Carbohydrate intake during a marathon is one of the most evidence-dense areas in sports nutrition, and also one of the most misapplied. Most recreational runners take far less than the research recommends. A smaller number take too much, without training their gut to handle it, and pay the price somewhere around kilometre 32.
Here is what the science actually says about how many carbs you need per hour in a marathon, how to structure your intake across the race, and why the cellular energy layer beneath your carbohydrate strategy matters just as much as the gels themselves.
What your body can actually absorb per hour
The human gut has a physical ceiling on carbohydrate absorption during exercise. Research by Jeukendrup (2014) established that a single carbohydrate source (glucose alone) can be oxidised at rates up to approximately 60g per hour. Push beyond that with glucose-only products and the excess carbohydrate sits in the intestine, drawing in water, causing bloating, and in many athletes triggering the kind of GI distress that turns a good race into a survival march.
The ceiling rises to approximately 90g per hour when you combine two types of carbohydrate that use different intestinal transporters. Glucose and fructose are absorbed via separate pathways (SGLT1 and GLUT5 respectively), which means using both simultaneously allows higher total absorption without gut overload. This is the science behind mixed-carbohydrate products and why "glucose:fructose" ratios appear on modern sports nutrition labels.
For a marathon specifically, research points to 60 to 90g per hour as the functional target for athletes racing at 70% VO2 max or above, with intake scaled by race duration and intensity.
What this means in practical terms
Most energy gels contain 20 to 25g of carbohydrate. To reach 60g per hour, you need to take two to three gels per hour, or one gel plus a sports drink with additional carbohydrate, or a combination of gels, chews, and liquid carbohydrate.
If you're aiming for 75 to 90g per hour for a faster effort, you need products with mixed glucose and fructose sources, and your gut needs to be trained to handle that volume. Gut training is exactly what it sounds like: practising high-carbohydrate intake during long training runs to upregulate your intestinal transporters. Jeukendrup's research also shows that consistent practice with high carbohydrate intakes during training can increase the gut's absorptive capacity over weeks.
If you have never practised taking gels or chews on a run, taking 90g per hour during a race is a reliable path to kilometre-32 regret. Your gut is a muscle. Train it.
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How to structure your intake across 42 kilometres
The most common fuelling error in a marathon isn't taking too little overall. It's front-loading. Most runners take their first gel too late (kilometre 15 to 20) and then try to catch up in the back half, which the gut handles badly when it's under physical stress at race pace.
A better structure looks like this. Start fuelling early: first gel or carbohydrate intake at kilometre 5 to 8, before you feel like you need it. Continue every 5 to 7 kilometres. By the time you hit the physiologically demanding stretch from kilometre 28 onwards, you've maintained blood glucose consistently and your glycogen stores are drawing down from a full tank rather than a depleted one.
Timing the final gel or carbohydrate intake at kilometres 35 to 38 gives a useful late-race blood glucose boost for the closing push, particularly in warm conditions where glycogen burns faster.
What happens when carbohydrates aren't enough
Glycogen depletion explains the bonk. But many athletes who fuel correctly still feel their cellular energy fade in the final 10 kilometres. They've taken their gels, kept the blood glucose up, done everything right by the carbohydrate playbook, and still experience a creeping fatigue that gels can't fix.
This is the cellular energy story that runs underneath the carbohydrate story. Mitochondria don't just need substrate (glucose); they need the structural and biochemical conditions to use that substrate efficiently. Sustained high-intensity exercise generates oxidative stress that degrades mitochondrial efficiency over hours. Gels replace the fuel. They don't address the engine.
Research from Lanfranchi et al. (2026) in the Journal of Physiology demonstrated that oleuropein-based olive leaf extract enhances mitochondrial bioenergetics response during moderate-intensity exercise in humans. This is the cellular layer of marathon preparation: supporting the mitochondria's capacity to convert available carbohydrate into ATP at the precision required for sustained race pace.
The Pre-Activity Shot, taken 60 minutes before the start, delivers that cellular layer. The Daily Shot, taken consistently in the weeks of training before the race, builds the mitochondrial foundation. Carbohydrate strategy sits on top of it. For a deeper look at how this plays out across distances, see the half marathon vs full marathon nutrition guide at OLEUS.
A practical carb-intake plan for a marathon
Target 60g per hour minimum for most recreational marathon runners, 75 to 90g per hour if you're trained in high-carb intake and using mixed glucose and fructose products. Start at kilometre 5 to 8. Fuel every 5 to 7 kilometres. Use sports drink alongside gels if you're targeting 75g per hour or above (liquid carbohydrate is easier on the gut than gels alone at high intake volumes). Practice the exact plan on your three longest training runs before race day. Carry one extra gel per hour as a buffer in case your pace or conditions demand more than expected.
The carbohydrate is the fuel. The mitochondria are the engine. Both need preparation, and both reward it.
Prime the engine before you fill the tank
The Pre-Activity Shot supports mitochondrial function before the start, so your cells are ready to convert carbohydrate into performance from kilometre one to kilometre 42.
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Sources
Jeukendrup, A. (2014). A step towards personalized sports nutrition: carbohydrate intake during exercise. Sports Medicine, 44(Suppl 1), 25-33. DOI: 10.1007/s40279-014-0148-z
Lanfranchi, C., et al. (2026). Oleuropein-based olive leaf extract enhances muscle mitochondrial bioenergetics response to moderate but not maximal intensity exercise in humans. Journal of Physiology. DOI: 10.1113/JP290316
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