What We Can Learn From Kilian Jornet's Body During a 100-Mile Race

A groundbreaking study tracked every calorie, every degree of body temperature,

and every drop of sweat during the 2025 Western States Endurance Run. Here's

what it means for you.

There's something almost mythical about watching elite ultra runners glide through

100 miles of mountain terrain. We see the highlights, the finishing photos, the

triumphant arms raised at the finish line. What we don't see is what's happening

inside their bodies—the furnace of metabolism, the constant negotiation between

fuel and fatigue, the quiet battle against overheating.

Until now.

A team of researchers from Loughborough University recently published a

remarkable case study following Kilian Jornet—arguably the greatest mountain

runner of our generation—through every moment of the 2025 Western States

Endurance Run. Using doubly labelled water (the gold standard for measuring

energy expenditure), ingestible temperature pills, and detailed nutrition tracking, they

captured what actually happens when a world-class athlete pushes through 161

kilometres of California mountains in temperatures reaching 40°C.

Kilian finished third that day, in 14 hours and 19 minutes—just ten minutes off the

course record. But the real story isn't his placing. It's what the data reveals about

human endurance, and what it can teach the rest of us.

The Numbers That Stopped Me in My Tracks

Let's start with the headline figure: Kilian burned 16,104 calories during the race.

That's not a typo. To put it in perspective, that's roughly a week's worth of food for an

average person, consumed by his body in a single day of running.

He ate 6,720 calories during the race—an impressive feat of in-race fuelling—yet still

finished with a deficit of over 9,000 calories. This tells us something important: in

ultra-endurance events, you simply cannot replace all the energy you expend. The

goal isn't perfect balance; it's smart management of an inevitable deficit.

What struck me most, though, was his carbohydrate intake: an average of 86 grams

per hour across the race, ramping up to 110 grams per hour in the final five hours.

For years, the standard advice has been to aim for 60-90 grams per hour. Kilian blew

past that ceiling—and did it without any gastrointestinal distress.

This doesn't happen by accident. It happens through deliberate gut training over

months and years, teaching the digestive system to absorb fuel under stress. If

you've been conservative with your race nutrition because you're worried about

stomach issues, this is your invitation to gradually push those boundaries in training.

A Masterclass in Phased Fuelling

Perhaps the most practical insight from the study is Kilian's nutrition strategy, which

evolved across three distinct phases.

In the first four hours, he relied on a 50/50 mix of fats and carbohydrates—avocado,

coconut oil, nuts, dates, overnight oats, banana. Real food. Calorie-dense but

relatively easy on the stomach when intensity is manageable.

From hours four to nine, he shifted toward 80 grams of carbs per hour with a smaller

fat component. A mix of gels, soft bars, rice cakes, and those early-race flasks.

Then, in the final five hours—when the body is screaming for quick energy and the

stomach struggles to process anything complex—he went pure carbohydrate. 110

grams per hour. Gels only.

This phased approach makes physiological sense. Early in a race, fat oxidation

contributes meaningfully to energy production. As intensity creeps up and glycogen

stores deplete, carbohydrate becomes increasingly essential. By matching fuel type

to race phase, Kilian maximised both absorption and energy availability.

The lesson for us: stop thinking of race nutrition as one fixed strategy. Think of it as

a progression that adapts to what your body needs at each stage.

Preparing for the Heat Before it Arrives

Western States is notorious for its canyons—exposed, relentless, and often

exceeding 40°C. Kilian didn't leave his heat tolerance to chance.

In the three weeks before the race, he completed 12 structured heat acclimation

sessions. Four times per week, for 1.5 to 2.5 hours each, he trained in a makeshift

heat chamber set to 40-45°C. The goal of each session was to raise his core

temperature to approximately 38.5-39°C—measured with a rectal thermometer—and

hold it there.

This isn't comfortable training. It's not fun. But the adaptations it triggers—improved

sweating efficiency, expanded plasma volume, better cardiovascular stability in

heat—are profound.

During the race, despite ambient temperatures hitting 40°C, his average core

temperature remained at 37.1°C. It peaked at 39.4°C in the final kilometres—high,

but manageable. He never entered dangerous territory.

For those of us racing in hot conditions: Heat acclimation works, and it doesn't require fancy equipment. A hot room, extra layers, and a thermometer can get you started. The key is structured, progressive exposure over two to three weeks.

Cooling: Early, Often, and By Every Means Available

What surprised me wasn't that Kilian used cooling strategies—it's how early and how

systematically he deployed them.

He didn't wait until he felt overheated. From the Duncan Canyon aid station at

kilometre 39—long before the hottest part of the day—he began implementing a

multi-pronged cooling approach: wet clothing (evaporative cooling), cold-water

immersion in rivers when available (conductive cooling), ice slurry ingestion, menthol

mouth rinse, and chilled gels.

He maintained this discipline throughout the remainder of the race. No single method

was magic; it was the combination and consistency that kept his core temperature in

check.

The researchers noted something interesting: his highest core temperatures came

not when ambient temperature peaked, but in the final kilometres when his pace

increased approaching the finish. Exercise intensity, it turns out, drives thermal strain

even more than environmental heat.

