Carbohydrates for Endurance Athletes: How Much You Need

Carbohydrates in endurance sport are not a matter of belief but an arithmetic problem with hard limits. Your body stores them in limited quantities, burns them almost exclusively at high intensity, and can only absorb a certain amount per hour — no matter how much you eat. Anyone who knows these three numbers can plan fuelling without guesswork, without panic and without the famous bonk at kilometre 30.

Why carbohydrates are the limiting factor

A trained runner stores roughly 400 to 600 grams of glycogen — around 80 to 120 grams in the liver and 300 to 500 grams in the muscles. That equals 1,600 to 2,400 kilocalories. A marathon burns 2,600 to 3,200 kilocalories depending on body weight and pace. The maths only works out if a substantial share of the energy comes from fat and if you top up along the way.

Fat supplies 50 to 60 percent of the energy at easy pace, which is exactly why aerobic base training gets so much attention. But the ratio shifts with intensity: above roughly 80 percent of maximum oxygen uptake, the body covers well over 80 percent of its demand from carbohydrates. At 5K pace, glycogen economy is effectively the whole race. That is why the carbohydrate question is not a question for the last three kilometres — it is decided from the start.

Daily needs: grams per kilogram, not percent of calories

The most useful control variable is grams per kilogram of body weight — not the carbohydrate share as a percentage of calories. Percentages are misleading because they do not scale with training volume: an athlete burning 6,000 kilocalories a day on “60 % carbs” eats 900 grams, a recreational athlete burning 2,400 kilocalories on the same percentage gets only 360 grams. Current position-stand logic (ACSM, IOC) instead brackets intake by load:

• Rest day, little training: 3–5 g/kg — 210 to 350 g at 70 kg. Everyday cooking is enough, no special treatment needed.
• One hour of moderate training per day: 5–7 g/kg (350–490 g). This is the band most recreational athletes should live in permanently.
• One to three hours of intense training daily: 6–10 g/kg (420–700 g). Here fuelling becomes part of the training itself.
• Four to six hours daily, training camps, ultra distances: 8–12 g/kg (560–840 g). Practically only reachable through liquid and highly tolerable sources.

What matters is the difference between the daily balance and acute availability. An athlete who chronically lives at 3 g/kg and then does a long run at the weekend is driving on a permanently half-empty tank — it feels like overtraining but is often just a nutrition problem. Conversely, carb loading achieves nothing if the weekly base is missing.

Timing: the four windows that count

• Before the session: For runs under 60 minutes a normal meal rhythm is enough. Before long or hard sessions, 1–2 g/kg two to three hours out — 70 to 140 grams at 70 kg. Oats with banana, rice with egg, bread with honey: all fine as long as it is low in fat and familiar.
• During: From about 75 to 90 minutes of duration, topping up becomes relevant. 30–60 g/h is the entry level, 60–90 g/h the standard for marathon and gran fondo, up to 120 g/h the ceiling for pros with a trained gut (see the next section).
• After the session: 1.0–1.2 g/kg per hour in the first two hours refills stores fastest. If you only train once a day, you can take this more casually — the 24-hour balance matters more than the golden window.
• The day before the race: Not a feast, but 8–10 g/kg spread across the day, with emphasis on easily tolerated, low-fat and low-fibre sources. Details in Carb loading: the protocol before the race.

Grams per hour: why 60 was the old ceiling

The gut absorbs glucose via the SGLT1 transporter, and that transporter saturates at roughly 60 grams per hour. Anyone eating more pure glucose or maltodextrin gains no extra benefit — only nausea, bloating and, in the worst case, diarrhoea. That was the state of practice for years.

The breakthrough came with a second door: fructose uses the GLUT5 transporter and therefore a parallel pathway. A glucose-fructose blend at a 2:1 ratio raises absorption capacity to 90 grams per hour, and in newer studies with more fructose-heavy blends up to 120 grams. The price: fructose takes getting used to for many guts, and the adaptation takes weeks, not days.

In practice that means: below 60 g/h the choice of source is almost irrelevant. Above 60 g/h the composition decides — and whether you have trained it. One gel delivering 30 grams of carbohydrate per hour is perfectly fine for a 1:45 half marathon; for a 3:30 marathon it is too little.

Gut training: 90 grams per hour is not learned in race week

Gut absorption capacity is trainable. Studies with repeated high carbohydrate intake during exercise show adaptations within two to four weeks — the number of SGLT1 transporters increases, gastric emptying speeds up, and reported gut problems drop markedly. That is precisely why the race is the wrong place for an experiment.

The build belongs in the long sessions of the base and build phases: start at 60 g/h, raise by 10 to 15 g/h every two weeks, and test every new step at least twice under realistic conditions — same gels, same fluid volume, similar intensity and temperature. Anyone who arrives at 90 g/h has one less worry on race day. Anyone who tries it for the first time on race day has one more.

Concrete sources: what delivers how much

• Gel: 20–40 g per portion depending on brand. At 90 g/h you need two to three gels per hour — more than most guts tolerate without training.
• Sports drink: 6–8 g per 100 ml, so 30–40 g per 500 ml bottle. Practical for fluid and carbohydrate at once, but a heavy gastric load at high intake. Homemade recipe in Homemade sports drink.
• Bananas: about 23 g of carbohydrate per 100 g; a medium banana delivers 25–27 g. Well tolerated, but hard to chew at 4:30 min/km.
• Dates: 60–75 g per 100 g — the densest natural source, with fibre and potassium. Great in training, often too sweet and sticky in a race.
• Rice, pasta, oats (cooked): 25–30 g per 100 g. The basis for daily needs — this is where nutrition is decided, not in your race belt.
• Bars: 30–45 g, usually with a fat and protein fraction. Good for long easy sessions, too slow to digest at high intensity.

How to fold this into race day is covered in detail in Fuelling the marathon. For the systematic distribution across the week — including low-intake rest days and deliberately carbohydrate-rich key sessions — Carbohydrate periodization is the natural follow-up.

Common mistakes

• Percent instead of grams. “I eat 60 % carbs” says nothing once training volume rises. Calculate in g/kg and you immediately see whether a 25-hour training camp is fuelled or not.
• Ramping up only in the race. Jumping from 40 to 90 g/h on race day ends in the bushes for most people. Build over four to six weeks.
• Glucose only. Above 60 g/h you need the fructose component, otherwise the transporter saturates and the rest ferments in the gut.
• Too little fluid with lots of gel. Concentrated carbohydrate solutions pull water into the gut. A gel without accompanying water is a recipe for cramps and nausea in the heat.
• Carb loading all week. Permanently high intake without matching training leads to weight gain with no performance gain. Periodization means: high on the days that count.
• Fibre on race day. Whole grains, legumes and plenty of vegetables are right in everyday life and wrong 24 hours before the start.

How Peakora factors carbohydrates into the plan

The Peakora engine does not calculate fuelling as an isolated table but from your actual training plan: every session has a duration and an intensity, which yields a TSS value — and from that the carbohydrate demand of the day. Rest days land automatically in the 3 to 5 g/kg band, long runs and key sessions higher up. On the Elite plan the weekly nutrition plan is added: training days get carbohydrate-rich meals, rest days everyday cooking, and race week switches to a carb-loading focus.

This is deliberately deterministic rather than “the AI guesses”: same week, same selection. You can trace why rice appears on one day and something else the next. Anyone who wants the definitions behind the numbers will find them in the Peakora glossary, and anyone who wants to see load management as a whole should look at the marathon training cluster.

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