Carb Loading for Endurance: The Science of Filling the Tank Before Race Day

For decades, runners and endurance athletes have tossed around the term “carb loading” like it’s common sense. Most picture it as a license to inhale pasta the night before a race. It’s a comforting ritual, a big dinner, maybe a beer, and the illusion that you’ve done something to guarantee energy for tomorrow. But in reality, that’s not carb loading. That’s just dinner with a hopeful narrative.

The real process is slower, more deliberate, and rooted in one of the most reliable findings in exercise physiology: the body’s capacity to store carbohydrates as glycogen and how manipulating that storage can change performance outcomes.

In the late 1960s, two Swedish scientists, Jon Bergström and Eric Hultman, were among the first to uncover this mechanism. Through muscle biopsies (yes, literally removing tiny pieces of muscle), they demonstrated that glycogen stores could be “supercompensated”, meaning that, under the right conditions, muscles could hold more glycogen than normal. They achieved this by having subjects perform exhaustive exercise to deplete their glycogen stores, then follow several days of a high-carbohydrate diet. Glycogen levels rebounded not just to baseline but beyond it. Those findings, published in Scandinavian Journal of Clinical and Laboratory Investigation in 1967, formed the foundation for everything we know about carb loading today.

About 15 years later, William Sherman and colleagues refined the idea. Their 1981 study, published in Medicine and Science in Sports and Exercise, found that athletes didn’t need an entire week of strict depletion and refueling to achieve glycogen supercompensation. A practical three-day depletion phase followed by three days of rest and high carbohydrate intake led to the same result. Muscles packed with glycogen and better performance during a 20.9 km run. Sherman’s work made carb loading accessible, reproducible, and applicable to competitive athletes outside a lab.

Physiologically, the concept is simple but powerful. Glycogen is the body’s most efficient energy source for sustained, high-intensity effort. You store it in both the liver and muscles, roughly 400 to 500 grams total for most trained individuals. During prolonged exercise, especially beyond 90 minutes, those stores deplete steadily. Once they run low, your body turns to fat oxidation for energy, which is slower and less efficient. That’s the infamous “bonk” or “wall” runners describe: not a lack of willpower, but a lack of fuel.

Here’s where the process gets interesting. Every gram of glycogen stored pulls about three to four grams of water with it into the muscle cell. This isn’t just a byproduct, it’s part of what makes carb loading effective. The additional water improves thermoregulation, blood plasma volume, and overall endurance capacity. The tradeoff is a temporary weight increase, typically two to four pounds, but that’s not bloat or fat, it’s stored energy and hydration ready to be used.

Executing a proper carb load involves strategy, not spontaneity. The classic method starts about six days before the event. For the first three days, the athlete trains at moderate to high intensity while consuming fewer carbohydrates, roughly 1.5 to 2 grams per pound of body weight. This depletes muscle glycogen and primes the body to store more later. Over the next three days, training volume drops dramatically, and carbohydrate intake rises to about 3.5 to 5.5 grams per pound per day. This is paired with adequate sodium and hydration, since water retention is a critical part of the process. The body responds by super-storing glycogen, reaching levels up to 50 percent higher than normal.

More recent research, like that of Bussau et al. (2002), has shown that a similar effect can be achieved without the depletion phase, simply resting and consuming a very high carbohydrate intake for 24 to 36 hours. This “rapid load” approach is less fatiguing and often more practical for athletes who train frequently. Still, the classical six-day strategy remains the gold standard for endurance races lasting longer than 90 minutes, especially when there’s enough time to taper training volume.

Hydration plays a central role in all of this. Glycogen doesn’t exist in isolation; it’s stored with water, and that water contributes to endurance performance by maintaining plasma volume and electrolyte balance. Athletes who carb load without paying attention to hydration or sodium often feel sluggish, not because of the carbs, but because of poor fluid management. Conversely, overhydrating with plain water can dilute sodium levels and cause hyponatremia, a dangerous drop in blood sodium concentration that impairs nerve and muscle function. The key is balance: hydrate consistently, salt meals, and include electrolytes, especially in warmer conditions.

There are, of course, drawbacks. The temporary weight gain from glycogen-associated water can feel uncomfortable, particularly for those sensitive to small changes in bodyweight. Digestive issues can also occur if the loading phase includes too much fiber or high-fat foods, which slow gastric emptying and can cause GI distress on race day. Carb loading also isn’t necessary for every athlete. If your event lasts less than about 75 to 90 minutes, your normal glycogen stores are sufficient. In those cases, focus instead on a balanced pre-race meal and steady fueling during the event.

But when applied correctly to long races, triathlons, or high-output endurance challenges carb loading works. It’s one of the few performance strategies that has stood the test of time, from biopsy needles in Swedish labs to modern sports science consensus statements.

What’s most misunderstood about carb loading isn’t the science; it’s the execution. People either do too little for too long or too much too late. The best approach is practiced, not improvised. Use a training week to test your plan (adjust fiber, fluid, and sodium) and find what your body tolerates best. Then, when it’s race week, you’ll know exactly how to fuel.

Carb loading isn’t about superstition or comfort food. It’s physiology in action, a methodical way to fill the tank before asking your body to go the distance. It doesn’t make the race easier. It just ensures you’re not running on empty.

Exceed Your Limits

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References

1. Bergström J, Hultman E. Muscle glycogen synthesis after exercise: an enhancing factor localized to the muscle cells in man. Scand J Clin Lab Invest. 1967; 19(3): 218–228.

2. Sherman WM, Costill DL, Fink WJ, Miller JM. Effect of exercise-diet manipulation on muscle glycogen and 20.9-km run performance. Med Sci Sports Exerc. 1981; 13(2): 90–95.

3. Bussau VA, Fairchild TJ, Rao A, Steele P, Fournier PA. Carbohydrate loading in human muscle: an improved one-day protocol. Eur J Appl Physiol. 2002; 87(3): 290–295.

4. Burke LM. Carbohydrates for training and competition. J Sports Sci. 2011; 29(Suppl 1): S17–S27.

5. Murray B. Fundamentals of glycogen metabolism for coaches and athletes. Nutr Rev. 2018; 76(4): 249–259.