Should I fuel differently on race day than in training?

No. Use the same carbs/hr target you've trained with. Gut absorption capacity adapts slowly — jumping from 60g/hr in training to 90g/hr on race day is the most common cause of mid-race GI distress. The principle every endurance coach drills in is 'nothing new on race day,' and it applies to fueling at least as much as kit. If you want to race at 90g/hr, train at 90g/hr in your key sessions for at least 4–6 weeks beforehand.

How many grams of carbs per hour should I aim for?

Under 60 minutes: usually nothing — your glycogen covers it. For a hard sub-hour effort (5K, criterium, threshold session) a carb mouth rinse, or a few sips, can produce a small CNS-mediated benefit even without ingesting meaningful carbs. 1–2 hours: 30–60 g/hr. 2–3 hours: 60–90 g/hr. 3–5 hours (most marathons, 70.3): 80–100 g/hr. 5+ hours: 60–120 g/hr, depending on whether you're cycling-pace or running-pace, and how trained your gut is. The number scales with duration and intensity — not with how 'important' the day is.

What is gut training?

The practice of progressively raising your in-session carb intake so your intestinal transporters (especially GLUT5 for fructose) up-regulate. Add 10–15 g/hr every 1–2 weeks during long sessions, using the exact products and concentrations you'll use in your event. Most race-day GI failures are gut-training failures, not product failures.

Does ratio matter at lower carb targets?

Not really. Below ~60 g/hr you're under the SGLT1 ceiling on glucose alone, so sucrose, maltodextrin, or dextrose all work fine without an engineered ratio. Once you push above 60 g/hr you start needing dual-transporter loading, which is why 1:0.8 (modern) or 2:1 (classic) becomes the right framing.

Do I bump my target up in heat?

No — bump fluid and sodium, not carbs. Heat doesn't raise your gut's carb-absorption ceiling; it usually lowers it. Keep your trained carbs/hr number, increase ml/hr, and add 200–500 mg/hr of sodium beyond your usual baseline. If you don't have a baseline yet, ~300–700 mg/hr is the typical range for a moderate sweater (sweat sodium varies enormously between individuals — roughly 200–2000 mg/L — so this is a starting point, not a prescription).

Should I dose my carbs all at once or in smaller increments?

Smaller, more frequent. Aim for a sip or chew every 15–20 minutes rather than a full gel or bottle dump on the hour. Steady delivery keeps the gut working at its absorption rate instead of asking it to clear a bolus, which is what triggers the cramping and bloat. Slamming 90g in one shot every hour is roughly the worst way to deliver 90g/hr.

Does body weight matter? Are these targets the same for a 55kg runner and a 90kg cyclist?

The traditional carb-rate framework is intentionally weight-independent — gut transporter ceilings don't scale linearly with mass. But common sense says a 55kg runner and a 90kg cyclist matched at the same intensity probably shouldn't sit at exactly the same g/hr. Use the table as a starting framework and adjust by feel: smaller athletes often settle toward the low end of each range, larger athletes toward the high end, with intensity and gut readiness still doing most of the work.

Caffeine is a separate lever, not a substitute for carbs. ~3 mg/kg taken 30–60 minutes before exercise (or split through long events) modestly enhances carb absorption and oxidation, and reduces perceived effort. It's a small but real performance edge that's standard in modern fueling. Test it in training — some athletes get GI distress from caffeine in events, which is exactly the kind of thing 'nothing new on race day' is meant to catch.

How many carbs per hour do I need?

The number depends on how long and how hard you're going, and how much your gut has been trained to absorb. Pick a target from the table below, train at it for several weeks, and use the same number on race day. That last part is the rule that most guides bury — and the one most often broken.

Last reviewed May 14, 2026·~5 min read

Targets by duration

A starting framework. Where you actually land inside each range depends on gut readiness (untrained = lower end, regardless of ambition), how much you're actually burning (higher intensity = higher need, lower tolerance — those pull opposite ways), and your body size. Adjustments below.

