How do you train in the heat and race a hot event?

Why does heat hurt endurance performance so much?

Endurance performance in the heat is limited by a specific chain of events. When you exercise, your muscles produce heat as a byproduct of work — far more heat than is typically useful to your body. To get rid of it, blood flow has to split between delivering oxygen to working muscles and delivering heat from the core to the skin, where it can be released through sweating and radiation. At some point those two jobs start competing.

In cool conditions the split is easy. Your core temperature stays stable around 37–38°C, sweating is efficient, and blood supply to working muscles is not significantly reduced. In hot conditions the same exercise produces the same muscle heat, but your body has a much harder time releasing it. Core temperature rises, sweating ramps up to try to compensate, and — critically — more of your cardiac output gets redirected to skin blood flow for cooling. That leaves less blood for working muscles, which is why heart rate climbs at the same power, perceived effort rises sharply, and sustainable output drops.

Above a core temperature of about 39°C, central nervous system protective mechanisms kick in. Your brain reduces the voluntary drive to your muscles independently of whether you feel like pushing harder — a built-in safety valve that keeps you from cooking yourself. You are not 'giving up'; your brain is protecting you. And no amount of motivation overrides it once the core temperature gets high enough.

What does heat do to race-day performance?

The performance impact of heat depends on temperature, humidity, and race distance. Short races are relatively protected because the race is over before the core reaches dangerous levels. Long races are where heat destroys people.

Research on marathon performance versus temperature consistently shows that marathon finishing times slow by roughly 1 to 3% for every 5°C above the performance-optimal range (about 10–12°C). A well-prepared amateur runner targeting a 3:30 marathon in cool conditions might realistically run 3:35–3:45 on a 25°C day, and 3:50+ on a 30°C day — even with the same fitness. In Ironman racing, the heat effect is larger still: hot Ironmans routinely produce finish times 20–60 minutes slower than cool ones for the same athlete.

This is not a matter of toughness. The physiology is the physiology. What a prepared athlete can do on a hot day is accept the slower target, pace accordingly, and finish. What an unprepared athlete does is pace for their cool-day target, blow up at the second half, and suffer through a much slower race than they'd have had if they'd adjusted from the start.

What is heat acclimation and how does it work?

Heat acclimation is the biological adaptation to repeated heat exposure, and it's one of the most effective performance interventions available to endurance athletes preparing for a hot race. The protocol is straightforward: expose yourself to heat during exercise, consistently, for 10 to 14 consecutive days, and your body will adapt.

The adaptations are well-studied and reliable across multiple decades of research from both military physiology labs (which cared about soldiers in hot environments long before sports science did) and endurance sport research groups. The main changes:

• Plasma volume expansion of 4 to 15% — the single most important heat adaptation. Your body increases the liquid portion of your blood, giving your cardiovascular system more volume to work with. This shows up as lower heart rate at the same workload, better sustained stroke volume during long efforts, and improved cardiovascular capacity.
• Lower core temperature at the same workload — your body produces the same mechanical work for less internal heat accumulation. This buys you more time before core temperature reaches the performance-limiting zone.
• Earlier and more efficient sweating — after acclimation, you start sweating at lower core temperatures, which means cooling starts earlier and is more effective. Sweat also becomes less concentrated in sodium, reducing the electrolyte cost of heavy sweating.
• Lower skin blood flow for the same cooling effect — meaning more of your cardiac output stays available for working muscles.
• Improved perceived effort and thermal comfort — the same workload in hot conditions feels subjectively easier after acclimation, even before any of the other measured adaptations are accounted for.

Collectively these adaptations can recover 3–8% of race-day performance in hot conditions — often more than anything else you could do in the two weeks before a race. The effect is large.

What does a heat acclimation protocol actually look like?

The classic protocol, refined across many studies, has a simple shape. It's boring and it works.

• 10 to 14 consecutive days of heat exposure during exercise. Skipping days slows the adaptation — the consistency matters.
• 60 to 90 minutes per session. Shorter than that and the heat exposure is insufficient. Longer is fine but adds diminishing returns.
• Hot conditions — typically 32 to 40°C ambient temperature. If you can't access real heat, you can create it artificially: a warm room with no airflow, extra layers during a trainer session, or a hot environmental chamber if you have access to one.
• Moderate intensity — roughly Zone 2, or a conversational endurance pace. You are not trying to do hard intervals in the heat. The point is repeated heat exposure while moving, not to destroy yourself.
• Stay hydrated, but not aggressively so. Drink to thirst with electrolytes. Part of the adaptation comes from allowing the body to experience the stress, so don't over-hydrate to eliminate it.

