Carb Loading: The Glycogen Protocol for Endurance

Carb loading is the reason marathoners eat a mountain of pasta the night before a race. The idea is real and the science is solid — but it’s also widely misapplied by people who don’t need it. Carb loading means deliberately overfilling your muscle glycogen stores before a long endurance event so you don’t hit the wall partway through. If your event lasts over about 90 minutes, it can genuinely save your race. If you’re doing a 45-minute gym session or a 5K, it does nothing but add calories. Here’s the protocol and who it’s actually for.

Quick answer

• What it does: supercompensates muscle glycogen so you have more stored fuel for long efforts.
• Protocol: roughly 8–12 g of carbohydrate per kg of body weight per day, for 1–3 days before the event.
• Who needs it: endurance athletes facing continuous efforts over ~90 minutes (marathon, long ride, triathlon).
• Who doesn’t: anyone doing short sessions, lifting, or events under ~90 minutes.
• Pair it with: reduced training (a taper) in the loading days so the glycogen actually accumulates.

Why glycogen runs the show in endurance

Your muscles store carbohydrate as glycogen, and that store is your high-octane fuel for sustained hard effort. You’ve got enough for roughly 90–120 minutes of moderate-to-hard work before levels get low — and when they do, that’s the dreaded “wall” or “bonk”: legs turn to concrete, pace collapses, everything feels twice as hard.

Carb loading attacks this by stuffing more glycogen into your muscles than they’d normally hold (called supercompensation), giving you a bigger tank to start with. For events that outlast your normal stores, a fuller starting tank directly delays fatigue.

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According to PubMed, the ISSN nutrient timing position stand states that endogenous glycogen stores are maximized by following a high-carbohydrate diet of 8–12 g of carbohydrate per kg of body weight per day.¹ That’s the headline number for loading.

The modern protocol

Old-school carb loading from the 1960s involved a brutal depletion phase — train to exhaustion, eat almost no carbs for days, then load — which left athletes miserable and was hard to execute. The good news: modern research shows you mostly don’t need the depletion misery.

Here’s the practical version:

For an 70 kg athlete, 8–12 g/kg means roughly 560–840 g of carbs per day — a large amount that usually means leaning on easy-to-digest, lower-fiber carbs (rice, pasta, bread, potatoes, sports drinks) rather than huge volumes of high-fiber food that’ll wreck your gut.

The taper matters. According to PubMed, a glycogen-loading study found that a protocol starting with a glycogen-depleting bout produced higher and longer-lasting muscle glycogen than a non-depletion taper — but that even light daily training during the loading days didn’t compromise glycogen supercompensation.² In plain terms: you can keep doing short easy sessions while you load; you don’t have to lie on the couch.

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How long to load

You don’t need a full week. Glycogen stores can be substantially elevated within 1–3 days of high carbohydrate intake paired with reduced training.

• 24 hours of high-carb intake plus rest already raises stores meaningfully.
• 2–3 days gets you closer to maximal supercompensation.
• Going longer mostly just adds calories and, often, water weight (glycogen is stored with water, so expect to feel a bit heavier and puffier — that’s normal and it’s fuel, not fat).

Who actually needs carb loading

This is where most people get it wrong. Carb loading only pays off when the event is long enough to threaten your glycogen stores.

Worth it:

• Marathon and ultra-marathon
• Long-distance cycling (century rides, road races)
• Triathlons, especially longer distances
• Any continuous endurance effort over ~90 minutes

Not worth it:

• Gym lifting sessions
• 5K and most 10K runs
• High-intensity interval workouts under an hour
• Team-sport games with lots of rest (though general high-carb fueling still helps)

If your event is short, loading just gives you extra calories and water weight to haul around. For shorter or normal training, your day-to-day carbs already cover you — see nutrient timing for why daily totals do most of the work. Runners planning race-week fueling should also read what to eat before running and our broader runner’s diet guide.

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Loading is only half the plan

Topping up before the start doesn’t mean you can ignore fuel during the event. For anything past ~90 minutes you’ll still want carbs on the move — see intra-workout nutrition for in-session dosing (roughly 30–60 g/hour, up to ~90 g/hour for ultra efforts). And going long means sweating, so don’t forget electrolytes to replace sodium and hold onto fluid.

After you cross the line, refilling glycogen quickly matters if you’re racing or training again soon; the post-workout nutrition guide covers recovery fueling. Before all of it, loosen up properly — a thorough dynamic warm-up routine helps you start strong.

Practical tips that actually matter

• Practice it in training. Do a full carb-loading rehearsal before a long training run or ride so race week holds no surprises — especially for your gut.
• Go lower-fiber as you get closer. High-fiber loading is a recipe for stomach trouble at the start line. Shift toward white rice, pasta, bananas, and sports drinks in the final 24 hours.
• Expect the scale to jump. A 1–2 kg gain from stored glycogen and water is expected and is exactly what you want.
• Don’t add fat and protein on top. Loading is about carbs; piling on a huge fatty meal just makes you sluggish. Keep the extra calories coming from carbs.
• Hydrate normally. Glycogen needs water to store, so steady fluid intake supports the process.

Bottom line

Carb loading works by supercompensating muscle glycogen so you start a long event with a bigger fuel tank and delay hitting the wall. The protocol is roughly 8–12 g/kg/day of carbohydrate for 1–3 days, paired with a training taper — and the brutal old depletion phase is optional, since light easy sessions during loading don’t hurt your glycogen. It genuinely helps continuous efforts over ~90 minutes and does nothing useful for short sessions or lifting, where it just adds calories and water weight. Practice it in training, lean on lower-fiber carbs near race day, and pair it with in-event fueling and electrolytes. For the rest of the endurance picture, see intra-workout nutrition, electrolytes, nutrient timing, and what to eat before running.