Our food choices supply the energy for our bodies to continue to function properly. These energy sources are: carbohydrate, protein and fats. The body can store these fuels in a form that allows immediate source of energy. Carbohydrates are readily broken down to glucose, the body’s main energy source. Glucose can be used immediately as fuel, or can be sent to the muscles and liver to be stored as glycogen. During exercise muscle glycogen is converted back into glucose. The liver converts its glycogen back into glucose, too, however it is released into the bloodstream to maintain your blood sugar levels. Blood glucose is also the main fuel for the brain when you rest as well as when you exercise. The body constantly uses and replenishes its glycogen stores.
The amount of energy the body can store is limited however. The body can store approximately 1800 – 2000 kcal worth of energy, enough to fuel about 90-120 min high intensity exercise. As we exercise, we gradually deplete our muscle glycogen stores, and blood glucose plays an increasingly important role in meeting the body’s energy demands. When the liver is also depleted of glycogen, you experience hypoglycaemia (low blood sugar) when your performance drops. You can avoid that by consuming carbohydrates during prolonged and high intensity exercise.
 Fat is the body’s most concentrated energy source. During exercise stored fat in the body is broken down into fatty acids. These fatty acids are transported through blood into the muscles for fuel. This process is slower than the mobilization of carbohydrates for fuel. Fat is also stored within the muscles where it can be accessed easier during exercise. In order for fat to fuel exercise, sufficient oxygen must be simultaneously consumed.

 As for protein, our bodies use protein to build, maintain and repair body tissues as well as synthesize important enzymes and hormones. Protein meets only 5 % of the body’s energy needs. In some situations, however, such as when we eat too few calories daily or not enough carbohydrate, as well as during latter stages of endurance exercise, when glycogen reserves are depleted, skeletal muscle is broken down and used as fuel to access certain amino acids that can be converted into glucose.

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