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Carbohydrate loading, commonly referred to as carb-loading or carbo-loading, is a strategy used by endurance athletes, such as marathon runners, to maximize the storage of glycogen (or energy) in the muscles.
Carbohydrate loading is also used in healthcare to optimise the condition of patients prior to colorectal surgery.
Carbohydrate loading is generally recommended for endurance events lasting longer than 90 minutes. For many endurance athletes the foods of choice for carbo-loading are those of low glycemic indices due to their minimal effect on serum glucose levels. Low glycemic foods commonly include fruits, vegetables, whole wheat pasta and grains. Because of this, many marathoners and triathlon participants have large pasta dinners the night before the race. Since muscles also extensively utilize amino acids when functioning within aerobic limits, meals should include adequate protein on top of carbohydrates. Large portions before a race can, however, be detrimental to race-day performance if the digestive system has not had the time to adequately process the food.
In the 1980s, research led to a modified carbo-loading regimen that eliminates the depletion phase, instead calling for increased carbohydrate intake (to about 70% of total calories) and decreased training for three days prior to the event. Most athletes now follow this modified regimen; it is recommended by many coaches, although there are some athletes who still follow the original carbo-loading regimen.
A new carbo-loading regimen developed by scientists at the University of Western Australia calls for a normal diet with light training until the day before the race. On the day before the race, the athlete performs a very short, extremely high-intensity workout (such as a few minutes of sprinting) then consumes 12 g of carbohydrate per kilogram of lean mass over the next 24 hours. The regimen resulted in a 90% increase in glycogen storage when compared to before the carbo-load, which is comparable or higher to the results achieved with other 2 day - 6 day carbo-loading regimes.
Carbohydrate ingestion less than 2 hours prior to aerobic exercise triggers elevated levels of insulin in the blood, which may dramatically decrease serum glucose levels. This can limit aerobic performance, especially in events lasting longer than 60 minutes. This is known as transient or reactive hypoglycemia, and can be a limiting factor in elite athletes. Individuals susceptible to hypoglycemia are especially at risk for elevated insulin responses and thus will likely suffer from performance-limiting, transient hypoglycemia.
The composition of carbohydrates in the athlete's diet during carbohydrate loading is as important as their share of the overall caloric intake.
Most dietary carbohydrates consist of varying proportions of two simple sugars, glucose and fructose. Fructose may be metabolized into liver glycogen, but it is ineffective at raising muscle glycogen levels (which is the objective of carbohydrate loading). Consequently, sources of high-fructose carbohydrates, such as fruit and sweets, are less than optimal for the task. The classic carb-loading meal is pasta, whose caloric content is primarily due to starch, a glucose polymer. Other high-glucose meals include bread, rice, and potatoes.