Key takeaways
- Carb-loading is eating extra carbohydrate in the days before a long event to maximize glycogen stores.
- Glycogen (in muscles and liver) is the body's most accessible fuel for sustained, harder efforts.
- Topping up glycogen helps delay depletion and the 'bonk' when fuel runs low.
- It mainly benefits events lasting roughly 90 minutes or longer.
What carb-loading is
Carb-loading (carbohydrate loading) is the practice of increasing carbohydrate intake in the days before a long endurance event to maximize the glycogen stored in the muscles and liver — the body’s most accessible fuel for sustained, harder efforts. The goal is to start the event with full fuel tanks.
How it works
Your muscles and liver store carbohydrate as glycogen, but storage is limited. By eating more carbohydrate (often while tapering so you’re not burning it off) for a few days before a race, you fill those stores to their maximum — so you can sustain harder efforts longer before glycogen runs low, which is when fatigue and the bonk set in.
In the two days before a mountain marathon, a runner bumps up their carbohydrate intake — more rice, pasta, bread, and fruit — while their training is tapered, topping off their glycogen stores so they can run strong deep into the race before fuel becomes a limiter.
When it helps
For long, continuous efforts — roughly 90 minutes or more, like marathons, long trail races, and ultras — where glycogen depletion limits performance. For a 5K or 10K, normal stores are plenty. And it doesn’t replace in-race fueling: even maximized stores can’t last forever in very long events, so keep taking in gels and electrolytes as you go.
The bottom line
Carb-loading is boosting carbohydrate intake in the days before a long event to maximize muscle and liver glycogen — your most accessible fuel — so you start with full tanks and delay the depletion and bonk that come when fuel runs low. It mainly helps efforts of ~90 minutes or more, pairs naturally with the taper, and sets you up for, but doesn't replace, fueling during the race itself.
Frequently asked questions
What is carb-loading?
Carb-loading (carbohydrate loading) is the practice of eating more carbohydrate than usual in the days leading up to a long endurance event, in order to maximize the amount of glycogen (stored carbohydrate) in your muscles and liver. Glycogen is the body's most readily usable fuel for sustained, harder exercise, so the goal is to start the event with full fuel tanks.
How does carb-loading work?
Your muscles and liver store carbohydrate as glycogen, but the storage capacity is limited. By increasing carbohydrate intake (often while tapering training so you're not burning it off) for a few days before a race, you fill those glycogen stores to their maximum. With fuller stores, you can sustain harder efforts longer before glycogen runs low — which is when fatigue and the 'bonk' set in. Modern approaches often emphasize a high-carb intake in the final day or few days rather than the old depletion-then-loading protocols.
When does carb-loading actually help?
For long, continuous endurance efforts — generally those lasting roughly 90 minutes or more, like marathons, long trail races, and ultras — where glycogen depletion is a real performance limiter. For shorter events (a 5K or even a 10K), your normal glycogen stores are plenty and carb-loading offers little benefit. It's also important to keep fueling during very long events, since even maximized stores can't last indefinitely; carb-loading sets you up, but it doesn't replace in-race nutrition.
Sources
- Sports nutrition — Academy of Nutrition and Dietetics
- Endurance fueling — American Council on Exercise