Marathon Fueling Guide
What to eat and drink during a marathon — carb rates, gel timing, hydration strategy, gut training, and how to pick a fuel brand. All backed by peer-reviewed sports science.
Quick summary
- →Start fueling at 40–45 min — not when you feel tired.
- →Target 60–90 g/hr of carbs for marathons (30 g/hr if you're new to fueling).
- →90 g/hr requires dual-transport fuels (glucose + fructose) + 4–6 weeks of gut training.
- →Chase standard gels with water, never sports drink.
- →Hydrate to thirst — 400–800 ml/hr is typical; heat raises this.
- →Train your gut on long runs 4–6 weeks before race day.
- →Caffeine gel at 45–65% of race time for peak effect at miles 18–22.
- →Never race with a fuel you haven't practiced in training.
How Many Carbs Per Hour?
Your muscles burn through glycogen fast at marathon pace — roughly 2,500–3,500 kJ over the race. Exogenous carbohydrate can't fully replace this, but it meaningfully extends your glycogen stores and delays the wall. The research is clear on three tiers:
One standard gel every 45–50 min. Ideal for first-timers and runners whose gut hasn't been trained on carbs during exercise. Prioritizes GI stability over maximum energy delivery. Start here and work up.
Brands: Any single-transport gel: GU, Clif, Huma, Honey Stinger
One standard gel every 25–30 min. The ceiling for single-transporter (SGLT1) absorption of glucose/maltodextrin. A solid target for experienced runners who have practiced fueling on long runs but haven't specifically trained for higher rates.
Brands: GU, Clif Shot, Maurten (100), SiS GO, Precision Fuel, NeverSecond
The current evidence-based standard for marathons over 2.5 hours with dual-transport carbs (glucose + fructose via SGLT1 + GLUT5). Requires 4–6 weeks of gut training. Single-transport fuels max out at ~60 g/hr — the excess draws water into the intestine and causes GI distress. Most elite and competitive age-group runners target this range.
Brands: Maurten 320, SiS Beta Fuel, NeverSecond C90, Precision Fuel 90, Tailwind
The science: single vs dual-transport absorption
The intestine absorbs glucose and maltodextrin via SGLT1 (sodium-glucose cotransporter 1), which saturates at ~60 g/hr. Exceeding this with single-transport gels means excess carbs sit in your intestine, draw water via osmosis, and cause cramping and diarrhea. Dual-transport fuels (glucose + fructose) add the GLUT5 fructose transporter, unlocking a second absorption pathway and allowing up to 90 g/hr — the current standard for competitive marathoners with trained guts (Jeukendrup 2014; Thomas et al. 2016).
Emerging research: A 2025 study in the Journal of Applied Physiology found elite male marathoners tolerated 120 g/hr at a near-1:0.8 glucose:fructose ratio with greater exogenous CHO oxidation than 90 g/hr. This is at the frontier of sports nutrition and requires extensive gut training — not recommended for recreational runners, but the direction of the science is clear: more carbs, more often, with dual-transport formulations.
When to Start Fueling
Take your first gel at 40–45 minutes into the race. Not when you feel hungry. Not when you feel tired. 40–45 minutes, regardless of how you feel.
Glycogen depletion is cumulative and invisible until it's catastrophic. By the time you feel the wall coming, your brain has already begun rationing blood glucose. You cannot replenish stores mid-race — you can only maintain them. That maintenance has to start early.
The old advice of "wait until you're hungry" or "your first gel at mile 6" reflects a misunderstanding of gut transit time and absorption. Research by Jeukendrup (2014) and the ACSM (Thomas et al. 2016) puts first fueling at 30–45 minutes into prolonged exercise.
Example timeline — 4:00 marathon (60 g/hr)
Hydration Strategy
The guiding principle from the ACSM (Sawka et al. 2007): drink to thirst, targeting replacement of 50–80% of sweat losses. Drink more and you risk hyponatremia (dangerous sodium dilution). Drink less and performance declines — body weight loss of 2% impairs thermoregulation and aerobic capacity.
Cool conditions (50–60°F)
400–600 ml/hr
Sweat rate is low. Sip at aid stations. Don't force fluids.
Mild (60–70°F)
600–800 ml/hr
Standard race conditions. One cup water at most aid stations.
Warm (70–80°F)
800–1,000 ml/hr
Sweat rate climbs. Prioritize water, consider electrolytes.
Hot (80°F+)
900–1,200 ml/hr
Drink at every aid station. Use ice when available. Slow down.
Sodium and electrolytes
Sweat contains ~900 mg/L of sodium on average (Baker 2017), with a huge individual range (200–1,600 mg/L). On hot days with high sweat rates, cumulative losses can exceed 2,000 mg — consider salt tabs or an electrolyte drink (not just water) if you're a heavy sweater or racing in heat. Most gels contain minimal sodium; sports drinks on course typically provide 200–400 mg/hr.
