The main sources of energy in the body are carbohydrates and fats. Carbohydrates yield about 4 kilocalories per gram; fat yields more than 9 kilocalories per gram. Protein yields about the same amount of energy as carbohydrates, but, except during starvation or prolonged fasting, only contributes up to 10 per cent of the energy requirements. The energy released from these nutrients during cellular respiration is used to make adenosine triphosphate (ATP), the only chemical that can be used directly by the body as a source of energy. Glycogen stored in muscles is the quickest source of ATP for muscles to use, especially for short bursts of vigorous activity. Fat is a less accessible source of energy. Fat stores in adipose tissue have to be broken down to glycerol and fatty acids, then transported in the blood stream to active muscles before being broken down. Consequently, fat is used mainly when physical activity is relatively gentle and prolonged. Because carbohydrates are the primary source of energy for intense, short bursts of muscular activity, they should constitute at least 50 per cent of the diet of physically active people.
Subjects: Medicine and Health.