Carbohydrates (CHO) are composed up of carbon, hydrogen and oxygen. In their simplest form (known as simple sugars), they exist as monosaccharides such as glucose and fructose. In their disaccharide form, they are made up of two monosaccharides molecules. For example, normal table sugar or sucrose, is composed of glucose and fructose. Complex carbohydrates are polysaccharides of which starch is the plant storage form and glycogen is the animal storage form (taken up and stored in the body through glucogenesis) found in the skeletal muscles and liver. Complex CHO (such as potatoes) often have a higher nutritional value due to them containing vitamins and minerals. Fibre is an indigestible form of CHO and is not used as a source of fuel. Indeed top players should be careful as too much fibre may slow CHO digestion and can reduce the absorption of valuable minerals and vitamins.
All carbohydrates contain roughly the same amount of energy - 16kJ/g. Foods which are high in carbohydrates include bread, potatoes, pasta, rice and cereals. For example, 400g of white bread or pasta (complex CHO) contain around 100g of CHO. Similarly a 150g chocolate bar or jam contains 100g of simple sugars. Foods are also based on their glycaemic index which is the ability of the food to raise blood glucose levels. Foods with a high index are absorbed quicker and hence elevate blood glucose quicker. Bread, potatoes and rice are amongst those with a high index, pasta, noodles and crisps belong to the moderate index and apples, beans and lentils to the low index. A more detailed list is shown below.
The recommended daily CHO intake for athletes in training is around 60% of their overall energy intake. The amount required depends upon the athlete’s total daily energy expenditure, type of sport performed, the athlete’s sex, and environmental conditions. However, it is more useful to base CHO intake on body weight (the amount of CHO per Kg of body weight). Recommendations for athletes range from 6 to 10 g/kg of body weight a day (i.e. between 400 - 600g). During heavy training, this may be increased to 70%. Various suggestions on increasing CHO intake are listed below.
- Eat a large
portion of potatoes, pasta or rice with your main meal.
Studies have generally demonstrated the importance of CHO in providing energy for athletic performance. However, CHO also acts as fuel for the central nervous system at rest and during exercise. A reduction in blood glucose (hypoglycaemia) leads to feelings of weakness, hunger and dizziness and will lead to a reduction in performance due to neurological fatigue. CHO also acts as a protein sparer. Protein, normally used for tissue development will be used as an energy source if glycogen stores are reduced. Similarly, CHO aid in the breakdown of fat for energy which is especially useful in endurance exercise.
As mentioned earlier, CHO play an important role in exercise performance. The use of CHO depends upon various factors such as the intensity of exercise, duration, the athletes fitness and nutritional status. CHO act as an energy fuel during high-intensity exercise and in the early minutes of exercise from the breaking down of blood glucose and muscle glycogen. For example, after 40 minutes of intense exercise, glucose uptake can increase up to 20 times compared to the uptake at rest. During high-intensity aerobic exercise, intramuscular glycogen is the preferential energy source as it supplies energy quicker than fat or protein.
During the first few minutes of moderate and prolonged exercise, muscle glycogen provides the required energy. Further into exercise, liver and muscle glycogen provide around 40-50% of energy requirements with the rest coming from blood glucose and fat breakdown. When muscle glycogen stores deplete, fat metabolism and blood glucose play a greater role with the latter gradually declining due to the liver being unable to provide the necessary output.
A high carbohydrate diet (i.e. for three days) results in an elevated muscle glycogen content which in turn increases the capacity to undertake prolonged exercise. Indeed, muscle glycogen plays an important part in soccer performance. Studies have shown that players starting matches with low glycogen levels ran and sprinted less than those with fully replenished stores. This shows the importance of correct CHO intake before and after training and competition to fully replenish glycogen stores. CHO loading (through increasing CHO intake) as mentioned earlier can help augment intense and prolonged aerobic performance. Athletes deplete glycogen stores over several days by initial exhaustive exercise then maintain a low CHO diet. They then load upon a high CHO diet during several days which increases glycogen stores past levels obtained on a normal diet. However, expert nutritional advice should be sought as this can have negative effects on performance.
The use of CHO energy drinks can help increase muscle glycogen stores and improve player work-rates. For more detailed information on such drinks click here. Drinking during a match helps reduce fatigue and prevents dehydration. The use of CHO drinks use in aiding post-match recovery is also important, especially as the most rapid restocking of glycogen stores occurs during the first few hours after exercise, with the first two hours being the most crucial. The only time CHO intake should be avoided in within 30-60 minutes of exercise as this can impair performance due to an increase in insulin which then reduces blood glucose levels.
Generally, when experimenting with changes in CHO intake and general diet, this should only be carried out with expert advice from a doctor or qualified nutritional expert to ensure the continuing good health and performance of the player. Likewise, trying out new drinks or diets should firstly be carried out during training before being used in competition.
Click here to go to the nutrition advice section of Soccer Performance.
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