Carb loading done right

Carbohydrate loading is one of the most researched nutritional strategies in sports. The principle is simple: maximize glycogen stores before competition so that the body starts with a full tank. However, it is often poorly executed. This article discusses what science actually shows, who benefits, and what practical program suits ultrarunners.

Does Carbohydrate Loading Work?

Yes — for events lasting over approximately ninety minutes. The evidence is consistent and clear.

The body stores about four to five hundred grams of glycogen in muscle depending on body size and training status, and about one hundred grams in the liver. On a typical diet, most athletes start a race with sixty to eighty percent of maximal glycogen capacity. Carbohydrate loading can elevate starting glycogen twenty to forty percent above this baseline.

In an event where glycogen depletion is a limiting factor — and that includes any ultramarathon — starting with fully loaded stores delays depletion and reduces reliance on exogenous carbohydrate during critical early hours.

The performance benefit of carbohydrate loading for events over ninety minutes has been consistently demonstrated in research since the 1960s. This is not an area of small improvements. It is one of the most impactful nutritional strategies available.

Who Benefits Most

Loading provides the biggest benefit for athletes competing at moderate to high intensity for two or more hours, which includes all competitive ultrarunners. It also helps athletes with stomach issues in races — starting with higher glycogen stores reduces how much carbohydrate per hour needs to be consumed to maintain blood glucose. And it helps athletes with a history of late-race energy crashes, as fully loaded stores extend the window before depletion begins.

The Current Protocol

The classic 1970s loading protocol included a depletion phase, several days of very low carbohydrate intake, followed by a loading phase. Research beginning in the 1980s has shown that the depletion phase is unnecessary. Trained athletes can supercompensate glycogen stores without prior depletion.

In the thirty-six to forty-eight hours prior to a race, aim for ten to twelve grams of carbohydrate per kilogram of body weight per day. For a seventy-kilogram athlete, this is seven hundred to eight hundred forty grams of carbohydrate per day. Use rice, pasta, bread, oats, potatoes, fruit. Keep fat and fiber intake low to reduce gut fill and minimize race day stomach distress. Drastically reduce training volume during this window — exertion will burn off the glycogen you are trying to store.

The night before the race, have a high-carbohydrate, low-fat, low-fiber, and lower-protein than usual dinner. Use foods that your gut tolerates well. Don’t introduce anything new.

On race morning, consume one to four grams of carbohydrate per kilogram of body weight one to four hours before the start, depending on gut tolerance and time available. For a seventy-kilogram athlete, this is seventy to two hundred eighty grams of carbohydrate pre-race. The closer to the start, the smaller the amount.

Common Mistakes

Too much fat alongside carbohydrate during the loading phase. Fat slows gastric emptying and reduces the rate at which carbohydrate is absorbed and stored as glycogen. Keep fat intake low.

High-fiber foods during loading. Fiber adds to gut fill, which can lead to bloating, discomfort, and stomach distress on race day. Avoid high-fiber vegetables, legumes, and whole grains during the final forty-eight hours.

Not reducing training volume. If normal volume continues during loading, you will just be replenishing the glycogen you burn rather than supercompensating above normal. Tapering is a prerequisite for effective loading.

Trying new foods. Race week is the worst time to experiment. Use carbohydrate sources you’ve consumed regularly throughout training.

Loading and Race Day Fueling are Complementary

Carbohydrate loading and race day fueling are complementary, not alternatives. Loading maximizes starting glycogen. Race day fueling maintains blood glucose and replenishes glycogen during the event. Together, they eliminate the energy deficit that leads to performance collapse in long ultras.

A fully loaded athlete who fuels properly throughout a one hundred-kilometer race is in a very different physiological state from one who either loads without fueling or fuels without loading.

Summary

Carbohydrate loading works. The protocol is straightforward: ten to twelve grams per kilogram per day for thirty-six to forty-eight hours pre-race, combined with reduced training, low fat and fiber, and a familiar pre-race meal. It is one of the highest-return nutritional strategies available to ultrarunners, and it takes less work to implement than most training blocks.

References

Bergström J, Hermansen L, Hultman E, Saltin B. (1967). Diet, muscle glycogen and physical performance. Acta Physiologica Scandinavica. 71(2-3), 140–150.

Sherman WM, Costill DL, Fink WJ, Miller JM. (1981). Effect of exercise-diet manipulation on muscle glycogen and its subsequent utilization during performance. International Journal of Sports Medicine. 2(2), 114–118.

Burke LM, Hawley JA, Wong SHS, Jeukendrup AE. (2011). Carbohydrates for training and competition. Journal of Sports Sciences. 29(Suppl 1), S17–S27.