1] At the Falmouth Road Race, an American race of about 11 km, 274 heatstroke cases over 18 years were plunged into cold water on site, and all of them survived. [34] But knowing whether someone needs an ice bath, salt or sugar takes a measurement, and that decision belongs to the medical team.

Can a supplement protect you?

No, and a supplement brand has every reason to say so clearly. No capsule replaces time spent adapting, sensible pacing and tested nutrition. That’s why no DIX product has a place in this article.

Test what you’ve learned

Running in the heat: true or false?

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Your questions about running in the heat

Is it dangerous to run in the heat?

For most healthy runners, the risk comes mainly from intensity, duration and not being used to it. Heatstroke is rare, but it remains a medical emergency. If you take medication or have a health condition, talk to your GP.

At what temperature should you stop running outside?

There’s no one-size-fits-all number. Humidity, sun, wind, intensity and how acclimatised you are matter as much as the thermometer. In very hot weather, the simplest approach is to go out early, cut the session short and slow down.

How long does it take to get used to the heat?

The main adaptations happen in 4 to 7 days for a trained athlete, and the International Olympic Committee advises at least two weeks before a big event. These gains fade within about two weeks without further exposure.

Should you take salt when running in the heat?

On an ordinary run, none of the studies cited here shows a benefit. On a very long effort where you’re drinking and sweating a lot, a little salt can help you hold on to sodium, but it doesn’t protect you from too much water. The first rule is not to drink more than you lose.

To go further

Carbs during exercise are the most practical lever in this article. This one explains how they’re absorbed, and what the trendy gels are really worth.

