Running a Marathon in the Heat: Fueling Guide

Heat and humidity fueling means raising attention to fluid and sodium while keeping carbohydrate on a practised g/h plan — not abandoning gels for ice pops you never trained. Hot races compete for blood flow between skin and gut, raise sweat rate, and increase GI symptom risk (Costa et al., 2017; de Oliveira et al., 2014). Being plant-based does not change that physiology; it still changes which packets are allowed.

Educational guidance only — not medical advice, not heat-illness treatment. Seek urgent care for heat-stroke signs, confusion, or collapse.

In-race drinking volume is covered in how much water during a marathon. Sodium sources are in electrolytes for marathon runners.

What stays the same in the heat

Published carbohydrate bands do not vanish because the forecast is 28 °C:

• About 30–60 g/h from a single carbohydrate source
• Toward about 90 g/h when glucose and fructose are combined and the gut is trained (Jeukendrup 2014; ACSM 2016)

Many runners still land near 60–75 g/h. Heat may force you to the lower end of your trained range if nausea rises — it is not a reason to invent a new product at kilometer 25.

Race-week food still targets about 8–12 g carbohydrate per kg for 36–48 hours (Burke et al., 2011) with the plantbaes.net ~15 g fiber/day heuristic so residue is not fighting you on top of heat (carb-loading guide).

What changes: fluid, sodium, concentration

Sawka et al. (2007) still frames fluid replacement as avoiding both dehydration and overdrinking. Hew-Butler et al. (2015) underline hyponatremia risk when athletes force large volumes of low-sodium fluid.

| Lever | Cool race habit | Hot/humid adjustment to test in training | | --- | --- | --- | | Fluid | Sip to thirst with a plan | More frequent sips; do not switch to forced plain-water gallons | | Sodium | Carb drink ~300–600 mg Na/L class | More deliberate sodium from practised drinks if you are a salty sweater | | Gel timing | Water with every gel | Non-negotiable in heat | | Hourly carbs | 60–75 g/h trained | Hold if tolerated; step down ~10 g/h if gut turns — do not zero out | | Product choice | Dual-source if targeting high rates | Same SKUs you trained in warmth | | Pace | Goal pace | Heat may require slowing so fueling stays possible |

We do not invent personalized sweat-sodium milligram prescriptions. Learn patterns on warm long runs. Put sodium beside carbohydrate grams in the vegan race fuel planner.

Heat-specific gut training

1. Schedule at least one long run in similar heat/humidity clothing and fluid access as race day.
2. Use the exact vegan-status-checked gels and mixes from the catalog.
3. Practise gel + water at aid-station frequency.
4. If using a stronger mix, confirm that concentration in heat — Skratch Super High-Carb is 100 g carbohydrate per serving in the catalog; that is a trained tool, not a heat-wave improvisation.
5. Do not debut LMNT-style stacks, new caffeine, or honey samples because “it’s hot.”

Vegan-specific heat traps

• Grabbing aid-station honey gels “because they look watery.” Honey Stinger is not vegan.
• Dairy recovery cartons mistaken for hydration (Tailwind Rebuild, Skratch Recovery).
• Dates or fruit mid-race as “natural cooling food” without practice — fiber/FODMAP risk (low-FODMAP race-week filter).
• Skipping salt because the carb load already feels heavy — panic tablets at 30 km cannot replace a practised bottle.
• Staying on high-fiber “light salads” in a heat-wave race week.

| Symptom pattern | More likely issue | First response (educational) | | --- | --- | --- | | Gel-paste nausea | Too concentrated / too little water | Slow, water, resume lower g/h | | Bloated, clear urine, headache | Overdrinking low-sodium fluid | Stop forcing water; practised sodium | | Salt crust, heavy sweat, crampy | High losses (cramp ≠ only sodium) | Practised sodium-forward drink | | Chills, confusion, stop sweating | Possible heat illness | Medical help — not another gel |

Humidity raises the stakes on water with gels, sodium planning, and practised concentrations. It does not authorize expo experiments. Keep finish-time expectations honest — fueling plans that assume a cool PR break in humidity.