What carb loading actually does to your muscles
The principle is straightforward, even if the execution trips people up. Your muscles store carbohydrate as glycogen, and they can hold roughly 400–600 grams of it in total, spread across your legs, liver, and other muscle groups. During a hard ride lasting more than 90 minutes, glycogen is the fuel that keeps your power output stable — and when it runs low, power drops whether you want it to or not. Carb loading is the deliberate act of filling those stores beyond their resting baseline, so you start with a fuller tank. A 2025 meta-analysis in Frontiers in Physiology, which pooled data from 30 studies, confirmed that cyclists achieve a greater magnitude of glycogen supercompensation than runners — meaning the biology genuinely works in your favour on the bike.
The mechanism ties back to what happens right after a period of glycogen depletion. When your muscles are starved of glycogen and then flooded with carbohydrate, the enzymes responsible for glycogen synthesis — GLUT4, hexokinase II, and others — upregulate strongly, allowing glucose to enter muscle cells faster than normal. That window of enhanced uptake is what makes carb loading effective. Outside that window, simply eating more pasta has much smaller effects on how much glycogen you actually store. This is why the protocol matters more than just eating big the night before a race.
Let's be direct about the evidence for performance gains: studies consistently show that carb loading extends time to exhaustion at a fixed workload by up to 20%, and improves time trial power output by roughly 2–3%. Those are real, meaningful numbers — not marginal gains territory. A 2–3% improvement in a 200-watt effort is 4–6 watts, which over the course of a four-hour sportive translates to arriving at the finish several minutes earlier. The gains are most reliable in events lasting longer than 90 minutes, where glycogen availability genuinely becomes the limiting factor. Short crits or criterium-style races? The evidence doesn't support loading the same way.
One side effect worth knowing about: for every gram of glycogen you store, roughly 2.7 grams of water comes with it. A well-loaded cyclist can gain 1–2 kg before an event, mostly water bound to glycogen. Some riders find this psychologically uncomfortable, especially when they track weight closely. Let it go. That extra weight disappears within the first hour of riding as the glycogen is metabolised — and the energy it provides is far more valuable than the marginal weight penalty at the start line.
The numbers that actually matter
The target carbohydrate intake for a proper loading phase is 10–12 grams per kilogram of body weight per day, maintained for 36–48 hours before your event. For a 70 kg rider, that's 700–840 grams of carbohydrate — considerably more than most cyclists eat on a normal training day, where 5–7 g/kg is typical. You need to spread it across the day rather than eating it all in one sitting; practically, that means carbohydrate at every meal and at least one or two high-carb snacks in between. Research confirms that distributing carbohydrate intake throughout the day improves glycogen synthesis rates compared to infrequent large boluses.
The type of carbohydrate matters more than most cyclists realise, and this is where a lot of well-intentioned riders make the wrong call. During the loading phase, high-glycaemic-index sources — white rice, white bread, regular pasta, potatoes, fruit juice — outperform low-GI alternatives for glycogen storage. The same property that makes low-GI foods valuable in everyday eating (slower digestion, more stable blood sugar) works against you when the goal is rapid glycogen resynthesis. Drop the brown rice and quinoa for these 48 hours. This isn't everyday nutrition advice; it's targeted preparation for a specific physiological goal. For a broader framework of how carbohydrate fits into your day-to-day training fueling, the cycling fueling and hydration guide is worth reading alongside this article.
Modern research has also collapsed the old 6-day protocol down significantly. The classic Scandinavian approach from the 1960s — a depletion phase followed by a loading phase spanning almost a week — is now considered unnecessary for trained cyclists. A single well-executed 24–36 hour loading phase achieves equivalent glycogen supercompensation, provided you've trained consistently in the preceding weeks and aren't arriving at the loading window already depleted from overtraining. The total carbohydrate number still has to hit 10–12 g/kg; you just don't need the elaborate multi-day structure around it.