The takeaway: Don't wait until you're suffering to cool down. Start early, use multiple methods, and maintain them consistently.

The Art of Pacing Below your Ceiling

The pacing data from this study is, frankly, beautiful.

The researchers built a speed-gradient-duration profile from Kilian's training data,

allowing them to calculate his "critical speed" —the threshold intensity above which

fatigue accumulates rapidly. Throughout the race, Kilian ran at an average of 84.8%

of his critical speed.

Here's what's remarkable: only 1.4% of race segments were run above critical

speed. He almost never touched the ceiling. He stayed consistently below it, hour

after hour, across every gradient.

The result? His performance declined by only 15% from start to finish. Compare that

to the average ultrarunner at UTMB, who sees a 32% decline between the first and

second halves. Kilian's fatigue resistance is extraordinary, but it's not

mysterious—it's the product of disciplined intensity management.

There's a lesson here that applies to every runner: Sustainable pace isn't about how fast you can go. It's about knowing your threshold and staying below it long enough for it to matter.

Hydration: Good Enough Is Sometimes Good Enough

Kilian consumed approximately 12.5 litres of fluid during the race—about 870

millilitres per hour—with 18.5 grams of sodium. That's substantial. Yet he still

finished 4.3% lighter than he started.

This tells us two things. First, in extreme conditions, complete fluid replacement is

often impossible. Sweat rates can exceed absorption capacity. Second, some body

mass loss is tolerable—perhaps even advantageous in a weight-bearing sport like

running.

The goal isn't perfect replacement. It's staying within a manageable deficit while

maintaining performance. Somewhere between 2-4% body mass loss is generally

considered acceptable for most athletes, though this varies individually.

Practical note: Don't forget electrolytes. Kilian's consistent sodium intake—roughly1.5 grams per litre of fluid—supported both fluid retention and nerve function. Plain water alone isn't enough in the heat.

Train Your Gut Like You Train Your Legs

We spend countless hours building aerobic capacity, muscular endurance, and

mental resilience. How much time do we spend training our gut to absorb fuel under

stress?

Kilian's ability to tolerate 86-110 grams of carbohydrate per hour without

gastrointestinal issues didn't emerge spontaneously. It's the result of systematic

practice—using race nutrition during training runs, gradually increasing quantities,

and learning what works.

The gut is trainable. Research shows that practicing high carbohydrate intake during

exercise improves gastric emptying, intestinal absorption, and tolerance. If you've

been holding back on race-day nutrition because of fear, start incorporating your

target intake into long training runs. Build up progressively. Find out what your gut

can actually handle when it's been prepared.

Use Your Data to Know Yourself

One of the more technical aspects of this study was the speed-gradient-duration

profile the researchers built from 77 of Kilian's training sessions. By analysing his

performance across different gradients and durations, they could predict his

capabilities on uphills, downhills, and flats—and then compare his race performance

against those predictions.

Most of us aren't going to commission a research team to build our profile. But the

principle applies: your training data contains valuable information about your

capabilities. GPS watches, power meters, heart rate monitors—they're not just for

logging miles. They can reveal your strengths and weaknesses across different

terrain.

Spend time understanding your own data. Know your sustainable pace on climbs

versus flats. Recognise when you're exceeding your threshold. Race with self-

knowledge, not just ambition.

Accept the Deficit

Perhaps the most liberating insight from this study is the confirmation that energy

deficit is inevitable in ultra-endurance events.

Kilian ate 6,720 calories and burned 16,104. That's a 9,384-calorie deficit. Roughly

40% energy replacement. And this is a world-class athlete with optimised absorption

and a carefully planned nutrition strategy.

You cannot out eat a 100-mile race. The gastrointestinal system has limits. What you

can do is maximise what you absorb, minimise wasteful expenditure, and accept that

your body will draw on stored reserves.

This is why pre-race fuelling matters. Kilian intentionally increased his dietary fat

intake in the week before Western States, gaining approximately one kilogram of

body mass. Extra glycogen stores. Extra fat reserves. A buffer against the deficit he

knew was coming.

What This Means for the Rest of Us

Case studies of elite athletes can feel distant from our own running. We're not Kilian

Jornet. We won't finish Western States in 14 hours.

But the physiology doesn't change. The principles that governed his body that

day—the relationship between fuel and fatigue, the impact of heat on performance,

the importance of pacing below threshold—apply to every human being who laces

up for a long run.

What this study offers isn't a template to copy. It's a window into what's possible

when preparation is meticulous, when nutrition is treated as seriously as training,

when heat is respected and addressed proactively, and when pacing is disciplined

rather than hopeful.

Your numbers will be different. Your race will be different. But the questions you

should be asking yourself are the same:

• How much can I absorb? Have I trained my gut to handle it?

• What's my threshold? Can I stay below it when it matters?

• Is it going to be hot? Have I prepared for that specifically?

• What does my data tell me? Am I listening to it?

The answers won't make you Kilian Jornet. But they might make you the best version

of yourself on race day. And that's the point, isn't it?

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Reference

Mougin et al. (2025). Physiological, nutritional and thermoregulatory responses of a world-class mountain ultramarathon athlete during the 2025 Western States Endurance Run 100. Journal of Applied Physiology.