Western States just showed us what heat really costs a runner

For over fifty years the Western States 100 has been defined by one thing. Canyon heat. Runners plan their whole race around surviving it.

Then the 2026 edition ran cool. Temperatures peaked around 23 degrees, roughly sixteen degrees Fahrenheit below the historical average for the day (Marathon Journal, 2026). What happened next was not a good day. It was a rewriting of the record book.

Vincent Bouillard ran 13:46:15 and took more than 23 minutes off Jim Walmsley's course record. Four men finished under the old record. Three broke 14 hours, which had never happened once before. The finish rate hit an all time high of 87 percent (Marathon Journal, 2026).

Same trail. Same climbs. Same distance. The single biggest variable that changed was the temperature, and the entire field got faster. That is the clearest real world demonstration you will ever see of what heat quietly steals from an endurance athlete. And most of what it steals runs straight through your gut.

Why heat slows you down

Every time your muscles contract, only about 20 to 25 percent of the energy becomes movement. The other 75 to 80 percent is released as heat inside your body (Hermand, Chabert and Hue, 2019; Bouscaren, Millet and Racinais, 2019). Your job during a long race is to shed that heat as fast as you make it.

In the cold that is easy. In the heat it is not. When the air is hot, you lose the ability to dump heat passively into the environment, so sweating becomes almost your only tool (Hermand, Chabert and Hue, 2019). And when the air is also humid, sweat cannot evaporate efficiently, so even that tool starts to fail.

As your core temperature climbs, your body sends more and more blood to the skin to try to cool you. That is blood being pulled away from your working muscles. Your heart rate drifts upward for the same pace, your perceived effort rises, and your aerobic capacity drops (Precision Hydration, 2025). The pace that felt easy in the cool now costs you far more.

The performance hit is real and it is measurable. At the Western States 100, researchers compared a hot edition, where temperatures ranged from 7 to 38 degrees, with a cool one ranging from 2 to 31 degrees. Runners were around 8 percent slower in the hot year, and the withdrawal rate was higher (Parise and Hoffman, 2011). Interestingly, over the 100 mile distance it was the faster runners who were hit hardest, partly because the slower runners spent more of their race moving through the cooler night (Parise and Hoffman, 2011).

The part most runners miss: heat wrecks your fuelling

Here is where it becomes a nutrition problem, not just a thermoregulation one.

Heat and dehydration attack your ability to eat and drink through several routes at once, and in an ultra that is often what ends the day.

First, blood gets diverted away from your gut. To cool you and to keep your muscles supplied, blood flow to the digestive system drops. Less blood to the gut means slower gastric emptying, impaired absorption, and a rising risk of nausea, cramping and vomiting. This is a big reason gastrointestinal symptoms are so common in ultras. At the 2025 Western States, 63 percent of finishers reported gut symptoms during the race, and most complaints clustered in the middle of the day, the hottest part (Mougin et al., 2026). Across the wider ultra literature, gut symptoms have been reported in roughly 35 to 96 percent of runners (Bouscaren, Millet and Racinais, 2019).

Second, the heat changes what your gut can actually do with the carbohydrate you take in. Under heat stress, the rate at which you burn the carbohydrate you have eaten drops by around 20 percent (Mougin et al., 2025). So the carbohydrate targets built from cool, sea level lab studies may overshoot what your body can use on a hot day, and pushing more in can simply add to the gut load without adding usable fuel (Mougin et al., 2026).

Third, this becomes a vicious cycle. You get hot, so you sweat hard. You cannot keep up with the fluid loss, so you drift into dehydration. Dehydration further slows gastric emptying and worsens the gut symptoms, which makes it even harder to take on fluid and calories, which deepens the dehydration. That spiral is one of the most dangerous patterns in ultra running, and it is exactly why you see so much ice, so many soaked shirts, and so many runners lying in creeks at hot races.

Sweat is not just water: the electrolyte side

When you sweat heavily for hours, you lose sodium, not just fluid. In ultras held in the heat, disturbances in sodium balance are common among finishers, and this cuts both ways (Hermand, Chabert and Hue, 2019).

