Marathon Fueling Guide:
How Many Carbs Do You Actually Need?

Carb loading: kill the old myth

If your carb-loading plan involves three days of eating nothing but chicken breast followed by three days of pasta, you're running a protocol from the 1960s. The depletion-loading model works, but it's unnecessary. And it can leave you feeling flat on race morning.

Bussau et al. demonstrated in 2002 that a single day of high-carbohydrate intake (10 g/kg body weight) combined with rest produces full glycogen supercompensation — equal to the old multi-day protocol.[1] No depletion phase. No miserable low-carb days leading into your taper. One day of eating a lot and resting your legs.

Hawley et al.'s review in Sports Medicine established the broader guideline: 10–12 g of carbohydrate per kilogram of body weight per day for 24–48 hours before the event.[2] For events lasting more than 90 minutes — which includes every marathon for every human alive — this reliably produces a 2–3% performance improvement. That's 3–6 minutes off a 3-hour marathon. Free speed from eating.

Let's put real numbers on this. A 70 kg (154 lb) runner targeting 10 g/kg needs 700 g of carbohydrates in a day. At 12 g/kg, that's 840 g. Here's what that actually looks like:

Food	Serving	Carbs
Plain bagel	1 large	50 g
Cooked white rice	1 cup	45 g
Banana	1 large	30 g
Pancakes with maple syrup	3 pancakes + 3 tbsp syrup	90 g
Cooked pasta	2 cups	80 g
Sports drink (e.g. Gatorade)	32 oz	56 g
White bread with jam	2 slices + 2 tbsp jam	55 g

That table adds up to about 406 g — and that's roughly one meal and two snacks. To hit 700 g, you need to eat like this across breakfast, lunch, dinner, and snacks. It's a lot of food. That's roughly 14 bagels or 16 cups of rice across the day. Most runners underestimate what 10 g/kg actually requires.

Dual transport: the breakthrough most runners miss

For decades, the advice was simple: 30-60g of carbs per hour. That ceiling came from a single intestinal transporter called SGLT1, which absorbs glucose and maxes out at about 60g per hour.[3]

Then Jentjens and Jeukendrup found that fructose uses a completely different transporter: GLUT5. Two independent absorption pathways, working in parallel.[4]

By combining glucose and fructose, Jeukendrup's lab showed total carbohydrate absorption could reach 90 g per hour — a 50% increase over glucose alone.[5] This wasn't a small finding. It fundamentally changed what's possible in endurance fueling.

More recently, Hearris et al. (2022) demonstrated that trained athletes can absorb and oxidize exogenous carbohydrate at rates consistent with 120 g/hr intake, and that the delivery format — gel, drink, chews, or a combination — doesn't significantly affect oxidation rates.[6] This isn't theoretical. Tour de France riders like Tadej Pogačar routinely consume 100-120g/hr during mountain stages — fueling that would have been considered impossible a decade ago. The science moved, the products caught up, and the best athletes in the world proved it works.

The ratio matters. Podlogar and Wallis's 2022 review in Sports Medicine clarified the optimal blend: at moderate intakes (60–90 g/hr), a 2:1 glucose-to-fructose ratio works well. At higher intakes above 90 g/hr, the ratio should shift closer to 1:0.8 — nearly equal parts — because you need more fructose to keep both transporters working at capacity.[7]

How much do YOU need?

The answer depends on how long you'll be out there. Stellingwerff and Cox's 2014 framework, widely adopted in sports nutrition, recommends 30–60 g/hr for events lasting 1–2.5 hours and 60–90 g/hr for events exceeding 2.5 hours.[8] Since almost every marathoner is racing for more than 2.5 hours, you're in the higher bracket.

But those recommendations are increasingly seen as conservative. Podlogar and Wallis's 2022 update pushed the upper bound to 80–120 g/hr for well-trained athletes with adapted guts using dual-transport carbohydrate.[7] Elite marathon runners are now routinely fueling at 90–100+ g/hr in competition.

A key insight from Pfeiffer et al.'s analysis of 221 endurance athletes: higher carbohydrate intake correlated with faster finish times, and — critically — the GI problems that scare runners away from aggressive fueling were associated with fiber, fat, and solution concentration, not with the amount of carbohydrate itself.[9] The runners who ate more carbs didn't get sicker. They got faster.