How to apply it in training versus race week
Most cyclists make one of two errors: they try to carb load before every hard group ride, or they do nothing at all until the morning of the event. Neither approach works. Carb loading is a specific pre-event tool, not a daily fueling strategy — it's designed for events longer than 90 minutes where glycogen depletion is a genuine performance constraint. For ordinary training rides, the goal is to arrive reasonably fuelled, execute the session, and recover with protein and carbohydrate afterwards. Supercompensating glycogen stores for a Tuesday evening two-hour ride means carrying unnecessary water weight and almost certainly overeating for the training stimulus you're about to deliver.
For race week, a clean structure works well. In the two to three days before your event, reduce training volume (this overlaps with your taper anyway) and begin shifting carbohydrate intake upward toward 8–10 g/kg. On the day before the event, hit the full 10–12 g/kg target. Keep fat and fibre low during this window — not eliminated, but significantly reduced — because both slow gastric emptying and can cause GI distress on race morning. Your pre-race meal, taken 2–4 hours before the start, should add another 1–3 g/kg of carbohydrate on top of what you loaded the previous day; this tops off liver glycogen, which depletes overnight even while you sleep. For everything that happens from the start gun onwards — how to structure on-bike intake, timing, formats — race fueling for cyclists covers the full protocol from breakfast to finish line.
One thing that catches riders off guard is treating loading and in-race fueling as separate, interchangeable strategies. They are not. Carb loading fills your muscles before the start — it doesn't replace the need to take on carbohydrate during the event. Even a perfectly loaded cyclist will deplete glycogen in a race lasting more than two hours without consistent in-race fueling. The two approaches are additive: a full tank at the start and 60–90 grams of carbohydrate per hour during the ride gives you the best possible conditions for sustained power output. Neither substitutes for the other, and treating them as if they do is one of the most common reasons athletes fade in the final hour of a long event.
The mistakes that cancel out the benefit
The most common and most avoidable error is practising carb loading for the first time on race day. Eating 750 grams of carbohydrate in 24 hours feels completely different from a normal eating day, and the GI consequences — bloating, urgent bathroom stops, general discomfort on the start line — are real and predictable. Practise the full protocol before a hard training day or a B-priority event so your gut knows what's coming. Gastrointestinal issues are the single biggest reason trained athletes underperform on race day, and most of them are preventable with deliberate rehearsal before it counts.
The second mistake is cutting protein and fat so aggressively that every meal becomes plain pasta and white bread. You need some fat and protein even during loading — they just take a back seat to carbohydrate volume. A chicken breast with white rice is fine. A large portion of spaghetti with tomato sauce is fine. A meal built around fried food, heavy cream sauces, or very high-fibre vegetables is not ideal. The goal is to prioritise carbohydrate volume without becoming one-dimensional about it. And avoid the temptation to eat a massive loading meal right before bed on race eve — you'll sleep badly and wake up feeling sluggish rather than charged.
The third mistake, and probably the least discussed, is over-relying on carb loading to fix poor in-race fueling habits. If you routinely bonk in long events because you're not eating enough on the bike, loading the night before adds maybe 60–90 minutes of extra glycogen buffer. After that, the same bonk happens, just slightly later in the race. Carb loading and in-race fueling are a package deal. If you haven't yet built a reliable on-bike nutrition structure — how to sequence intake from kilometre one, what formats work for your gut — that's worth prioritising before you worry about the loading phase. The guide to building a race fueling plan walks through exactly that process.
Related reads
Race fueling for cyclists · Cycling fueling and hydration · Build a race fueling plan
Sources
Vigh-Larsen, J.F. et al. (2025). Glycogen supercompensation in skeletal muscle after cycling or running followed by a high carbohydrate intake the following days: a systematic review and meta-analysis. Frontiers in Physiology.
Escalante, G. et al. (2025). A review of carbohydrate supplementation approaches and strategies for optimising performance in elite long-distance endurance. PMC / NCBI.
Thomas, D.T., Erdman, K.A., Burke, L.M. (2016). Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance. Journal of the Academy of Nutrition and Dietetics.