Drink too little and you risk dehydration. Drink too much plain water out of fear of dehydration, and you can dilute your blood sodium and drift toward exercise associated hyponatremia, a genuinely dangerous condition that has affected a meaningful share of ultra finishers across studies (Hermand, Chabert and Hue, 2019; Bouscaren, Millet and Racinais, 2019). Slower runners who over drink are particularly exposed.

This is why blindly hammering water is not a strategy. Sodium alongside your fluid helps you hold onto what you drink, supports fluid balance, and can help stimulate thirst and voluntary drinking (Hermand, Chabert and Hue, 2019). Real ultra runners already lean this way. At the Grand Raid de la Réunion, a large tropical ultra, around 59 percent of runners added sodium to their food or drink during the race (Bouscaren et al., 2021).

Cooling: the lever the fast guys are pulling

If heat is the tax, cooling is how you pay less of it. And the evidence here is genuinely strong.

A meta analysis of 28 studies found that cooling strategies improved performance in the heat by around 6.7 percent on average. Cooling before exercise gave about a 5.7 percent gain, and cooling during exercise gave about 9.9 percent (Bongers et al., 2015). One honest caveat: those studies were all in men and were shorter than an ultra, so we should treat the exact numbers as a direction of travel rather than a promise for a 100 miler (Hermand, Chabert and Hue, 2019). But the principle holds up well, and cooling during the event looks at least as valuable as cooling beforehand.

What actually works in the field, without needing a lab:

• Water dousing. Pouring water over your head and body is one of the most practical mid race strategies going, because most aid stations have cups. It cools your skin, and it partly replaces sweat, which lowers how much you need to sweat and drink (Precision Hydration, 2025). Neck cooling during running in the heat has increased distance covered in trials (Hermand, Chabert and Hue, 2019).
• Ice on the skin. Ice bandanas, ice towels, or stuffing ice into your pack or sleeves cools you first by direct contact, then keeps cooling as the melt water evaporates. Ice towel application has been shown to blunt the rise in core temperature and lower heart rate (Precision Hydration, 2025). Use a setup that lets the melt evaporate off your skin rather than trapping it.
• Ice slurries and cold drinks. Drinking an ice slurry cools you from the inside as the ice melts, and it doubles as fluid and, if you mix it right, carbohydrate and electrolytes. It reliably improves how cool you feel and your perceived effort. Its effect on actual core temperature is less consistent, and it may be most useful in humid conditions where sweating is already limited (Precision Hydration, 2025; Hermand, Chabert and Hue, 2019).
• Menthol. A menthol mouth rinse does not lower your core temperature, but it tricks your cold receptors so you feel cooler, and that has improved running performance in the heat in trials (Hermand, Chabert and Hue, 2019).

One nuance worth knowing. The single most valuable moment to cool is often at the start, or whenever conditions suddenly ramp up the heat load. It is always easier to avoid storing heat than to shed it once it is in you (Precision Hydration, 2025).

Prepare your body before you get there: heat acclimation

Cooling manages the heat on the day. Heat acclimation changes how well your body copes with it in the first place, and the good news is you do not need weeks.

In a group of ultra runners, just two bouts of two hours of running in 30 degree heat were enough to trigger meaningful adaptations, including a lower heart rate and lower core temperature at the same effort, driven largely by an expanded blood volume (Costa et al., 2014). Full adaptation still favours a longer run in, with most benefit accruing over about two weeks (Hermand, Chabert and Hue, 2019). But if a proper block is not possible, even a couple of solid heat sessions in the days before a hot race is worth doing.

The catch is that most runners do not do it. At a large tropical ultra, only about a quarter of runners from cooler climates had trained in the heat before racing (Bouscaren et al., 2021). At the hot 2025 Western States, by contrast, the field was unusually well prepared, and that preparation, alongside race infrastructure and tested nutrition, likely helped keep severe gut issues lower than in previous hot editions (Mougin et al., 2026). Preparation is a difference maker, and it is one almost entirely in your control.

What to actually do for your next hot race

Turn all of that into a plan you can run.

