Running Ultras in Extreme Heat. The Golden Rule — Eat Early, Drink Early

Ultra marathons are already a test of physical and mental resilience. Add extreme heat or desert-like conditions, and you’re stepping into survival-level endurance. The rules of ultra running don’t change, but in these environments, they become non-negotiable. At the heart of it all is one mantra: Eat early, drink early.

In the opening kilometres of an ultra, your body is fresh, glycogen stores are high in your muscles and liver, your gut is functioning at full capacity, and you should be fully hydrated.

At this point you feeling good, full of confidence and the last thing on your mind is eating or drinking as you are probably not hungry or thirsty YET!

There is a narrative that athletes should only drink to thirst, as there is a condition called hyponatremia. This occurs when an athlete drinks too much WATER.

Hyponatremia in athletes is a serious and can sometimes be life-threatening. It occurs when the level of sodium (salt) in the blood becomes abnormally low, usually below 135 mmol/L. Sodium is an essential electrolyte that helps regulate fluid balance, muscle contractions, and nerve impulses. During endurance events, especially marathons, ultras, or long triathlons, athletes can develop hyponatremia if fluid intake and electrolyte balance aren’t managed correctly.

As mentioned above, this condition occurs when the following happens:

Excessive water intake: Drinking large amounts of plain water without replacing sodium dilutes blood sodium concentration. In my opinion, water is great to swim in, but is not so great in ultra running. Its great to wash out your mouth, and for a change to avoid taste fatigue, but NOT for continuous hydration.

Sweat losses: Sodium is lost through sweat, especially in hot or humid conditions. If those losses aren’t replaced, blood sodium drops further.

Hormonal effects: Intense exercise can increase antidiuretic hormone (ADH) levels, reducing urine output. This means extra water is retained rather than excreted.

The opposite of hyponatremia is dehydration, which can also be very dangerous and is a huge cause of DNF in ultras.

Why you can’t “fix dehydration” on the run:

Once your body is dehydrated, blood flow to the stomach slows, gastric emptying stalls, and large volumes of fluid simply slosh in your gut instead of being absorbed. This is why trying to chug water mid-race often makes runners nauseous rather than rehydrated. As soon as you begin sweating, plasma volume drops, heart rate rises, and core temperature climbs. Waiting until you’re parched means you’re already on the slippery slope toward dehydration.

Your plasma volume shrinks, making the blood thicker, which makes it harder for your heart to pump the blood around your system. Less blood in the capillaries means less cooling of the skin, thus overheating occurs.

Due to the thicker plasma and the overheating, your HR raises.

When your cooling system fails, you run the risk over overheating and heat stroke.

Your sodium imbalance worsens, increasing the risk of cramping or hyponatremia. (Please note that cramping is generally caused by fatigued muscles, but a lack of electrolytes and an unbalanced sodium household certainly also play a role.)

As I have already said, as soon as you start sweating your core temperature rises, your HR rises, and your plasma levels drop so drinking to thirst can and will cause dehydration in very hot or windy conditions. As mentioned, dehydration cannot be fixed on the run, so try to not get to that point.

In desert-like conditions, where sweat loss can exceed 1.5 litters per hour, dehydration isn’t just performance-limiting, it’s life-threatening.

As much as drinking is important, so is eating. Eating before your run and fuelling on the go is the secret to preserving your glycogen stores in your muscles and liver.

Glycogen is a stored form of carbohydrate. Think of it as your body’s energy “savings account.” It’s a large, branched molecule made of glucose (sugar) units, and your body can quickly break it down to release glucose when you need energy. Most of your glycogen is stored in skeletal muscle, where it’s used locally to fuel movement, with a smaller amount in the liver. This glycogen helps maintain blood glucose levels, keeping your brain and other organs supplied with energy.

For endurance runners, glycogen is your body’s premium fuel. Unfortunately, your body has limited glycogen storage (roughly 1,500–2,000 calories), barely enough for 1 hour to 3 hours of running (depending on the intensity). Beyond that, you must rely on continuous fuelling.

