Marathon Fueling For Hot or Humid Conditions

Let's face it — "just hydrate more" is not a fuelling plan. When race morning arrives hot and humid, you need specific numbers, not vague intentions. I'd recommend you raise your sodium target to 900-1200mg/hour once temperature crosses 80°F or dew point crosses 65°F, and drop your carb concentration to 45-60g/hr rather than pushing your usual 60-90g/hr — because heat shifts blood flow away from your gut and raises your GI-distress risk sharply. Below are the exact sodium and fluid numbers by heat band, why carbs should drop rather than rise, and the decision you need to make the morning of the race.

You built your fuelling plan on cool-weather long runs. Now the forecast says otherwise. The adjustment is specific — and it isn't simply more of everything.

Notice that's banded by temperature and dew point together — not temperature alone. Humidity is the bigger driver of how hard your body has to work to cool itself. And in the high-heat band, this isn't a fuelling-alone fix. Pair it with a slower goal pace.

Why Carb Concentration Should Drop, Not Rise

As your core and skin temperature climb, blood flow shifts toward the skin for cooling and away from your gut. Your carb absorption capacity drops — and your GI-distress risk climbs at the same concentration that felt completely fine in training. My rule for athletes racing in the high-heat band: keep carb intake at the low end of your trained range — 45-60g/hr rather than 60-90g/hr — and prioritise diluted sports drink or water-plus-gel combinations over concentrated gels taken without fluid.

My reasoning is straightforward. GI failure risk outweighs the marginal energy-availability benefit of hitting your higher gram target in the heat. I'd recommend you under-fuel carbs slightly rather than push your trained number. It's a trade I'd make every time — and I'd encourage you to make it too.

The Morning-Of Decision Rule

Turn the table above into an action — not just a number to memorise. First, check the forecast temperature and dew point at your expected finish-window time, not just the start. If you're a later finisher, you'll hit the hottest part of the day, not the coolest. Second, map that to the heat band and pre-load the right electrolyte concentration into your flasks, or plan your aid-station grabs accordingly. Third — and this is the one athletes most often skip — if dew point crosses 65°F, pre-commit to a slower goal pace before the gun. Don't leave that decision to mid-race, when you're already under heat stress and your decision-making has measurably degraded.

I coach a runner who fell apart in the final 10K of a humid race because she kept her cool-weather gel concentration despite the forecast. Since then, every athlete of mine facing a hot-forecast race runs a pre-race dew-point check the morning before — not the morning of. It changes what goes in the flask, not just what goes in the legs.

When Fueling Adjustments Aren't Enough

Mild heat-fuelling discomfort — nausea, needing more frequent small sips — is something you can work through with the adjustments above. Red-flag heat illness is a different matter entirely. Stopping sweating, confusion, goosebumps in the heat, or dizziness mean you stop and seek medical attention. Full stop. You don't adjust your fuelling further. If you're experiencing anything beyond normal discomfort, see warning signs that go beyond a fuelling fix.

Pair this heat-adjusted fuelling plan with a heat-adjusted goal pace — heat changes both, and fixing only one leaves you exposed. If you're also racing a slower finish time, scale your total fuelling to your finish time first, then layer the heat adjustment on top.

Back to the full marathon fuelling strategy hub for the rest of your race-day nutrition plan.

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