Finish Time	Target (g/hr)	Per 20 min	Example
Sub-3:00	80–120 g/hr	27–40 g	1 gel every 15–20 min + drink mix
3:00–4:00	60–90 g/hr	20–30 g	1 gel every 20 min or gel + chews
4:00–5:00	50–80 g/hr	17–27 g	1 gel every 25 min + sports drink
5:00+	40–70 g/hr	13–23 g	Mix of gels, chews, and real food

Why the variation by finish time? Faster runners are working at higher intensities with less time to absorb fuel. They need the highest rates per hour but for fewer hours. Slower runners have more total time on course, more accumulated fueling opportunities, and typically lower per-hour requirements — but they need to fuel for longer, so total carbohydrate consumed may actually be similar.

When to start and how often

The most common fueling mistake is waiting too long to start. By the time you feel like you need carbs, glycogen is already critically low and blood sugar is dropping. You're playing catch-up in a game you can't win.

Start fueling within the first 30 minutes of the race. Then take in carbohydrate every 20–30 minutes for the remainder. This keeps a steady stream of exogenous glucose entering your bloodstream before your glycogen tank gets dangerously low.

Pre-race fueling

Your race-morning nutrition sets the stage. The goal is to top off liver glycogen (which depletes overnight during sleep) without causing GI distress at the starting line.

• 3 hours before start: Eat a carb-rich, low-fiber, low-fat meal — roughly 150 g of carbohydrate. Examples: 3 plain bagels with jam, a large bowl of white rice with honey, or pancakes with maple syrup. This is well-established practice across elite endurance sport.
• 60 minutes before start: Take in 30 g of easy carbohydrate — a gel, a sports drink, or a few swigs of maple syrup. This bridges the gap without triggering a large insulin response.
• 15 minutes before start: Sip water. Don't chug. You want hydration, not sloshing.

Race-day fueling timeline

Here's a practical mile-by-mile plan for a runner targeting 60–90 g/hr (adjust quantities for your specific target):

Mile	Approx. Time	Action
1–2	0:08–0:18	First gel or serving of drink mix (25–30 g)
4–5	0:35–0:45	Gel or chews (25–30 g) + water at aid station
7–8	1:00–1:10	Gel or chews (25–30 g) + water
10–11	1:25–1:35	Gel (25–30 g) + water. Check in with how your gut feels.
13–14	1:50–2:00	Gel (25–30 g) + water. Halfway. You should feel strong.
16–17	2:15–2:25	Gel (25–30 g) + water. Don't skip this one.
19–20	2:40–2:50	Gel (25–30 g) + water. The wall zone. Fuel through it.
22–23	3:05–3:15	Gel (25–30 g) + water if tolerated.
24+	3:20+	Final gel if needed. Sip water. Close it out.

Train your gut (it's faster than you think)

Here's the part that stops most runners from fueling aggressively: the fear of GI distress. And it's not irrational — if you've never practiced taking in 80 g of carbs per hour while running at race pace, your gut will absolutely rebel. Nausea, cramping, bloating, urgent bathroom stops. It happens.

But the gut is a trainable organ. Cox et al. showed in 2010 that 28 days of high-carbohydrate feeding during training significantly increased the rate at which athletes could oxidize exogenous carbohydrate.[10] The intestinal transporters — SGLT1 and GLUT5 — literally upregulate in response to repeated demand. Feed them more, and they get better at absorbing more.

Even more encouraging: Miall et al. demonstrated in 2018 that just two weeks of gut training reduced GI symptoms and carbohydrate malabsorption during exercise.[11] You don't need months. You need a few long runs with deliberate practice.

Martinez et al. (2025) added further support, showing that a repetitive gut-challenge protocol — consuming carbohydrate during exercise on successive days — improves gut function and reduces symptoms even over short intervention periods.[12]

The gut-training protocol

• Weeks 1–2: Start at roughly 60% of your race-day target. If you're aiming for 80 g/hr, practice at 45–50 g/hr during long runs.
• Weeks 3–4: Increase by 10–15 g/hr per week, building toward your full race-day target.
• Use the same products you'll use on race day. Your gut adapts to specific formulations. Switching brands or formats on race morning is asking for trouble.
• Practice at race pace, at least for portions of your long runs. Gut tolerance is worse at higher intensities because blood flow diverts away from the GI tract. Easy-pace practice doesn't fully prepare you.
• Include mid-week runs with fueling practice, not just the weekend long run. More frequent exposure accelerates adaptation.