Caffeine Strategy
Caffeine is one of the most well-researched performance enhancers in endurance sport. The Southward et al. (2018) meta-analysis confirms a meaningful benefit at 3–6 mg/kg body weight for endurance events.
Dosing by weight
Timing for peak effect at miles 18–22
Note: carbohydrate co-ingestion (taking caffeine with a gel) delays Tmax by ~30–60 minutes compared to fasted state. Factor this in when you want peak effect during the final miles.
Fuel Brand Comparison
There is no objectively best gel — the best fuel is the one you've practiced with and your stomach tolerates at race intensity. That said, formulation differences matter:
The most widely available on-course gel. 20mg caffeine option (Roctane: 35mg). Hypertonic — take with water.
Buy on The Feed →Hydrogel technology. Isotonic — no water needed. Dual-transport. Premium price, premium performance. Caffeine variant: 100mg.
Buy on The Feed →Isotonic — can be taken without water. Thin, easy to consume at pace. Dual-transport formulation.
Buy on The Feed →40g carbs per gel — fewer servings needed for 90 g/hr. Dual-transport at 1:0.8 ratio. 200mg caffeine variant.
Buy on The Feed →50g carbs per gel — the highest on the market. Dual-transport at 1:0.8 ratio. Two gels covers a full hour at 90+ g/hr.
Buy on The Feed →Dual-transport (glucose + fructose). Clean taste, popular with elites. 75mg caffeine variant.
Buy on The Feed →Research-led brand. Dual-transport, 90 g/hr capable. 100mg caffeine option. Used by elite athletes.
Buy on The Feed →Cluster dextrin + fructose dual-transport. Canadian brand, growing in the ultra/marathon crossover space. 50mg caffeine variant.
Buy on The Feed →2:1 maltodextrin:fructose. The gel behind Pogacar's fueling strategy. 50mg caffeine variant.
Buy on The Feed →89mg caffeine from guarana in caffeinated flavors. Thin, easy to take at pace. UK-based brand.
Buy on The Feed →Honey-based, natural ingredients. 32mg caffeine in Ginsting flavor. Thicker consistency.
Buy on The Feed →Real-food ingredients with chia seeds. Easy on the stomach, slower energy curve. Good for sensitive GI.
Buy on The Feed →Whole-food gels (rice, real fruit). Lower sugar spike, longer digestion. Popular with ultra runners.
Buy on The Feed →Natural ingredients, 75mg caffeine from guarana. Italian-inspired, premium feel.
Buy on The Feed →Chewable format — 3 bloks per serving. 50mg caffeine in Black Cherry. Good for runners who dislike gel texture.
Buy on The Feed →Fruit-drop chews. Lower carbs per serving — need more frequent intake. 25mg caffeine per half-pack.
Buy on The Feed →Standard gel format from Clif. Turbo variant: 100mg caffeine. Widely available.
Buy on The Feed →OG endurance gel. 25-50mg caffeine depending on flavor. Thicker consistency.
Buy on The Feed →Drink mix — replaces both carbs and electrolytes. Dual-transport, isotonic when mixed. No gel texture.
Buy on The Feed →Pure maple syrup in a packet. Simple, natural, no caffeine. Easy on the stomach.
Buy on The Feed →Classic endurance brand. Espresso variant: 50mg caffeine. Available in flask-refill sizes.
Buy on The Feed →Isotonic vs hypertonic — what it means for you
Standard gels are hypertonic (500–1,200 mOsm/kg) — higher osmolality than body fluids. They need water to dilute them for absorption. Isotonic gels (Maurten, SiS GO Isotonic) are formulated at body-fluid osmolality and absorb without extra water — useful when you can't easily carry or access water. Never mix a hypertonic gel with a hypertonic sports drink; the osmolality spike draws water into the intestine instead of out.
Practical Race Day Tips
Practice your exact race fueling
Rehearse this exact schedule on your long training runs — same brand, same timing, same water strategy. Never try new fuel on race day.
Gel + water, not gel + sports drink
Standard gels are hypertonic — combining them with sports drink overloads gut osmolality and causes nausea. Chase gels with water. If you sip sports drink at an aid station, skip the gel at that stop. Exception: isotonic gels (Maurten, SiS GO Isotonic) are designed to bypass this and can be taken with minimal water.
Chew your fuel completely
If using chews or bloks, chew them fully before swallowing — even if it means slowing for 10 seconds. Half-chewed solids sit in your stomach, absorb slower, and are the most common cause of mid-race nausea with solid fuel formats.
Nausea protocol
If you feel nauseous, switch to small sips of sports drink instead of another gel. Walk through an aid station if needed — 30 seconds of walking won't wreck your race. Don't force another gel when your stomach is already full.
Carry your own fuel
Don't rely on what's on the course unless you've trained with that exact brand. Carry your own gels in a race belt, front pocket, or taped to your bib.
Start early, not late
Begin fueling at 40–45 minutes into the race, not when you feel hungry. By the time you feel depleted, you're already behind. Glycogen stores are limited and won't recover mid-race — only maintenance is possible.
