Carb loading is a nutrition strategy where you deliberately eat a high-carbohydrate diet in the days before a long endurance event to maximize your body’s fuel stores. The goal is to pack as much glycogen (your muscles’ primary energy source) into your muscles and liver as possible, so you have more fuel available during prolonged exercise. It’s most useful for events lasting 90 minutes or longer, like marathons, long-distance cycling races, and triathlons.
How Your Body Stores Carbohydrates
When you eat carbohydrates, your body breaks them down into glucose and stores the excess as glycogen, a densely packed molecule that can hold up to 50,000 glucose units in a single particle. Your skeletal muscles hold roughly 500 grams of glycogen, while your liver stores about 100 grams. Together, that’s your fuel tank for intense exercise.
The catch is that your muscles have a built-in feedback mechanism that normally prevents them from stockpiling more glycogen than they need. Carb loading works by combining a high carbohydrate intake with reduced physical activity, which overrides that limit and pushes glycogen levels above their normal ceiling. This is sometimes called glycogen supercompensation.
There’s a side effect worth knowing about: every gram of glycogen pulls roughly 3 grams of water along with it into storage. So if you successfully load an extra 300 to 400 grams of glycogen, you can expect to gain 2 to 3 pounds of water weight in the process. This is temporary and actually beneficial, since that water becomes available during exercise. But it can feel unfamiliar if you’re not expecting it.
Who Benefits From Carb Loading
Carb loading is designed for sustained endurance efforts, not short workouts. If your event lasts under 60 to 90 minutes, your normal glycogen stores are sufficient and loading won’t offer a meaningful advantage. The strategy becomes relevant for marathons, half-Ironman and Ironman triathlons, long cycling events, cross-country ski races, and similar activities where you’ll be working hard for 90 minutes to several hours.
For events lasting one to three hours, current sports nutrition guidelines recommend consuming 30 to 60 grams of carbohydrates per hour during the activity itself. When events stretch beyond three hours, that recommendation increases to 60 to 90 grams per hour using a mix of carbohydrate types. Carb loading before the event complements these during-exercise fueling strategies by ensuring you start with a full tank.
How Much to Eat and When
The modern approach to carb loading is straightforward: eat about 10 to 12 grams of carbohydrates per kilogram of body weight per day, starting 24 to 36 hours before your event. For a 70-kilogram (154-pound) athlete, that works out to 700 to 840 grams of carbohydrates in a single day. That’s a lot of food, and it’s one reason the strategy requires some planning.
Older protocols from the 1960s and 70s called for a depletion phase first, where athletes would eat very few carbs and exercise hard for several days to drain glycogen stores, then switch to heavy carb eating for the final days. This approach worked but was miserable and carried a higher risk of injury and illness during the depletion phase. The modern one-to-two-day loading window produces comparable results without that ordeal.
To hit those carbohydrate targets, you’ll need to prioritize calorie-dense, carb-rich foods: white rice, pasta, bread, potatoes, oatmeal, fruit juice, sports drinks, and low-fiber cereals. The emphasis on low-fiber and lower-fat options is deliberate, and it brings us to the most common mistake people make.
Common Mistakes That Cause Problems
The biggest pitfall during carb loading is gastrointestinal distress, and it usually comes from choosing the wrong types of food rather than eating too many carbs in general. Fiber, fat, and excess protein all slow digestion and increase the risk of bloating, cramping, and nausea. If you try to hit your carbohydrate targets by eating large plates of whole-grain pasta with heavy sauce, beans, or high-fiber vegetables, you’re likely to feel terrible on race morning.
Instead, lean toward refined carbohydrates during the loading window. White rice over brown, white bread over whole wheat, fruit juice over whole fruit. This is one of the rare situations where lower-fiber choices are the smarter option. Concentrated carbohydrate drinks and gels can help you reach your targets without the sheer volume of solid food, but hypertonic (very concentrated) beverages can also trigger GI problems by drawing water into the intestines. Sipping isotonic sports drinks throughout the day is a gentler approach.
Another common error is not reducing your training volume enough. If you keep training hard while loading, your muscles burn through the glycogen you’re trying to store. The loading phase should coincide with a taper, where you cut your training volume by roughly 40 to 60 percent while keeping the intensity of any remaining sessions the same. Research on tapering shows this range of volume reduction significantly improves performance, while cutting volume by more than 60 percent can backfire by reducing fitness.
What Carb Loading Looks Like in Practice
Say you have a marathon on Sunday morning. Your loading phase would start Friday evening or Saturday morning. During that window, you’d aim for carb-rich meals and snacks throughout the day: a large bowl of white rice with a light protein source, pancakes with syrup, toast with jam, pretzels, bananas, and sports drinks between meals. You’d keep fat and fiber low and spread your eating across the day rather than trying to consume everything in one or two sittings.
Your training on Friday would be a short, easy session, and Saturday would be a rest day or a very brief shakeout jog. This combination of high intake and low output is what creates the supercompensation effect. On race morning, you’d eat a familiar pre-race meal two to three hours before the start, then shift to your during-exercise fueling plan once you begin.
Practicing the loading protocol before a long training run is a good idea. Individual tolerance for high-carb eating varies, and you want to learn what foods sit well with you before race day. Athletes who regularly train their gut to handle higher carbohydrate loads tend to absorb more and experience fewer GI issues during competition.
Why It Doesn’t Work for Everyone
Carb loading is less effective for people doing shorter or lower-intensity activities, where glycogen stores don’t become the limiting factor. Recreational exercisers training for 30 to 60 minutes have no practical reason to load. It’s also worth noting that people with insulin resistance may synthesize glycogen less efficiently, since the main pathway for storing glucose as glycogen depends on insulin signaling working properly.
The strategy also requires eating a genuinely large amount of food, which some athletes find difficult or uncomfortable. If you weigh 55 kilograms, your target is still 550 to 660 grams of carbs in a day, roughly 2,200 to 2,640 calories from carbohydrates alone. Spreading this across six or more eating occasions, including calorie-dense drinks, makes it more manageable. Athletes who can’t comfortably reach those targets still benefit from increasing their intake above normal levels, even if they fall short of the full recommendation.