Sources and studies cited

  1. Roberts WO et al. ACSM Expert Consensus Statement on Exertional Heat Illness: Recognition, Management, and Return to Activity. Current Sports Medicine Reports, 2021. Consensus.
  2. Laursen PB et al. Core temperature and hydration status during an Ironman triathlon. British Journal of Sports Medicine, 2006. 10 triathletes.
  3. Olcina G et al. Core Temperature Response During the Marathon Portion of the Ironman World Championship (Kona-Hawaii). Frontiers in Physiology, 2019. 15 amateurs.
  4. Nybo L, Nielsen B. Hyperthermia and central fatigue during prolonged exercise in humans. Journal of Applied Physiology, 2001. 14 men, laboratory.
  5. James C et al. Thermal and Biomechanical Responses of Amateur, Elite and World Cup Athletes During a World Cup Sprint Triathlon in the Heat. Sports Medicine, 2025. 66 triathletes.
  6. González-Alonso J et al. Influence of body temperature on the development of fatigue during prolonged exercise in the heat. Journal of Applied Physiology, 1999. 7 cyclists, laboratory.
  7. Asplund CA et al. Exercise-associated collapse: an evidence-based review and primer for clinicians. British Journal of Sports Medicine, 2011. Review.
  8. Hew-Butler T et al. Statement of the Third International Exercise-Associated Hyponatremia Consensus Development Conference, Carlsbad, California, 2015. Clinical Journal of Sport Medicine, 2015. Consensus.
  9. Noakes TD et al. Three independent biological mechanisms cause exercise-associated hyponatremia: evidence from 2,135 weighed competitive athletic performances. PNAS, 2005.
  10. Hew-Butler TD et al. Sodium supplementation is not required to maintain serum sodium concentrations during an Ironman triathlon. British Journal of Sports Medicine, 2006. Randomised trial, 413 triathletes.
  11. Speedy DB et al. Diagnosis and prevention of hyponatremia at an ultradistance triathlon. Clinical Journal of Sport Medicine, 2000.
  12. Siegel AJ et al. Exertional dysnatremia in collapsed marathon runners: a critical role for point-of-care testing to guide appropriate therapy. American Journal of Clinical Pathology, 2009.
  13. Barnes KA et al. Normative data for sweating rate, sweat sodium concentration, and sweat sodium loss in athletes: An update and analysis by sport. Journal of Sports Sciences, 2019. 1,303 athletes, study from an institute linked to a beverage manufacturer.
  14. Hoffman MD et al. Exercise-associated hyponatremia and hydration status in 161-km ultramarathoners. Medicine & Science in Sports & Exercise, 2013.
  15. Johnson KB et al. Clinical presentation of exercise-associated hyponatremia in male and female IRONMAN® triathletes over three decades. Scandinavian Journal of Medicine & Science in Sports, 2023. 3,138 records.
  16. Sharwood KA et al. Weight changes, medical complications, and performance during an Ironman triathlon. British Journal of Sports Medicine, 2004.
  17. Mougin L et al. The Effect of Heat Stress and Dehydration on Carbohydrate Use During Endurance Exercise: A Systematic Review and Meta-Analysis. Sports Medicine, 2025. 51 studies; two authors funded by beverage and supplement manufacturers.
  18. Jentjens RL et al. Heat stress increases muscle glycogen use but reduces the oxidation of ingested carbohydrates during exercise. Journal of Applied Physiology, 2002. 9 cyclists.
  19. Snipe RMJ et al. The impact of exertional-heat stress on gastrointestinal integrity, gastrointestinal symptoms, systemic endotoxin and cytokine profile. European Journal of Applied Physiology, 2018. 10 runners.
  20. Pfeiffer B et al. Nutritional intake and gastrointestinal problems during competitive endurance events. Medicine & Science in Sports & Exercise, 2012.
  21. Jeukendrup A. A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Medicine, 2014. Author then employed by a beverage manufacturer.
  22. Costa RJS et al. Gut-training: the impact of two weeks repetitive gut-challenge during exercise on gastrointestinal status, glucose availability, fuel kinetics, and running performance. Applied Physiology, Nutrition, and Metabolism, 2017. 25 runners.
  23. Jentjens RL et al. Exogenous carbohydrate oxidation rates are elevated after combined ingestion of glucose and fructose during exercise in the heat. Journal of Applied Physiology, 2006.
  24. Périard JD et al. Adaptations and mechanisms of human heat acclimation: Applications for competitive athletes and sports. Scandinavian Journal of Medicine & Science in Sports, 2015. Review.
  25. Karlsen A et al. Time course of natural heat acclimatization in well-trained cyclists during a 2-week training camp in the heat. Scandinavian Journal of Medicine & Science in Sports, 2015. 9 cyclists.
  26. Racinais S et al. IOC consensus statement on recommendations and regulations for sport events in the heat. British Journal of Sports Medicine, 2023. Consensus.
  27. Lundby C et al. Training wearing thermal clothing and training in hot ambient conditions are equally effective methods of heat acclimation. Journal of Science and Medicine in Sport, 2021. 34 cyclists.
  28. Cubel C et al. Time-course for onset and decay of physiological adaptations in endurance trained athletes undertaking prolonged heat acclimation training. Temperature, 2024.
  29. Watson P et al. Acute dopamine/noradrenaline reuptake inhibition enhances human exercise performance in warm, but not temperate conditions. The Journal of Physiology, 2005. 9 men.
  30. Bongers CC et al. Precooling and percooling (cooling during exercise) both improve performance in the heat: a meta-analytical review. British Journal of Sports Medicine, 2015.
  31. Siegel R et al. Ice slurry ingestion increases core temperature capacity and running time in the heat. Medicine & Science in Sports & Exercise, 2010.
  32. Jay O, Morris NB. Does Cold Water or Ice Slurry Ingestion During Exercise Elicit a Net Body Cooling Effect in the Heat?. Sports Medicine, 2018. Thermal balance calculation.
  33. Byrne C, Lim CL. The ingestible telemetric body core temperature sensor: a review of validity and exercise applications. British Journal of Sports Medicine, 2007. Review.
  34. Demartini JK et al. Effectiveness of cold water immersion in the treatment of exertional heat stroke at the Falmouth Road Race. Medicine & Science in Sports & Exercise, 2015.
  35. VG. Statements from the Norwegian team after the race. October 2026.
  36. 220 Triathlon. Should there be tighter controls in Ironman races?. October 2026.
  37. Tri247. Athlete’s reaction after the world championship. October 2026.
  38. Iowa Environmental Mesonet. METAR readings from station PHKO, Kona airport. 10 October 2026.

Article based on Cyril Certain’s video, with a review of the studies cited. The sections on dehydration, Boston and New Zealand are additions. Four points from the video are corrected here using the publications: the authors of the 2010 crushed ice study put the gain at 19%, not 23%; swallowed sugar was used about 10% less efficiently in the reference trial, not 10 to 20%; the 6 to 136 g an hour range covers all the events in the study, not Hawaii alone; the 0.78 score is classed as a medium effect by its authors. This content is for information only and does not replace advice from a healthcare professional.