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 Carbohydrate is the body's preferred source of fuel. Carbohydrates are the only energy source that can be used by the brain. They are the energy source used to provide muscles with the ability to work without oxygen. Therefore carbohydrates are the primary energy source for high intensity exercise. Carbohydrate should make up around 60% of daily energy intake.
There are two main types of carbohydrate, complex and simple.

Complex Carbohydrates.
Complex carbohydrates can be found within starchy food such as rice, potatoes, vegetables, cereal, pasta and bread. Complex carbohydrates are known as slow release foods; this is because they take some time to be broken down in the digestive system and are more steadily absorbed by the body. Complex carbohydrates provide us with that feeling of being full and a satisfying meal. Most people would benefit from eating a higher proportion of starchy carbohydrate in their daily diet. This tends to result in a diet that is lower in fat, especially if wholegrain varieties are chosen.

Simple Carbohydrates.
The rest of our carbohydrate intake comes from foods and drinks that contain sugar, including fruit and vegetables, milk, confectionery, sugar and soft drinks. Simple carbohydrates are quickly absorbed into the bloodstream and cause rapid rises in blood sugar levels. However these surges are usually followed by rapid falls. Foods high in refined sugar such as table sugar, sugary drinks and confectionery items provide "empty calories". This means that apart from the energy the sugar provides, there is often very little else of nutritional value. Sugar also contributes to tooth decay and gum disease. It is a good idea to limit your daily sugar intake.