Train Your Gut (Start Now)
Your stomach is a muscle you can train. Research shows that repeated exposure to carbohydrate during exercise increases gut tolerance and absorption capacity over 2–6 weeks (Costa et al., 2017). If you haven't been fueling during long runs, start immediately.
Take 1 gel during your long run. Just one. See how your stomach handles it at marathon effort.
Add a second gel. Space them 30–40 minutes apart, same timing as race day.
Practice your full race fueling schedule on your longest run. Same brand, same timing, same water.
No new experiments. Trust the plan you've practiced.
Gut training works best when you progressively increase carbohydrate intake during long runs — starting with 30 g/hr and working up to your race target. The discomfort you feel early in training (bloating, loose stools) is your gut adapting. Most runners adapt within 3–4 weeks of consistent practice.
Your Fueling Targets
Enter your weight and goal time to see how much fuel you need at each rate tier, plus your estimated calorie burn, sweat rate, and caffeine range.
Enter your weight and goal time to see your personalized fueling targets.
Half Marathon Fueling
Do you need to fuel during a half marathon? It depends on how long you'll be out there. Glycogen depletion begins at around 90 minutes of sustained effort. If you're finishing under 75 minutes, your stored glycogen plus a solid pre-race meal is generally enough.
Pre-race meal is sufficient. Optional: mouth rinse with sports drink at aid stations for a small CNS boost.
30–60 g/hr. One gel at 30–40 minutes, a second at 60–70 minutes. Don't overthink it — just get something in.
Treat it like a slow marathon — 30–60 g/hr with gels every 30–35 minutes starting at 30–40 min. You're on your feet long enough for glycogen depletion to be a real factor.
The most common mistake in half-marathon fueling: carrying gels but never taking them. A 2024 study in Sports Medicine — Open found gels had the highest "leftover rate" of any fuel format — runners pack them and forget or avoid them. Decide before the race exactly when you'll take each one.
Fueling for 4:30+ Marathoners
If your marathon takes 4+ hours, you need more total fuel than faster runners — not less. You're on your feet longer, burning more total calories, and glycogen depletion is just as real (it's time-based, hitting around 2 hours regardless of pace).
3:00 marathoner at 60 g/hr
~180g total
5–6 gels over 3 hours
4:30 marathoner at 50 g/hr
~225g total
7–8 gels over 4.5 hours
The hourly rate can be slightly lower (40–60 g/hr) because intensity is lower, reducing GI stress. But keep fueling consistently through the entire race — don't stop at mile 20 just because you're tired. Also watch your fluid intake: slower runners are at higher risk of hyponatremia (dangerous sodium dilution) from overdrinking. Drink to thirst, not to a schedule.
Get a plan built for your race
Weather, course, and body weight change everything
The plan above is a starting point. But running Boston in 45°F on a hilly course is completely different from Miami in 82°F heat. A racecast.io premium dossier generates a personalized fueling timeline for your specific race — adjusted for race-day temperature, WBGT heat stress, course grade, your body weight, and experience level. Including sweat rate estimates, sodium targets, GI risk scores, and terrain-aware gel timing.
Find your race →Research Sources
Jeukendrup (2014) — A Step Towards Personalized Sports Nutrition. Sports Medicine 44(Suppl 1):S25–S33.
Primary basis for 30/60/90 g/hr carb rate tiers and dual-transport theory.
Thomas, Erdman & Burke (2016) — ACSM Joint Position Statement: Nutrition and Athletic Performance. Medicine & Science in Sports & Exercise 48:543–568.
60–90 g/hr with dual-transport for events >2.5 hr.
Pfeiffer et al. (2012) — CHO oxidation from a CHO gel compared with a drink during exercise. Med Sci Sports Exerc 44(11):2038–2045.
Gel + water = similar oxidation to sports drink. High-osmolality combinations risk GI distress.
Costa et al. (2017) — Gut-training: the impact of two weeks repetitive gut-challenge during exercise. European Journal of Applied Physiology 117(12):2483–2494.
2–6 weeks of training with carbs during exercise improves gut tolerance and absorption.
Sawka et al. (2007) — ACSM Position Stand: Exercise and Fluid Replacement. Medicine & Science in Sports & Exercise 39(2):377–390.
Sweat rate models, fluid replacement targets (50–80% of losses).
Baker (2017) — Sweating Rate and Sweat Sodium Concentration in Athletes. Sports Medicine 47(Suppl 1):111–128.
Population sweat sodium ~900 mg/L, sex differences in sweat rate.
Guest et al. (2021) — ISSN Position Stand: Caffeine and Exercise Performance. Journal of the International Society of Sports Nutrition 18(1).
3–6 mg/kg caffeine is ergogenic for endurance. Mid-race top-ups of 50–100 mg supported.
Hearris et al. (2025) — Exogenous CHO oxidation rates at 90 vs 120 g/hr in elite marathoners. Journal of Applied Physiology.
Emerging: 120 g/hr at 1:0.8 glucose:fructose ratio tolerated by elite athletes with trained guts.
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