Carbo Loading: What It Is, Why It Works and How to Do It Properly

Carbo loading is a strategic nutrition approach used by endurance athletes to maximize glycogen stores before prolonged strenuous exercise or races. It is indicated for events lasting longer than 90 minutes. The purpose of carbo loading is to increase the amount of stored carbohydrate available to working muscles so that fatigue is delayed and performance can be sustained for longer.

Glycogen is the stored form of carbohydrate in the body. It is primarily stored in skeletal muscle and the liver. Muscle glycogen provides fuel directly to the working muscles during moderate to high intensity exercise, while liver glycogen helps maintain stable blood glucose levels and supplies energy to the brain and central nervous system. During prolonged aerobic exercise, especially when intensity rises above easy conversational pace, glycogen becomes the dominant fuel source. Once these stores decline significantly, performance begins to suffer.

Total glycogen storage capacity depends on body size, muscle mass and training status. Skeletal muscle typically stores between 300 and 700 grams of glycogen, while the liver stores approximately 80 to 120 grams. This results in a total storage capacity of roughly 400 to 800 grams, equivalent to around 1600 to 3200 calories of carbohydrate energy. Endurance trained athletes often store more glycogen than untrained individuals due to muscular adaptations and improved glycogen synthase activity.

"Carbo loading works by increasing muscle glycogen stores above normal resting levels, a phenomenon known as glycogen supercompensation."

When glycogen stores become severely depleted during prolonged exercise, athletes experience what is commonly referred to as “hitting the wall” or “bonking”. This usually occurs after 90 to 120 minutes of continuous endurance effort, although it varies depending on intensity and fuelling strategy. The symptoms are distinctive and often abrupt. Athletes may feel sudden and profound fatigue, heavy legs, dizziness, shakiness, reduced concentration and an increased perception of effort. Heart rate may feel higher at a previously manageable pace, and maintaining race speed becomes increasingly difficult.

From a physiological perspective, when glycogen becomes limited the body shifts toward a greater reliance on fat oxidation. While fat is an important fuel source, it cannot generate energy at the same rate as carbohydrate during higher intensity exercise. This reduced rate of ATP production contributes to the sharp drop in performance. In addition, low carbohydrate availability is associated with increased cardiovascular strain, higher core temperature, and reduced blood flow to the gastrointestinal system. These changes can increase the risk of dehydration, thermal stress and gastrointestinal discomfort, particularly in hot conditions.

Carbo loading works by increasing muscle glycogen stores above normal resting levels, a phenomenon known as glycogen supercompensation. Research has shown that when athletes reduce their training load in the days before competition and simultaneously increase carbohydrate intake, muscle glycogen concentrations rise significantly above baseline. Starting an endurance event with elevated glycogen stores has been consistently associated with improved time to exhaustion and better maintenance of pace during the later stages of the race.

This is where tapering becomes important. Tapering refers to the deliberate reduction in training volume in the days or weeks before a race. The goal of tapering is to reduce accumulated fatigue while preserving fitness. From a metabolic standpoint, tapering reduces daily glycogen utilization and muscle damage. At the same time, carbohydrate intake is increased. The combination of lower glycogen usage and higher carbohydrate availability creates optimal conditions for glycogen supercompensation. In simple terms, the body is using less fuel but storing more, resulting in maximized energy reserves on race day.

Carbo loading is generally recommended for continuous endurance events lasting longer than 90 minutes, such as marathons, long distance triathlons, cycling races and ultra endurance events. It is not necessary for shorter events where glycogen depletion is unlikely to be the primary performance limiter.

Current sports nutrition guidelines suggest consuming between 7 and 12 grams of carbohydrate per kilogram of body weight per day for two to three days before competition. For a 70-kilogram athlete, this equates to approximately 490 to 840 grams of carbohydrate daily. The exact amount depends on the duration and intensity of the event, as well as individual tolerance. During this period, carbohydrate becomes the dominant macronutrient in the diet, while fat intake is reduced and protein is kept moderate rather than high.

Food choices during carbo loading should prioritize easily digestible carbohydrate sources. Lower fibre and lower fat options are preferred in the final 48 hours before competition to reduce gastrointestinal strain. Foods such as white rice, white pasta, potatoes without skin, white bread, pretzels and rice cakes are commonly used because they replenish glycogen efficiently while being relatively easy to digest. Vegetable intake may be slightly reduced in the final days before racing, particularly high fibre vegetables such as cabbage, broccoli and legumes, to minimize bloating or discomfort.

Liquid carbohydrates can also be useful for athletes who struggle to meet high intake targets through solid food alone. Fruit juice, smoothies and sports drinks can help increase carbohydrate intake without excessive fullness. However, most intake should still come from whole food sources, with higher sugar foods used strategically rather than excessively.

The final pre-race meal, usually consumed three to four hours before the start, should contain a moderate amount of carbohydrate that has been practiced during training. It should be low in fibre and fat to reduce the risk of gastrointestinal upset. Individual tolerance is key, and athletes should never experiment with new foods on race day.

In summary, carbo loading is a scientifically supported strategy that enhances endurance performance by maximizing muscle glycogen stores. When combined with appropriate tapering, it allows athletes to begin competition with optimal fuel availability, delay fatigue and maintain performance during the later stages of prolonged exercise. Like any nutrition strategy, it should be practiced during training and individualized to ensure comfort and effectiveness on race day.