• Acclimate before you go. Aim for repeated heat exposure in the days to weeks before a hot race. If time is tight, even two good heat sessions of around two hours help.
• Cool early and cool often. Do not wait until you are cooked. Douse with water at aid stations, keep ice on your neck and in your sleeves, and start cooling from the moment conditions turn.
• Get sodium in, not just water. Pair your fluid with sodium so you hold onto it and protect against both dehydration and over drinking. Do not chug plain water to the point your stomach sloshes.
• Respect the gut in the heat. Expect your stomach to handle less when it is hot. Do not force feed carbohydrate. Spread intake out, lean on fluids and slurries when solids stop going down, and back off before nausea turns into vomiting.
• Test everything in training. Every cooling method, every drink, every gel. Almost every runner in that well prepared 2025 Western States field had trialled their nutrition beforehand (Mougin et al., 2026). Race day is not the place to experiment.

Western States 2026 proved the point in the cleanest possible way. Take the heat away and even the best in the world go dramatically faster. You cannot control the forecast. But you can control how ready your body is, how well you cool, and how smartly you fuel when it is hot. That is the difference between surviving a hot race and racing it.

Heat management and fuelling are deeply individual. Your sweat rate, your sodium losses, your gut, and your event are all your own. If you want a hot weather fuelling and cooling plan built around your body and your next race rather than a generic template, book a discovery call with us [Book Call Here] and we will build it with you.

References

Bongers, C.C.W.G., Thijssen, D.H.J., Veltmeijer, M.T.W., Hopman, M.T.E. and Eijsvogels, T.M.H. (2015) 'Precooling and percooling (cooling during exercise) both improve performance in the heat: a meta-analytical review', British Journal of Sports Medicine, 49(6), pp. 377–384. Available at: https://doi.org/10.1136/bjsports-2013-092928.

Bouscaren, N., Faricier, R., Millet, G.Y. and Racinais, S. (2021) 'Heat acclimatization, cooling strategies, and hydration during an ultra-trail in warm and humid conditions', Nutrients, 13(4), p. 1085. Available at: https://doi.org/10.3390/nu13041085.

Bouscaren, N., Millet, G.Y. and Racinais, S. (2019) 'Heat stress challenges in marathon vs. ultra-endurance running', Frontiers in Sports and Active Living, 1, p. 59. Available at: https://doi.org/10.3389/fspor.2019.00059.

Costa, R.J.S., Crockford, M.J., Moore, J.P. and Walsh, N.P. (2014) 'Heat acclimation responses of an ultra-endurance running group preparing for hot desert-based competition', European Journal of Sport Science, 14(sup1), pp. S131–S141. Available at: https://doi.org/10.1080/17461391.2012.660506.

Hermand, E., Chabert, C. and Hue, O. (2019) 'Ultra-endurance events in tropical environments and countermeasures to optimize performances and health', International Journal of Hyperthermia, 36(1), pp. 752–759. Available at: https://doi.org/10.1080/02656736.2019.1635718.

Marathon Journal (2026) The ultimate limit: what happens when you remove the heat from Western States 100? Available at: https://marathonjournal.com/western-states-100-removing-heat/ (Accessed: 1 July 2026).

Mougin, L., Stennett, R., Ely, B.R., Taylor, L., Adams, W.M., Blagrove, R.C., Longman, D.P. and Mears, S.A. (2026) 'Heat preparation and nutrition strategies for a 100-mile ultramarathon in extreme heat: a questionnaire study at the Western States Endurance Run', Journal of Science and Medicine in Sport [in press]. Available at: https://doi.org/10.1016/j.jsams.2026.05.014.

Mougin, L., Horner, M., Edwards, D. et al. (2025) 'Heat stress impairs exogenous carbohydrate oxidation during prolonged running when maintaining euhydration', Journal of Applied Physiology, 139(6), pp. 1436–1446. Available at: https://doi.org/10.1152/japplphysiol.00873.2025.

Parise, C.A. and Hoffman, M.D. (2011) 'Influence of temperature and performance level on pacing a 161 km trail ultramarathon', International Journal of Sports Physiology and Performance, 6(2), pp. 243–251. Available at: https://doi.org/10.1123/ijspp.6.2.243.

Precision Hydration (2025) What is the best cooling strategy for athletes? Available at: https://www.precisionhydration.com/performance-advice/performance/a-comparison-of-cooling-strategies-for-athletes/ (Accessed: 1 July 2026).