When you’re running at moderate to high intensities, your body relies heavily on glycogen because it can be broken down quickly for energy. Fat is abundant, but it burns slower and can’t keep up with the demands of faster running. (see comment on fat burning at the end of this chapter)

Glycogen stores deplete quickly, even at moderate intensity. Eating early keeps your fuel topped up and your body uses that fuel to power your run, not your glycogen stores. Eating early also trains your gut to process calories continuously throughout the run. Once muscle glycogen is depleted, runners experience the dreaded “bonk” or “wall.” Energy levels plummet, legs feel heavy, concentration drops, and pace slows dramatically because the body is forced to rely mostly on fat.

Athletes who manage glycogen well—by starting conservatively, fuelling before the run and early in the run and pacing correctly—can run longer without crashing.

Most athletes require 90-100 calories per hour depending on size, effort, and conditions.

Once glycogen is gone, your body attempts to shift to fat metabolism. But switching gears mid-ultra is clunky, inefficient, and often comes with a wall of fatigue and brain fog. You cannot “power through” glycogen depletion — it’s like trying to drive a sports car on fumes.

So how do you protect your glycogen stores?

• Start fuelled: Taper training before race day and carb-load to maximize glycogen stores.

• Eat early, eat often: Taking in carbohydrates from gels, drinks, or food helps spare glycogen.

• Pacing: Running too fast too soon burns through glycogen quickly. A steady, controlled pace preserves it.

Fat burning in endurance athletes is difficult because it’s a slower, less efficient process compared to using carbohydrates. The body prefers carbohydrates since they can be quickly broken down into glucose for energy. We also always train with carbs,so switching to fat burning is not so simple. When carbohydrate intake is high, insulin levels rise and this suppresses fat oxidation, making it harder for the body to switch to burning fat—even when glycogen stores start running low.

Bottom Line for Endurance Runners
Glycogen is limited—like a small fuel tank. Once it runs dry, performance collapses. Protecting your glycogen stores with smart pacing, fuelling strategies, and training adaptation is the key to finishing strong instead of crawling to the line.

Ultras aren’t won on memory or willpower. In the heat, your brain is busy just keeping you moving forward. I try and eat every 30 minutes, even if it is only a bite or two and depending on the temperature, sip on my bottles continuously. It eventually becomes a habit.

If taste fatigue sets in and you really cannot stomach any more carbs in liquid form, just swirling the drink in your mouth is enough, the carbs are absorbed through the membranes under your tongue. A great way to fuel when you feel bloated.

Modern GPS watches (Garmin, Coros, Suunto) are ultra-running gold. Use them intentionally. As I have said, ultras are not won or finished on memory, so use the tools you have to your advantage. Your biggest hammer in your toolbox is your watch. Make technology your crew chief.

• GPS screens: See pace, distance, time, and heart rate at a glance.

• Lap screens: Auto-lap every kilometre for consistent feedback.

• Navigation: Use GPX routes in the desert to avoid navigational errors.

• Heart rate and pace zones: Monitor effort to avoid redlining early.

• Custom alerts: Nutrition, hydration, pacing, and HR reminders built in.

When the race stretches into double-digit hours under blazing heat, your watch becomes a silent pacer, nutritionist, and strategist — all on your wrist.

Set an eating alarm every 30 min and listen to it. If you must set a drinking alarm too. The irritation of an alarm is a small price to pay if it stops you from dehydrating and a DNF as a result!

The most common mistake in ultra running is going out too hard. In extreme heat, it’s a recipe for disaster!

• Start conservatively: Keep your heart rate low, stride relaxed and focus on efficiency.

• Even effort strategy: Run the first half of your race slightly under effort. Use heat as a cue to keep yourself in check.

• Finish strong: A well-paced ultra often means passing carnage in the final 20%.

In the unforgiving environment of ultra marathons in extreme heat, survival and success come down to discipline. Eat early, drink early. Protect your glycogen. Respect hydration. Pace conservatively. Use your technology wisely. Master these fundamentals, and you give yourself the best chance not just to finish, but to finish strong.