Duration	Carbs / hr	Intensity	Notes
Under 60 min	0 (or rinse)	Any	Glycogen covers the work. For a hard sub-hour effort, a carb mouth rinse or a few sips delivers a small CNS-mediated lift even without meaningful ingestion.
1 – 2 hr	30 – 60 g/hr	Easy to moderate	Sub-ceiling on glucose alone. Sucrose works fine.
2 – 3 hr	60 – 90 g/hr	Moderate to hard	Dual-transporter starts to matter. 2:1 or 1:0.8.
3 – 5 hr	80 – 100 g/hr	Hard / sustained	1:0.8 ratio, trained gut. Standard 70.3 / 3-to-4-hour-marathon range. Slower marathoners (5hr+) sit in the next row down.
5 hr +	60 – 120 g/hr	Pacing-driven	Full-IM and high-intensity cycling ultras tolerate 90–120 g/hr. Long-format ultra running (8+ hr at lower intensity) usually settles at 60–90 g/hr — tolerability beats maximum delivery when you're on your feet for that long.

Caveats the table doesn't capture

Body weight matters a little. The standard framework is intentionally weight-independent — gut transporter ceilings don't scale linearly with mass — but a 55kg runner and a 90kg cyclist matched at the same intensity probably shouldn't sit at exactly the same g/hr. Use the ranges as a starting framework and adjust toward the lower or higher end accordingly.
Concentration matters as much as grams. 90 g/hr in a 4–8% solution behaves very differently than 90 g/hr in a 12% one — the latter slows gastric emptying and pulls water into the gut. The dilution math is on /how-it-works.
Cycling vs. running ultra distances diverge. The 5+ hr row groups them by hours, but a high-intensity cycling ultra and a 10-hour ultramarathon are different fueling problems — the runner usually needs to prioritise tolerability and texture variety over peak g/hr.

The rule everyone breaks

Race day uses the same target as training

Race day is not the day you find out whether your stomach can handle 90 g/hr. That experiment belongs in a Tuesday long session in week six of your build. By race morning, the answer should be boring.

The reason: your gut's capacity to absorb carbs adapts the same way a muscle does — slowly, with repeated exposure. The fructose transporter (GLUT5) up-regulates in cells lining the small intestine when you train it. Skip that work, and the extra carbs just sit in your gut on race day, drawing water in. That's the cramping, the bloating, the porta-potty.

So pick a target you can actually train at consistently. Hit that number in your long sessions. Race at the number you trained at — not the number you wish you could absorb.

Stepping up your gut, week by week

One example progression for an athlete who currently fuels around 30 g/hr (or not at all) and wants to race at 90 g/hr. Treat it as an upper bound on pace, not a recipe — many athletes need longer at each step. Practice on a long session each weekend with the same products and concentrations you'll use on the day, and spread the intake out: a sip or chew every 15–20 minutes is far easier on your gut than a single hourly bolus.

Weeks 1–2
30→45g/hr
Weeks 3–4
45→60g/hr
Weeks 5–6
60→75g/hr
Weeks 7–8
75→90g/hr
Weeks 9+
90→100+g/hr

If a step gives you GI distress, hold for another week before advancing. If you blow up entirely, drop a step and rebuild — the progression is real but the floor is genuine, and pushing through symptoms is how you train your gut to associate fueling with pain.

What about heat? What about intensity?

Heat

Doesn't raise your carb ceiling — usually lowers it, because blood flow gets diverted to the skin and away from the gut. Keep your trained carbs/hr number. Bump fluid (often by 50%) and add 200–500 mg/hr of sodium beyond your usual baseline.

No baseline yet? A typical moderate sweater needs roughly 300–700 mg/hr. Sweat sodium varies enormously between individuals (~200–2000 mg/L), so this is a starting point, not a prescription — heavy or salty sweaters can run well above the top of that range.

Intensity (tolerance vs. need)

Two different effects pull in opposite directions, and most advice runs them together:

Tolerance falls with intensity — gut blood flow drops, so threshold or VO2 sets often have to sit 10–15 g/hr below your long-session number.
Need rises with intensity — you're burning more carbs per minute, so the metabolic case for fuel is stronger.

