Racing in the heat is harder because your body has to cool itself and fuel your muscles at the same time, using the same limited supply of blood and fluid. As core temperature rises, more blood is sent to the skin, sweat losses shrink plasma volume and heart rate climbs at the same pace or power. The athletes who perform best in hot conditions arrive heat-acclimated, drink and take sodium to a tested plan, fuel with enough carbohydrate and start more conservatively than they feel they need to.

That applies equally to a midsummer full-distance race in southern Europe, a July gran fondo or a city marathon in an unexpected heatwave.

Key term — heat acclimation: a set of physiological adaptations produced by repeated exercise or exposure in the heat, typically over 7–14 days. It is for any endurance athlete racing in conditions warmer than they train in, and it is most useful in the two to four weeks before a hot event.

Why is racing in the heat so much harder?

In hot conditions the same pace or power costs more, because cooling and performance compete for the same blood. Working muscle generates heat continuously, so your body sends blood towards the skin and increases sweating. In humid air evaporation slows, heat storage rises and the cooling demand keeps growing.

The longer you sweat without replacing enough fluid, the smaller your plasma volume becomes. Less blood returns to the heart, so each beat pumps a little less, and your heart compensates by beating faster. This is cardiac drift (also called cardiovascular drift): heart rate rising over time while your output stays the same.

The practical signs are easy to recognise:

• Rising heart rate at fixed watts — the first signal that internal load is climbing faster than external load
• Higher perceived effort — a steady pace starts to feel like a hard session
• Faster glycogen use — studies in trained endurance athletes suggest carbohydrate use increases as body temperature rises
• Reduced gut tolerance — blood diverted to skin and muscle leaves less for digestion

Dehydration amplifies drift, drift raises effort and effort raises carbohydrate demand, which is why preparing for heat means working on several systems together.

SHOP FUEL, HYDRATION & RECOVERY

BUILD YOUR FREE FUEL PLAN

How long does heat acclimation take?

Most of the benefit arrives within 7–14 days of regular heat exposure, with some changes visible after just four or five sessions. Peer-reviewed research shows heat acclimation expands plasma volume, lowers heart rate at a given workload, triggers sweating earlier and makes sweat less salty.

For athletes in Scandinavia and northern Europe, that heat usually has to be created indoors.

Heat acclimation protocol: two to four weeks before a hot race

1. Choose your method — indoor trainer sessions in a warm room with minimal airflow, extra clothing layers, post-exercise sauna or a hot bath
2. Do 60–90 minutes of easy to moderate exercise in the heat — on 7–14 days, ideally consecutive or close together
3. Or add passive heat after training — 20–30 minutes in a sauna, or 30–40 minutes in a bath at around 40°C, straight after a normal session
4. Keep intensity low at first — heat adds load, so progress duration before intensity
5. Weigh yourself before and after — use the data to build your sweat rate profile
6. Maintain the adaptation — two or three heat sessions a week until race week, as gains fade within a few weeks without exposure

Safety note: stop immediately if you feel dizzy, confused or nauseous, or if you stop sweating. Passive heat exposure is not suitable for everyone; speak to a medical professional first if you have a heart condition or other health concerns.

How much should you drink when racing in the heat?

Drink enough to keep body mass loss to roughly 2–3% across the race, but never so much that you gain weight. Individual sweat rates vary enormously, from under 500 ml to more than 2 litres per hour, so your own data is the only reliable starting point.

How to measure your sweat rate

• Weigh yourself nude before a 60–90 minute session — in conditions close to race temperature
• Record everything you drink — in millilitres
• Weigh yourself nude afterwards — towel off first
• Calculate — weight before minus weight after (kg), plus litres drunk, divided by hours = litres lost per hour

Many athletes settle on 400–800 ml of fluid per hour in the heat, with heavy sweaters needing more. Drinking beyond your losses is not safe: overdrinking, particularly plain water, raises the risk of hyponatraemia (low blood sodium).

How much sodium do you need?

Sweat sodium concentration is largely individual and can vary several-fold between athletes. A practical starting range for long, hot races is 500–1,000 mg of sodium per litre of fluid, adjusted upwards if you are a salty sweater (white residue on kit, stinging eyes) or racing for many hours. A high-strength option such as Precision Hydration PH 1000 electrolyte tablets, which provide 1,000 mg of sodium per litre, makes the upper end of that range simple to reach.

How much carbohydrate per hour do you need in hot conditions?

For races lasting more than two and a half hours, aim for 60–90 g of carbohydrate per hour, and up to around 90 g or more on the bike if you have trained your gut to tolerate it. Heat does not reduce your carbohydrate needs; the evidence points the other way.

