How Should You Fuel For Training And Racing In The Heat?

The human body is remarkably inefficient. Only around 20 to 25 percent of the energy you produce during exercise becomes movement. The remaining 75 to 80 percent becomes heat. That is why endurance performance is a heat management problem as much as it is a fitness problem. Think of your body as an engine.

The objective isn't simply to produce power. It's also to stop the engine overheating. In cool conditions, your body does this remarkably well. Blood is redirected towards the skin, sweat evaporates and heat escapes into the environment. As temperatures rise, however, this becomes increasingly difficult. Your body now has two competing priorities:

• Deliver oxygen and fuel to working muscles.
• Dissipate heat to keep core temperature under control.

The result is predictable. Heart rate rises. Sweat rate increases. Carbohydrate utilisation increases. Electrolyte losses increase. The same pace suddenly feels much harder. Heat is not simply uncomfortable. It becomes part of the event.

Heat increases the physiological cost of exercise. That means the same pace or power output now places greater strain on the body. In practical terms, hot conditions can lead to:

• Higher heart rate at the same pace
• Faster glycogen depletion
• Greater sweat losses
• Higher sodium losses
• More gastrointestinal stress
• Increased perceived effort
• Earlier fatigue

This is why a pace that feels controlled in cool conditions can become unsustainable in hot weather. Your fitness has not disappeared. The event has changed.

I hear this all the time. The forecast says it's going to be hot. Most people solve the problem by carrying another bottle. Unfortunately, that's only solving one part of the problem. Heat doesn't just increase fluid loss. It also increases:

• Carbohydrate utilisation
• Electrolyte losses
• Cardiovascular strain

Water replaces fluid. It does not replace the extra carbohydrate your muscles are using. It does not replace the sodium you're losing through sweat. And it does not prepare your body for exercising in the heat. Many athletes think they have a hydration problem. In reality, they have a heat management problem. Water solves hydration. It doesn't solve heat.

There is rarely one solution. Avoiding overheating during a race is about keeping the whole system under control. That means:

• Arriving heat adapted
• Starting fuelling early
• Replacing electrolytes
• Drinking appropriately
• Pacing for the conditions
• Using cooling tactics where available
• Practising the strategy before race day

Many athletes fail in the heat because they wait until the problem appears. By then, it is often too late. The best cooling strategy for runners, cyclists and triathletes is the one that starts before the race begins.

Pre-cooling can help some athletes reduce thermal strain before exercise, especially in hot conditions. Examples include:

• Cold drinks or ice slurries
• Cooling vests
• Shade before the start
• Cold towels
• Ice in aid stations

But pre-cooling is not a replacement for preparation. It does not replace heat adaptation. It does not replace carbohydrate. It does not replace electrolytes. It is one tool in the system. The goal is not to find one magic cooling strategy. The goal is to stack small advantages that help you manage heat for longer.

Heat adaptation is the process by which your body becomes better at exercising in hot conditions. Heat training is the practice that creates that adaptation. It does not have to mean training in a hot country. It can be any controlled method that raises core body temperature enough to create a heat stimulus. There are two main approaches.

Active heat training means exercising in hot conditions or in extra clothing so that training itself raises core temperature.

Passive heat training means using heat exposure after or around training, such as saunas, hot baths, heated rooms or heat tents.

I used different approaches over my career, including saunas, heated bathrooms, plastic tents and training in hot environments. The method mattered less than the outcome. The goal was to elevate core temperature during repeated sessions and give the body a reason to adapt. In sports science, this is often discussed as raising core temperature above approximately 38.5°C, although the exact threshold and response will vary between athletes. Over repeated sessions where core temperature is elevated, the body adapts to that stimulus. These adaptations include:

• Increased plasma volume
• Earlier onset of sweating
• Higher sweat rate
• Reduced cardiovascular strain
• Better ability to dissipate heat
• Lower heart rate at the same workload

Most athletes begin developing meaningful adaptations within approximately one week, with further improvements over the following days. Just like fitness, these adaptations require practice. Heat adaptation is earned, not bought.

Meaningful changes can begin within approximately 7 to 10 days, although responses vary between individuals. Some athletes adapt quickly. Others need longer. The important point is that heat adaptation is not something you can create on race morning. For important hot races, start preparing early and maintain exposure where possible.

Electrolyte needs vary enormously between athletes. Some people are light sweaters. Some are heavy sweaters. Some lose far more sodium in sweat than others. But the direction of travel is simple. As conditions become hotter, electrolyte needs usually increase.

As a practical guide:

Water replaces fluid. Electrolytes replace what sweat takes away.