Carb Loading for Endurance Events: What the Numbers Actually Say
"Carb loading" gets thrown around loosely, but the underlying physiology is specific: the goal is to supercompensate muscle glycogen stores to roughly double normal resting levels before an endurance event. It only pays off for continuous efforts longer than about 90 minutes — for a 30-minute 5K, normal eating is enough.
The Loading Protocol
The classic approach pairs a 2–3 day taper in training volume with a high-carbohydrate intake of roughly 8–12 g of carbohydrate per kg of body weight per day. Cutting training volume matters as much as the food itself: glycogen resynthesis competes with ongoing muscle breakdown from hard training, so easing off lets stores fill closer to their ceiling. Done correctly, muscle glycogen can reach roughly 500–900 mmol/kg dry weight, extending the time before you hit the wall.
How Much Carbohydrate During the Event Itself
Fueling during exercise is a separate question from loading beforehand, and recent analyses help clarify the dose-response curve. A meta-analysis found that carbohydrate ingestion spares muscle glycogen with a modest effect, and that 60–90 g of carbs per hour captures most of the practical benefit for most athletes — typically as a glucose-fructose blend, since combining sugar types raises the ceiling on gut absorption compared with glucose alone. A 2025 study in elite marathoners found 120 g/h outperformed 60 g/h for running economy late in the race, though the gap versus 90 g/h wasn't statistically significant — suggesting real gains taper off well before triple-digit intakes for most non-elite athletes.
Practical Takeaways
- Reserve full glycogen loading for continuous events over ~90 minutes — marathons, long triathlons, century rides, multi-hour hikes
- In the 2–3 days before the event, aim for roughly 8–12 g carbohydrate per kg body weight per day while training volume tapers down
- During the event, target 60–90 g of carbs per hour from a mixed glucose-fructose source (sports drink, gels, chews) rather than glucose alone
- Practice your fueling plan in training — gut tolerance for high carb intakes is trainable and shouldn't be tested for the first time on race day
Helpful products
- Glucose-fructose energy gels — a practical way to hit 60-90g of carbs per hour during long events.
- Electrolyte sports drink mix — pairs carbohydrate intake with sodium replacement for long training sessions.
This article summarizes general sports-nutrition evidence and is not individualized medical or dietary advice, especially for people with diabetes or GI conditions — consult a registered dietitian or physician.
Source: International Society of Sports Nutrition (ISSN) / Journal of Nutrition
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