Energy as fuel for the body comes from four main sources. Carbohydrate, protein, fats and alcohol. The right mix of these four nutrients will allow our bodies to function well and remain at a healthy state. An unbalanced diet will be to the detriment of a healthy body and body function.

During exercise, the body uses different fuel sources predominantly depending on the intensity and duration of the exercise being undertaken.

At low intensity, the body primarily uses fat stores as a source of energy. These are made into energy, or metabolised, only in the presence of oxygen. 




Medium intensity exercise increases the use of muscle glycogen and plasma glucose. As the intensity of the exercise increases, the body needs to shift the primary energy sources as the amount of energy being metabolised from fats is insufficient to meet the body's needs. It is also important to note that the amount of energy being used also increases.



High intensity exercise alters the amount and type of fuel sources being used for energy even further. The main source of energy now is muscle glycogen. High intensity exercise tends to only last for a short period of time, as there is only a small amount of readily available glycogen.


Energy is stored at different sites around the body, depending on the type.

Carbohydrate is stored primarily in the muscles and the liver, in the form of glycogen. This is a readily available fuel source as it is already in the muscles and is easily metabolised. Carbohydrate contains 3.75kcal/g.

Fats are generally stored around the abdomen and underneath the skin. They have 9kcal/g, which means that they are much higher in energy density but they are more difficult to metabolise, which means that they are not as easily used during exercise. Fats tend to be used more as a fuel in prolonged exercise at medium and low intensity.

Protein is generally not used as a primary source of energy for the body. Small amounts of protein are broken down and used, more so in endurance sports. It is dangerous for the body to use a large amount of protein and fuel, as this leads to muscle wasting. There are 4kcal/g protein. Protein is largely stored in the muscles and plays a important role as the building blocks of the body.

Alcohol is not a major contributor of energy during exercise but it does provide us with additional calories which are then stored and used as fuel. There are 7kcal/g alcohol, this is a fairly high amount and should not make up more than 5% of our energy intake.

For more information on carbohydrate, protein and fats, please see the relating pages (links from the nutrition front page).

Return to Homepage
Return to Nutrition Homepage