Carbohydrate, in the form of glucose, is the preferred fuel for working muscles. It is particularly important during high intensity activity but whatever exercise is performed some carbohydrate will be used.
Glucose is stored in the muscles and liver as a substance known as glycogen and is rapidly converted back to glucose when is it required. The capacity for glycogen storage is limited - a 70kg individual has glycogen reserves of approximately 400g. Once these stores have been used, the ability to perform exercise is reduced.
In order to maximise the body's glycogen stores, athletes should habitually consume a high carbohydrate diet, contributing approximately 60% to 70% of total energy, equivalent to 6g to 10g carbohydrate/kg body weight/day. This is about 5-15% greater than the level of carbohydrate recommended for the general population.
Although complex carbohydrates such as cereals, pasta and bread are generally recommended in preference to simple sugars, a diet providing 70% of energy as complex carbohydrate can be excessively bulky. Moreover, there may be situations, for example, immediately post-exercise, where foods containing complex carbohydrate are not suitable. The use of food and drink containing simple sugars e.g. jam or honey may therefore be necessary on occasion.
In addition to a habitually high consumption of carbohydrate containing food, carbohydrate intake before, during and after exercise is important for optimising energy supply and maximising performance.
Studies have shown that the point of fatigue may be delayed and, ultimately, performance improved by ensuring high initial levels of muscle glycogen.
In the past, endurance athletes have taken steps to maximise glycogen stores in the run-up to a competition by a procedure known as super-compensation. This involves the depletion of glycogen stores (through a combination of exhaustive exercise and a low carbohydrate diet) followed by a period of 'carbohydrate loading', involving an extremely high carbohydrate intake. However, this extreme method is no longer considered necessary and has been replaced by a gradual tapering of training over the five to six days prior to a competition combined with an increase in carbohydrate. The modified version still results in an increase in glycogen stores of 20% to 40% above normal.
In addition to glycogen-boosting procedures in the days preceding a competition, carbohydrate consumption immediately pre-exercise can also affect performance. It is generally recommended that an easily digested, high carbohydrate meal be consumed two to three hours before an exercise session. To avoid the risk of gastric discomfort, the meal should not be too high in fibre. For some high intensity activities it has also been suggested that consumption of readily available carbohydrate immediately (five to 15 minutes) prior to performance may be beneficial. However, there are some concerns that this may cause insulin-induced hypoglycaemia (low blood sugar) in susceptible individuals.
For exercise lasting for more than an hour, studies have shown that carbohydrate consumed during the activity will provide additional fuel and increase endurance capacity. (For events lasting less than an hour the benefits of carbohydrate ingestion are negligible.)
One of the most convenient ways to ingest carbohydrate during exercise is by consuming sports drinks, which have the added advantage of also providing fluid.
Following exercise, muscle glycogen stores are low and need to be replenished before the next session of activity. Failure to fully refuel can result in earlier fatigue and poorer performance.
Refuelling should begin as soon as possible after exercise as the rate of glycogen synthesis is most rapid in the first one to two hours post-activity. Ideally, at least 50g of carbohydrate should be consumed during this period.
Again, sports drinks can be a useful way to provide carbohydrate initially, especially if solid food is unpalatable straight after exercise. However, solid and liquid carbohydrates are equally effective in promoting muscle glycogen storage post-exercise.
Complete refuelling takes around 24 hours, except following very intense exercise when muscle glycogen repletion may take longer.
Sources of carbohydrate
|Jam or honey
|Isotonic sports drink