Above 18°C (65°F), your race plan changes. The body diverts blood from the gut to the skin to dissipate heat, gastric emptying slows by 20–40%, and sweat rates jump 50–100%. Running the same fueling protocol you trained in cool weather is the most common reason warm-weather marathons end at mile 20 instead of mile 26.

The one-sentence answer

Dilute your fuel, raise your sodium, slow your pace, and start cooling from the gun.

What heat does to the gut

Splanchnic blood flow drops by up to 70% in significant exercise heat. The gut becomes ischemic — under-perfused — which means it absorbs slower and complains harder. The same gel that went down at mile 10 in 13°C training sits like a rock at mile 14 in 28°C heat. The fix is not more carbs; it is the same carbs delivered more dilute and slower.

The five adjustments

1. Split each gel

Take half a gel, chase with 4–6 oz of water, wait 3–5 minutes, take the second half, chase again. Total carb count for the race is unchanged; the delivery rate matches the slower gastric clearance. See full marathon gel timing in our marathon gel count guide.

2. Raise sodium

Hot-weather sweat rates of 1.5–2.0 L/hr are normal for trained runners. At 600–1200 mg sodium per litre of sweat, hourly replacement jumps from ~500 mg/hr to 700–1500 mg/hr. The full sodium dosing guide covers personalisation by phenotype.

3. Drop carb concentration in drink mixes

Stock fueling bottles use 8–10% carb solutions. In heat, drop to 4–6%. Higher concentration means slower stomach emptying, which means more gut sloshing and a higher chance of distress.

4. Pre-cool

An ice slurry (500 mL of crushed ice + sports drink) consumed 30 minutes before the gun lowers core temperature by 0.3–0.5°C and buys you 10–15 minutes before heat-strain symptoms set in. A cold towel around the neck during warmup does similar work.

5. Pace down 2–3% per 5°C above 13°C

At 23°C wet-bulb, goal-paced effort is 4–6% harder than the same pace in cool weather. Hold goal effort, not goal pace. Negative-split the back half if temperature drops at sunset.

Hydration math in heat

Sweat rate of 1.5 L/hr × 4-hour marathon = 6 L. You cannot replace 100% during the race; the gut cannot absorb that fast. Aim for 70–80% replacement — roughly 750 mL/hr with sodium — and accept some post-race rehydration. If you do not know your sweat rate, run our 90-minute sweat-rate test before race week.

Cooling during the race

Ice in cap or sports bra. Sponges over the chest and quads, not the neck (drips down). A cold cup over the head at every aid station. If race organisers have hoses, take them.

Cold-weather caveat

The same plan applies in reverse below ~10°C: sweat drops 20–30%, so fluid intake drops proportionally, but carb and sodium targets do not change. The biggest cold-weather mistake is under-drinking because you do not feel thirsty.

Warning signs — slow or stop

  • Pounding headache that does not respond to fluids
  • Goose flesh in hot conditions (early heat stroke)
  • Slurred speech, dizziness, confusion
  • Stopped sweating despite high exertion
  • Sudden chills

Any of these means walking to the next aid station and asking for medical. The wall in heat can quickly become a hospitalisation event.

If the gut still shuts down

Switch from gels to liquid carbs (cola, sports drink) at aid stations. Smaller more frequent sips. If you have already bonked, follow the 30-minute bonk rescue protocol — walk 5 minutes, dose 30 g of carbs, sodium, then reassess.

Frequently asked questions

What if it is humid not just hot? Humidity is worse than dry heat because sweat does not evaporate. Drop pace 1% extra per 20% humidity above 60%.

Salt tablets vs sports drink? Both work. Tablets are easier to dose precisely; drinks make sipping habits easier. Use what you trained with.

Can I race a marathon at 30°C? Yes — but with a modified time goal and the protocol above. Sub-3 attempts above 25°C are statistical long shots.

Pre-race coffee in heat? Yes, but pair with extra sodium — caffeine raises urinary fluid loss.