What is carb loading and how do you actually do it?

Why does carb loading work?

The science of carb loading comes down to a single concept: muscle glycogen supercompensation. Your muscles store carbohydrate in the form of glycogen, which is your primary fuel during hard endurance exercise. A normal well-fed trained athlete has roughly 400 to 500 grams of muscle glycogen plus 80 to 100 grams of liver glycogen — enough to fuel about 90 to 120 minutes of hard endurance exercise before the tank runs low and performance degrades.

The mechanism behind bonking at kilometre 30 of a marathon is exactly this: your glycogen has dropped below a critical level and your central nervous system starts throttling muscle output to protect itself. The race-day fueling you take during the event helps, but it can only slow the depletion, not prevent it, because your gut can only absorb carbs at a limited rate (90 grams per hour for gut-trained athletes, less for most).

Carb loading exploits a specific quirk of muscle physiology: if you eat much more carbohydrate than normal for 24 to 48 hours, your muscles don't just refill to their normal glycogen level — they pack in extra, storing 20 to 40% more glycogen than they would at normal intake. The extra glycogen is real, measurable, and functional. Research going back to the original Bergström studies in the 1960s and 70s has consistently shown that carb-loaded athletes perform better in long endurance events than athletes who entered races with normal glycogen stores.

The performance benefit scales with race duration. For a 90-minute race, the effect is small. For a marathon, it's meaningful — often worth 2 to 4 minutes depending on how close to the wall you would have been without it. For an Ironman, it's the difference between bonking on the run and finishing strong.

What was the old depletion protocol, and why did it go away?

The original carb loading protocol from the 1960s, developed from the Bergström research, was more extreme than the modern version. It went like this: seven days before the race, deplete your glycogen with a hard long session. Then eat a low-carb diet for three to four days to keep glycogen suppressed. Then switch to a very high-carb diet for the final three days before the race. The resulting supercompensation was larger than what you got from just eating high-carb — about 50% above baseline rather than 20 to 40%.

The problem was that the depletion phase was miserable. Three to four days of low-carb eating in the week before your most important race of the year makes you tired, irritable, mentally flat, and prone to illness, while also making every training session feel terrible. Many athletes got sick during the depletion phase and arrived at the start line worse off, not better. The added 10 to 20% of glycogen was not worth the cost of the rest of the week.

In the 1980s and 90s, a series of studies showed that skipping the depletion phase entirely and just eating high-carb for the final 24 to 48 hours produced almost the same supercompensation without the miserable depletion period. The original Sherman and Costill research on this found 90% of the benefit from the simpler protocol. More recent research has refined it further: 24 to 48 hours of high-carb feeding, with a complete taper so training volume is low, produces supercompensation of 20 to 40% — which is enough to deliver most of the performance benefit without torching your last week.

The old protocol is essentially obsolete now. Almost no credible coach or sports nutritionist prescribes it anymore. The modern version is simpler, easier to execute, and almost as effective.

What is the modern carb loading protocol?

The current evidence-based protocol is straightforward. It has three parts: how much carbohydrate, how long, and what kind.

• How much. 10 to 12 grams of carbohydrate per kilogram of body weight per day. For a 70 kg athlete, that's 700 to 840 grams of carbs per day — which is a lot more food than it sounds. Most amateurs who think they're carb loading are actually eating half of this.
• How long. 24 to 48 hours before the race, depending on race distance. For a marathon, 48 hours is standard. For a half marathon, 24 hours is enough. For an Ironman, 48 to 72 hours of high intake is appropriate. For an ultra of 6+ hours, consider 72 hours to make sure glycogen is fully topped up.
• What kind. Simple, easily digested, low-fibre carbs. White rice, white pasta, white bread, bagels, oatmeal (without bran), potatoes, bananas, fruit juices, sports drinks, sugary cereals, crackers, pretzels. This is not the week to eat brown rice, quinoa, lentils, or bran. Fibre slows gastric emptying and adds bulk without adding much glycogen-useful carbohydrate.

The biggest surprise for most athletes trying this protocol for the first time is the sheer volume of food required. 700+ grams of carbs per day is roughly 4–5 substantial meals plus several snacks. Eating three pasta dinners does not count.

What does 48 hours of carb loading actually look like on a plate?

For a 70 kg athlete targeting 10 g/kg/day (700 g), here's a concrete example of what a day might look like. Each meal or snack is engineered for carbohydrate density with low fat and low fibre.

