Fuelling in the heat: what actually changes

How should you fuel in the heat? Start eating and drinking early, keep it steady, take on more fluid and sodium than you would on a cool day, and use cold drinks to help keep your temperature down. Heat makes your body burn through its stored carbohydrate faster while absorbing less of what you eat, so a fuelling plan that works in spring can fall apart in July.

The bigger point, and the one most people skip, is that the best heat strategy isn't something you swallow. It's getting used to the heat before race day.

What heat does to your fuel

When it's hot, your body has two jobs at once: power your muscles and get rid of heat. Both need blood. More of it goes to your skin to cool you, and your core temperature still climbs. That changes how you use fuel in a few ways.

You burn through stored carbs faster. In a study of trained men exercising at 40°C compared with 20°C, their muscles broke down more stored glycogen in the heat. Another study found muscle glycogen use was about 25% higher in the heat in the final 30 minutes of exercise.

You use less of the carbs you eat. That same study found the body burned less of the carbohydrate athletes were drinking during exercise in the heat. So on a hot day you're emptying the tank faster and topping it up less efficiently.

Your gut is under more stress. The leading explanation, from research on running in the heat, is that the drop in blood flow to the gut and the rise in body temperature make it harder for the gut to do its job. In practice, that's why stomach problems are so common in hot races.

Put those together and the risk is clear. You run low on fuel sooner, and it's harder to fix by eating more late on.

What to change on a hot day

Carbs: same target, smaller doses

I wouldn't raise your carbs per hour in the heat. You're already absorbing less, and a stressed gut is the last place to dump a bigger dose. What I'd change is how you deliver it: start early, before you feel you need it, and take smaller amounts more often.

This is the same on the bike, on the run or in a long open-water swim with feeds. If anything, it matters more when you're running, where the gut is already having a harder time. Our gut training guide covers how to build your tolerance in training, and in summer it's worth doing some of that practice in the heat.

Fluid: drink more, but don't overdo it

You'll sweat more in the heat, so you'll need more fluid than on a cool day. But the way to get there is still to drink to thirst, not to force down a fixed amount. Overdrinking on a hot day, especially in a long, slow event, is how people end up with low blood sodium. Our hyponatremia guide explains why.

The best way to know your number is to measure it. A sweat rate test in hot conditions tells you roughly how much you lose per hour when it's warm, which is often very different from your winter number.

Sodium: matters more when it's hot

More sweat means more sodium lost. For short, cool sessions it barely matters. For long, hot ones, and especially if you're a heavy or salty sweater, it's worth planning. Our sodium guide covers how much.

Cold drinks: a free advantage

Cold fluid does more than hydrate you. It also takes heat out of your body. In one study, athletes who drank an ice slurry before running in 34°C heat ran for 50.2 minutes on average before stopping, compared with 40.7 minutes after cold water. That's about 19% longer.

To be honest about it, the research on ice slurry isn't all this positive, and a later review questioned how big the effect really is. But cold drinks cost nothing, and there's little downside. In a race, grab ice at aid stations, freeze a bottle the night before for the bike, and drink cold where you can.

The thing that works best: get used to the heat

Here's what I'd tell anyone with a hot race on the calendar. The biggest gains don't come from a product. They come from your body adapting to heat.

When you train in the heat regularly, your body changes:

• You sweat more. A review of seasonal heat studies found sweat rate went up by anywhere from 0.03 to 0.53 litres an hour after a summer of exposure.

• Your sweat gets less salty. The same review found sweat sodium fell by between 22% and 59%.

• Your blood volume rises. A large 2024 analysis of heat training found it increases resting plasma volume and lowers heart rate during exercise.

• You may rely less on stored carbs. In the 40°C study, a week of heat training partly reduced how much the athletes leaned on carbohydrate as fuel.

So heat training helps on almost every front: better cooling, less salt lost and less of the extra carb burn. If your race is somewhere hot and you train somewhere cool, building in heat exposure in the weeks before is worth more than any drink mix.

Most people don't bother. They show up to a hot race having trained in mild weather, then try to fix it with more gels and more water on the day. That rarely works.

A simple hot-day plan

• Arrive well fuelled and hydrated, not overfilled.

• Start eating and drinking early, before you feel you need to.

• Keep your carbs per hour at your trained target, in smaller, more frequent doses.

• Drink to thirst, knowing you'll be thirstier than usual.

• Take sodium if it's long and hot, especially if you sweat a lot or sweat salty.

• Drink cold when you can, and use ice.

FAQ

Do I need more carbs in the heat? You burn through stored carbs faster, but you also absorb less of what you eat. So rather than raising the dose, start earlier and take smaller amounts more often.

How long does it take to get used to the heat? Changes in sweating and blood volume come with regular heat exposure over days to weeks. Summer training in warm weather does it naturally. If you train somewhere cool, deliberate heat sessions in the weeks before a hot race help.

Is it better to drink a lot before a hot race? Arrive hydrated, not overloaded. Drinking far more than you need before or during a long race doesn't protect you, and can lead to low blood sodium.

Do cold drinks really help? They help take heat out of your body, and one study found an ice slurry before running in the heat extended running time by about 19%. The overall evidence is mixed, but there's little downside.

Sources

• Febbraio MA, et al. Muscle metabolism during exercise and heat stress in trained men: effect of acclimation. 1994.

• Jentjens RLPG, et al. Heat stress increases muscle glycogen use but reduces the oxidation of ingested carbohydrates during exercise. Journal of Applied Physiology, 2002.

• 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.

• Siegel R, et al. Ice slurry ingestion increases core temperature capacity and running time in the heat. Medicine and Science in Sports and Exercise, 2010.

• Seasonal heat acclimatisation in healthy adults: a systematic review. 2021.

• Effects of heat adaptation on physiology, perception, and exercise performance in the heat: an updated meta-analysis. 2024.

In MOVE, a sweat test logged on a hot day gives you a fluid target for each hour based on what you actually lose, and every planned session gets its own carbs per hour, so you can rehearse your hot-race numbers in training.