The majority of what we eat each day has one main purpose: to provide energy for the body. The energy in food that we can use comes from the chemical bonds between the atoms in carbohydrate, fat, protein and even alcohol. Each of these nutrients can be broken down (metabolised) into simpler molecules through different biochemical pathways to make energy. Every cell in our body has the ability to use nutrients to make energy; in fact, every cell in our body needs to make energy from nutrients or else it cannot function!

The main pathway where carbohydrate, fat and protein metabolism meet to produce energy is called the Kreb's cycle (also known as the citric acid cycle). The Kreb's cycle takes place inside the cell in a specialised structure called mitochondria. The mitochondria are quite rightly called 'the powerhouses' of the cell. The Kreb's cycle allows electrons to be taken from nutrients to help drive pumps that produce ATP (adenosine triphosphate). ATP is the main energy currency of all cells in the body and is used to pump molecules into and out of cell, allow enzymes to work and muscles to contract. Metabolism of nutrients to make ATP results in by-products such as carbon dioxide, water and heat. For a typical adult eating a normal diet, almost all of the ATP we make each day comes from the carbohydrate and fat we eat; only small amounts of ATP come from protein.