• Breakfast: Large bowl of oatmeal (80 g oats dry weight) cooked with banana and honey, white toast with jam, glass of orange juice. ≈ 180 g carbs.
• Mid-morning snack: Bagel with jam, banana, sports drink. ≈ 120 g carbs.
• Lunch: Large bowl of white rice (150 g dry weight), small portion of lean protein (chicken or white fish), vegetables cooked soft and in small amount, orange juice. ≈ 180 g carbs.
• Afternoon snack: White toast with honey or jam, bowl of cereal, sports drink. ≈ 120 g carbs.
• Dinner: White pasta (150 g dry weight), tomato sauce, small amount of lean meat or fish, bread, small dessert. ≈ 180 g carbs.
• Evening snack (optional): Toast with jam, small bowl of cereal. ≈ 80 g carbs.

That's roughly 860 grams of carbohydrate, slightly above the target — and for most athletes, hitting 700 grams requires this level of deliberate planning. Carb loading is not eating 'more or less normal, with extra pasta at night'. It is a dedicated 24–48 hours of systematic high-carb feeding.

Which races benefit from carb loading?

Carb loading is worth the logistical trouble for events where glycogen depletion is a real performance concern — which means any race lasting longer than about 90 minutes. Below that threshold, you won't deplete your glycogen stores during the race anyway, and carb loading is wasted effort.

• 5K and 10K — not worth carb loading. Your glycogen stores have plenty for these distances without any manipulation. A normal healthy dinner the night before and a familiar breakfast are sufficient.
• Half marathon — borderline. For most amateurs, a standard dinner the night before and a good breakfast covers it. Competitive athletes racing at high intensities for 90+ minutes benefit from a mild 24-hour carb emphasis.
• Marathon — absolutely worth carb loading. 48 hours of 10–12 g/kg/day is the standard and produces a meaningful performance benefit on race day.
• 70.3 / half Ironman — worth carb loading. 48 hours of high intake with reduced fibre. The 4–6 hour race duration is squarely in the glycogen-depletion danger zone.
• Full Ironman — essential. 48–72 hours of maximized carb intake. Full Ironmans are won and lost on how much glycogen you bring to the start line and how well you keep topping it up during the race.
• Ultras (50K–100 mile) — similar to Ironman. 48–72 hours of carb loading, though the longest ultras shift somewhat toward continuous fueling rather than pre-race glycogen stores.
• Cycling stage races — complicated. Multi-day races are more about daily recovery than pre-race loading, though the first stage benefits from carb loading like any other event.

How does carb loading fit with tapering?

Carb loading and tapering work together. The taper reduces your training volume while keeping intensity, which means your daily glycogen usage drops. If you keep your normal carb intake during a taper, glycogen stores naturally start refilling above baseline over the taper week. The explicit carb loading on top of that, in the final 24–48 hours, pushes the supercompensation further.

One practical consequence is that tapering athletes often gain 1 to 2 kilograms of weight in the final few days before a race. This is not fat — it is the combination of extra glycogen (which binds about 3 grams of water per gram of carbohydrate stored) and the simple fact that you're eating more while moving less. Athletes who panic about this and restrict eating in race week are defeating their own taper and their own carb loading. The extra weight is exactly the weight you want. It will disappear within 24 hours of the race finishing.

The coordination between taper and carb loading is one of the reasons race week nutrition deserves to be planned as deliberately as the training block that preceded it. Eating slightly less than you normally would during a taper — because you're training less — is one of the most common self-inflicted wounds in amateur endurance sport.

What are the most common carb loading mistakes?

Five mistakes catch almost every amateur athlete at least once.

• Under-eating the target. Thinking you carb-loaded because you had pasta for dinner, when the actual target is 700+ grams of carbs across the full day for two days. Without measuring, most athletes hit about 400–500 grams of carbs and assume that's enough.
• Eating high-fibre carbs. Brown rice, quinoa, whole-wheat pasta, oats with bran, legumes — all nutritious normally, all a bad idea in the final 24 hours before a race. High fibre slows gastric emptying, adds bulk, and can produce GI distress on race morning. Save them for the week after.
• Starting too early. Glycogen stores can only hold so much. Starting carb loading 5 to 7 days before a race means the extra glycogen has time to deplete before the race starts, and you lose the benefit.
• Fearing weight gain. The 1–2 kg you gain during carb loading is glycogen and water, and it is exactly what you want. Athletes who restrict to avoid weight gain during race week sabotage their own preparation.
• Trying new foods during the load. Race week is for eating familiar, tested foods. Experimenting with a new breakfast cereal or a new pasta brand in the 48 hours before a marathon is how GI disasters happen on race morning.