In practice: Z2 base sessions have plenty of tolerance but modest need — they're your gut-training sessions, where you practice race-day intake even though the workout itself doesn't require it. Threshold and tempo work has high need but limited tolerance (sit toward the middle of your range and dose it in small increments).

Common questions

Should I fuel differently on race day than in training?: No. Use the same carbs/hr target you've trained with. Gut absorption capacity adapts slowly — jumping from 60g/hr in training to 90g/hr on race day is the most common cause of mid-race GI distress. The principle every endurance coach drills in is 'nothing new on race day,' and it applies to fueling at least as much as kit. If you want to race at 90g/hr, train at 90g/hr in your key sessions for at least 4–6 weeks beforehand.
How many grams of carbs per hour should I aim for?: Under 60 minutes: usually nothing — your glycogen covers it. For a hard sub-hour effort (5K, criterium, threshold session) a carb mouth rinse, or a few sips, can produce a small CNS-mediated benefit even without ingesting meaningful carbs. 1–2 hours: 30–60 g/hr. 2–3 hours: 60–90 g/hr. 3–5 hours (most marathons, 70.3): 80–100 g/hr. 5+ hours: 60–120 g/hr, depending on whether you're cycling-pace or running-pace, and how trained your gut is. The number scales with duration and intensity — not with how 'important' the day is.
What is gut training?: The practice of progressively raising your in-session carb intake so your intestinal transporters (especially GLUT5 for fructose) up-regulate. Add 10–15 g/hr every 1–2 weeks during long sessions, using the exact products and concentrations you'll use in your event. Most race-day GI failures are gut-training failures, not product failures.
Does ratio matter at lower carb targets?: Not really. Below ~60 g/hr you're under the SGLT1 ceiling on glucose alone, so sucrose, maltodextrin, or dextrose all work fine without an engineered ratio. Once you push above 60 g/hr you start needing dual-transporter loading, which is why 1:0.8 (modern) or 2:1 (classic) becomes the right framing.
Do I bump my target up in heat?: No — bump fluid and sodium, not carbs. Heat doesn't raise your gut's carb-absorption ceiling; it usually lowers it. Keep your trained carbs/hr number, increase ml/hr, and add 200–500 mg/hr of sodium beyond your usual baseline. If you don't have a baseline yet, ~300–700 mg/hr is the typical range for a moderate sweater (sweat sodium varies enormously between individuals — roughly 200–2000 mg/L — so this is a starting point, not a prescription).
Should I dose my carbs all at once or in smaller increments?: Smaller, more frequent. Aim for a sip or chew every 15–20 minutes rather than a full gel or bottle dump on the hour. Steady delivery keeps the gut working at its absorption rate instead of asking it to clear a bolus, which is what triggers the cramping and bloat. Slamming 90g in one shot every hour is roughly the worst way to deliver 90g/hr.
Does body weight matter? Are these targets the same for a 55kg runner and a 90kg cyclist?: The traditional carb-rate framework is intentionally weight-independent — gut transporter ceilings don't scale linearly with mass. But common sense says a 55kg runner and a 90kg cyclist matched at the same intensity probably shouldn't sit at exactly the same g/hr. Use the table as a starting framework and adjust by feel: smaller athletes often settle toward the low end of each range, larger athletes toward the high end, with intensity and gut readiness still doing most of the work.
What about caffeine?: Caffeine is a separate lever, not a substitute for carbs. ~3 mg/kg taken 30–60 minutes before exercise (or split through long events) modestly enhances carb absorption and oxidation, and reduces perceived effort. It's a small but real performance edge that's standard in modern fueling. Test it in training — some athletes get GI distress from caffeine in events, which is exactly the kind of thing 'nothing new on race day' is meant to catch.

What we're drawing on

The dose-rate framework on this page comes from Asker Jeukendrup's 2014 review — rinse or small amounts for sub-hour efforts, up to 30 g/hr for 1–2 hr, up to 60 g/hr for 2–3 hr, and 90 g/hr with multiple transportable carbs at 2.5 hr and beyond. Podlogar & Wallis (2022) updates that ceiling to ~120 g/hr in trained guts at 1:0.8 glucose:fructose. The gut-training and tolerance literature behind these numbers is summarised alongside the source papers.

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