A simple practical version for most athletes: for 10 to 14 days before a hot race, do one daily indoor session on a bike trainer in a warm room with no fan, wearing extra layers, for 60 minutes at easy endurance pace. If you live in a hot climate, just doing your normal endurance sessions outside in the heat is sufficient.

How long does heat adaptation last?

Heat acclimation is reversible — the adaptations start to fade within days of removing heat exposure. The main plasma volume expansion typically holds for 1 to 2 weeks and is largely gone after 3 to 4 weeks without continued heat exposure. The other adaptations (earlier sweating, lower skin blood flow, improved comfort) fade on similar timescales.

The practical implication is that heat acclimation has to be timed around your race. Doing a heat block in May for a July race doesn't work. The adaptations need to still be present on race day, which means the acclimation block should finish within the final week or two before the race, and then a very short taper (2 to 5 days) preserves the benefits.

If you're training through a hot summer for races that fall inside the summer, you're already accumulating and maintaining heat adaptation through your regular training. The formal protocol exists mainly for athletes who train in cool environments and have to race somewhere hot.

How should you pace a race in the heat?

The single most important rule is that race-day targets should be adjusted downward for heat, and the adjustment should happen from the start of the race, not as a reaction to falling apart at kilometre 20.

The practical tools:

• Pace by perceived effort or heart rate, not by power or pace. In the heat, the absolute power or pace you can sustain drops, and forcing your cool-day target means a predictable collapse. Heart rate and effort self-regulate better because they reflect what your body is actually paying.
• Expect your heart rate to run higher than normal at the same effort. Cardiac drift will be more pronounced than in cool conditions. Accept it and stay at the effort level, not the number.
• Adjust target times downward. For a marathon in 28–30°C, plan on 3–5% slower than your cool-day target. For an Ironman in those conditions, assume 30–60 minutes longer than a cool-day finish is normal.
• Drink and use electrolytes aggressively. Sweat rates in a hot race can hit 1.5–2.5 litres per hour, with sodium losses over 1000 mg per hour. Aim for 500–1000 ml per hour of fluid with 500–1000 mg of sodium.
• Use active cooling wherever available. Ice in hats, cold water on the head and neck, ice in jersey pockets, wet sponges at aid stations — all have small but real effects on performance and thermal comfort. Pre-race ice-water immersion or cold showers reduce core temperature going into the race.

The athletes who race well in the heat are almost always the ones who accepted the slower target from the start and executed it consistently. The athletes who race badly in the heat are the ones who ignored it, went out at cool-day pace, and discovered the physiology in real time.

What about humidity?

Humidity makes heat substantially worse. The body cools through sweat evaporation, and evaporation slows dramatically as the air becomes saturated with water vapour. A 30°C day at 40% humidity is uncomfortable but manageable; a 30°C day at 90% humidity is physiologically closer to 38°C and produces far more severe heat stress at the same effort.

This is why race-day forecasts should be read with the wet bulb globe temperature (WBGT) or heat index, not just the dry air temperature. The Hawaii Ironman in Kona — famous for its heat — typically runs at 27–30°C with high humidity, which gives it a WBGT around 29–32°C, a genuinely dangerous range for long-duration exercise. A 30°C dry day in Arizona with 15% humidity is a very different physiological stress from a 30°C day in Florida with 85% humidity, even though the thermometer says the same thing.

Heat acclimation helps with both, but the protocols don't distinguish much between them. Train in whatever heat you can access; the adaptations transfer.

What are the most common heat-training and hot-race mistakes?

Five mistakes catch most athletes who meet heat unprepared.

• Ignoring the forecast and planning for a cool-day time. The forecast is the first data point of your race. Adjust accordingly.
• Not heat-acclimating when racing somewhere hot. Travelling from a cool climate to a hot race with no acclimation block leaves a large amount of performance on the table.
• Over-drinking plain water. In heavy sweat conditions, plain water without electrolytes can lead to hyponatremia (low blood sodium), which has killed endurance athletes. Use electrolyte drinks or add salt to your intake.
• Trying to race at cool-day power or pace. Your body can't produce the same output in the heat — forcing it means blowing up. Pace by effort or HR, not by the target number your fitness says you should be hitting.
• Skipping active cooling strategies because they 'feel unnecessary'. Ice in hats and cold sponges at aid stations produce measurable performance gains in hot races. Take them.