One more tool worth knowing about: Rowe et al. (2022) found that hydrogel-format carbohydrate (like Maurten's gels) improved gastric emptying and GI tolerance compared to conventional formulations at the same carbohydrate concentration.[13] If you're struggling with gut issues at high intake rates, hydrogel products may help — though they're not required. Gut training matters more than the specific product.

Pacing is a fueling problem

We need to talk about pacing, because the best fueling plan in the world can't save you from going out too fast.

Brooks and Mercier's crossover concept is the key to understanding why.[14] As exercise intensity increases, your fuel mix shifts from predominantly fat to predominantly carbohydrate. At 60% VO2max, roughly half your energy comes from fat. At 85% VO2max, it could be 80%+ from glycogen. The relationship between pace and glycogen burn rate isn't linear — it's exponential at the margins.

What does this mean practically? Going out 10% too fast in the first half doesn't cost you 10% more glycogen. It costs you dramatically more, because you've shifted the fuel mix toward a much higher carbohydrate dependence. You're burning through your finite glycogen supply at a rate your fueling strategy was never designed to replace.

Ely et al. analyzed 137,000 marathon results and found that pacing errors — going out too fast relative to fitness — were the strongest predictor of late-race collapse, more so than ambient temperature or other environmental factors.[15] The runners who slowed most in the final miles weren't the ones who fueled poorly. They were the ones who went out too fast.

This is why it's worth modeling different scenarios before race day. What happens to your glycogen timeline if you go out at 7:45/mile instead of 8:00/mile? The difference might be the wall at mile 20 versus a strong finish.

Your race-day checklist

Everything above distilled into actionable steps. Print this out, tape it to your mirror, screenshot it — whatever works.

The day before

• Eat 10–12 g/kg body weight in carbohydrates across the full day. Prioritize low-fiber, low-fat sources: white rice, bagels, pasta, pancakes, sports drinks, juice.
• Hydrate normally — don't overdo it. Sip water and electrolytes throughout the day. Urine should be pale yellow, not clear.
• Lay out all race-day nutrition the night before: gels, chews, drink mix packets, flask. Count them. Know exactly what you'll carry and when you'll take each one.
• Avoid high-fiber vegetables, salads, heavy sauces, dairy (if sensitive), and anything you haven't eaten before.

Race morning

• 3 hours before start: ~150 g carbs. Familiar foods only. Bagels + jam, white rice + honey, or oatmeal with banana and maple syrup.
• 60 min before start: 30 g easy carbs (1 gel or sports drink).
• 15 min before: Sip water. Visit the porta-potty one last time.
• Pin your gels to your waistband or shorts. Have your first gel accessible — don't bury it.

During the race

• Mile 1–2: First fuel. Don't wait. This feels early — that's the point.
• Every 3–4 miles: Gel, chews, or drink mix — whatever you practiced. Chase gels with water, not sports drink (to avoid over-concentrating sugar in your gut).
• Don't skip the wall zone (miles 16–22). If anything, be more disciplined about fueling here. Your brain will tell you it doesn't want a gel. Your brain is wrong.
• Total target: 60–90+ g/hr depending on your finish time and what you've trained your gut to handle. See the table above.

What to carry

• Sub-3:00 runner (~2.5 hrs): 7–10 gels (or equivalent chews/flask). That's 175–300 g total. You may use aid station cups of sports drink to supplement.
• 3:00–4:00 runner: 8–12 gels. Consider a handheld flask with drink mix to supplement gels.
• 4:00+ runner: 10–14 gels or a mix of gels, chews, and real food (pretzels, gummy bears). More time on course means more total fuel needed.
• Always carry 1–2 more gels than you think you'll need. Dropping one, missing an aid station, or wanting extra late in the race — the backup is cheap insurance.

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