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The various forms of energy in the body are all ultimately derived from chemical energy present in the food ingested. This energy is needed to carry out various functions - body metabolism, thermoregulation, and physical activity.
The three metabolically active ingredients of food are fat, carbohydrates and proteins and the body uses the following three kinds of fuel derived from them - fatty acids, glucose, amino acids. Excess fatty acid is stored in fat cells called adipocytes in the form of triacylglycerol and an average person has enough fat energy to survive for a month. The body rapidly utilizes most ingested sugar and small carbohydrates. When glucose levels rise in the blood, insulin is released which helps the liver and muscles to absorb glucose from the blood. Furthermore, glucose metabolism is enhanced and organs start to use glucose as a fuel instead of fatty acids. All excess sugars are converted to glycogen (the storage form) in liver and muscle, which can store only a limited amount (about 150 gms). Thus any carbohydrate not immediately used for energy is stored in the liver as glycogen for short-term use and as the body has only a limited number of liver cells to store the glycogen, whatever is left over is converted to fat. This stored glucose as glycogen can generally be depleted in a single day, making it a very short-term fuel. Proteins are broken down to amino acids in the intestine and then brought to the liver, where they are partly reassembled and partly released into the bloodstream. Protein is primarily used to build up the body and it is only under extreme starvation that the body as a fuel uses protein. Excess protein and fat in the diet are also stored as fat.
Metabolizable energy is the chemical energy of carbohydrates (4 cals/gm), fats (9 cals/gm), and proteins (4 cals/gm) liberated by oxidation in the living body. Most of this energy appears as heat, but ~35-40% is converted to high-energy phosphate bonds of adenosine triphosphate (ATP) that is used for cellular work. The energy currency of the body is ATP and all fuels i.e. fatty acids, glucose or amino acid have to be converted to ATP for utilisation by the body. ATP molecules cannot be stored by the body and are generated within cells in an organelle called mitochondria.
The rate of energy metabolism is regulated by both the nervous and endocrine systems. The nervous system regulates the activity of skeletal muscle while the endocrine system regulates energy metabolism mainly through the action of the thyroid hormones and adrenaline. The thyroid hormones affect many cells in the body to increase the rate of energy metabolism.
Of the three dietary ingredients, fat is the most concentrated source of energy because it furnishes more than twice as much energy for a given weight as protein or carbohydrate and is the major source of energy for the body. If too little food is eaten to meet energy demands, the body's stored fat serves as an energy source and a weight loss will result.