Marathon Fueling: How Many Carbs You Really Need

Hitting the wall at kilometre 30 is not fate — it is usually a fueling mistake. The body's glycogen stores last for roughly 90 minutes of hard running; a marathon takes considerably longer. Anyone who does not refuel systematically runs into a wall that can be avoided.

The 60–90 g rule

The current sports-science recommendation for efforts longer than 2.5 hours is: 60 to 90 grams of carbohydrates per hour. For shorter races (up to about 2.5 hours), 30 to 60 grams per hour are sufficient. The source matters: the gut can absorb glucose via a single transporter at only about 60 g/h. Higher intakes only work with a mix of glucose and fructose (typically in a 2:1 or 1:0.8 ratio), because fructose uses a second transporter. Modern gels and sports drinks are formulated exactly this way — a look at the label is worthwhile.

Two worked examples: a 4-hour runner at 60 g/h needs around 240 g of carbohydrates during the race — depending on the product, that is eight to ten gels. A 3-hour runner at 80 g/h also ends up at 240 g, spread over less time. Sounds like a lot? It is — and that is exactly why it so often goes wrong.

Sodium and fluids

Carbohydrates are only one part of the equation:

• Fluids: a guideline of 400–800 ml per hour, depending on temperature and sweat rate. In the heat, aim for the upper end — but never drink more than the body can process (overhydration is dangerous too).
• Sodium: 300–600 mg per hour replaces the most important electrolyte losses and supports fluid absorption. Many gels contain little sodium — sports drinks or salt tablets close the gap.
• Caffeine (optional): around 3 mg per kilogram of body weight in the final third of the race can reduce the perception of fatigue. Test it in training first.

Gut training: the underestimated factor

The gut cannot handle 90 g/h by default — but it can learn to. Studies show that regular carbohydrate intake during training significantly increases absorption capacity over several weeks. In concrete terms: starting eight to ten weeks before the race, practise race-pace fueling in every long run and in hard sessions. Begin at 40–50 g/h and gradually work up to your target amount. Stomach problems on race day are almost always a training problem, not a product problem.

Practical example: a 3-hour marathon

A concrete hourly plan at 80 g/h (gels with a glucose:fructose ratio of 1:0.8, 40 g of carbs each):

• Before the start (−15 min): 1 gel + 200 ml water. The stores start full.
• Kilometre 7 (approx. 30 min): 1 gel, plus water at the aid station.
• Kilometre 14 (approx. 60 min): 1 gel + sports drink (sodium).
• Kilometre 21 (approx. 90 min): 1 gel, water.
• Kilometre 28 (approx. 2:00 h): 1 gel + sports drink.
• Kilometre 35 (approx. 2:30 h): 1 caffeinated gel for the final section.

That adds up to six gels at 40 g each = 240 g of carbohydrates over three hours, plus 500–700 ml of fluid per hour. Important: take the gels early and at short intervals — if you wait until you feel tired, it is too late, because absorption takes 20–30 minutes.

Common mistakes

• Starting too late. The first gel at kilometre 20 achieves little — absorption takes 20–30 minutes, and by then the stores are already empty. Start within the first 30 minutes.
• Drinking only water. Without sodium and carbohydrates in your drink, half the strategy is missing — and in the heat, cramps loom.
• A new product on race day. Every gel and every drink needs to be tested in training beforehand. Race day is not an experiment.
• Only taking fueling seriously from the half marathon onwards. Intake matters in training under two hours as well — if only as gut training.

The Peakora fueling calculator

Peakora works this out individually for you: based on the estimated finish time of your target event, the fueling calculator computes carbohydrates, sodium and fluids per hour — including a caffeine recommendation for the final third. You export the result as a PDF for your race bag, and matching recipe ideas for the days before are included right away.