Muscle and liver glycogen fall steadily across a long race, so keeping carbohydrate coming through to the final hour of the run matters as much as starting with full stores.

• Use multiple carbohydrate types — blends of glucose (or maltodextrin) and fructose allow higher absorption than a single sugar
• Front-load on the bike — eating and drinking is easier while cycling, so build your largest intake there
• Keep fuelling on the run — many athletes drop to 60–80 g per hour, but should not stop
• Train the gut — practise your exact race intake on long rides and brick sessions, ideally in warm conditions

Combining fluid and carbohydrate in one bottle, for example with the Precision Fuel & Hydration Carb & Electrolyte Drink Mix, keeps intake regular. Higher-dose gels such as the PF 90 Energy Gel reduce how many items you need to carry and open. Carbohydrate-electrolyte solutions contribute to the maintenance of endurance performance during prolonged endurance exercise and enhance the absorption of water during physical exercise.

Why does the run fall apart late in a long-course triathlon?

After many hours of exercise, both the muscles and the brain lose some of their capacity to sustain output, and heat accelerates both. Falling glycogen, metabolic by-products and fibre damage reduce the force each stride produces, so the same marathon pace costs a greater share of what you have left.

Meanwhile the brain responds to rising body temperature, falling blood glucose and signals from tired muscle by reducing drive to the muscles, so perceived effort climbs.

The ability to resist this decline is called durability: how well you hold power, pace and running form late in a long effort.

• Long, steady rides of 3–5 hours — finished with a 20–45 minute run off the bike at race effort
• Track heart rate drift — a smaller rise at the same power across a session signals improving durability
• Strength training twice a week — heavy, low-repetition work supports force production late in a race
• Sleep and recovery — fatigue resistance is built in training but only expressed when you arrive rested

Race-day protocol for racing in the heat

Your race-day plan should limit heat storage, protect plasma volume and keep carbohydrate flowing from the first hour.

1. Pre-load fluid and sodium — drink around 500 ml with a high-sodium electrolyte in the 90 minutes before the start
2. Eat a familiar, carbohydrate-rich breakfast — 2–3 hours before the start
3. Ride the first hour below target — if heart rate is already high early, ease back, because drift will only add to it
4. Fuel and drink to a clock — set reminders every 15–20 minutes rather than waiting for thirst or hunger
5. Cool at every opportunity — ice in the suit and cap, cold sponges, cold water over the head and forearms
6. Adjust the plan, do not abandon it — if your gut struggles, switch to drink-based carbohydrate in smaller, more frequent amounts

Frequently asked questions

How long before a hot race should I start heat acclimation?
Begin two to four weeks before the event. Most adaptations develop over 7–14 days of exposure, and you can keep them with two or three heat sessions a week until race week. Finishing your main heat block a few days before travel avoids carrying extra fatigue into the taper.

Does sauna use count as heat acclimation?
Yes. Post-exercise sauna or hot baths can produce meaningful heat adaptations, particularly an increase in plasma volume. They work best straight after a normal training session: 20–30 minutes in a sauna or 30–40 minutes in a hot bath. Active heat sessions on the bike or treadmill add sport-specific benefit.

How much sodium should I take per hour in the heat?
It depends on your sweat rate and sweat sodium concentration. A common starting point is 500–1,000 mg of sodium per litre of fluid, which often works out at roughly 300–800 mg per hour. Salty sweaters and athletes racing for many hours may need more. A sweat test removes the guesswork.

Can I drink too much during a hot race?
Yes. Drinking more than you sweat, especially plain water, dilutes blood sodium and raises the risk of hyponatraemia, which can be serious. Use your measured sweat rate to set intake, include sodium, and avoid finishing heavier than you started. Aim to lose no more than around 2–3% of body mass.

Should I eat less carbohydrate when it is hot?
No. Heat tends to increase reliance on carbohydrate, so intake matters more, not less. What changes is tolerance, because blood flow to the gut falls in the heat. Practise your race-day fuelling in warm conditions so your gut is accustomed to 60–90 g per hour before race day.

What is cardiac drift?
Cardiac drift is the gradual rise in heart rate during long exercise at a steady output. As fluid is lost and blood is directed to the skin for cooling, each heartbeat pumps less, so heart rate increases to compensate. Heat and dehydration make it larger, which is why early pacing restraint matters.

SHOP FUEL, HYDRATION & RECOVERY

BUILD YOUR FREE FUEL PLAN

Stay consistent. Fuel with purpose. Race better.