The most common sources of energy for laboratory animals are fats and carbohydrates. Energy is also derived from protein, but to a lesser extent. The unit of energy is expressed in terms of heat units (calories). A "small calorie" is defined as the amount of heat required to raise one gram of water 1°C. The "large Calorie," or "kcal," is the amount of heat required to raise one kilogram of water 1°C. The "kcal" is the unit of energy commonly used in expressing the energy content of food.
Gross energy is the energy of complete combustion measured as heat when a material is burned to its oxidation products.
Digestible energy is equal to the gross energy minus the energy remaining in the fecal matter.
Metabolizable energy equals the gross energy minus the energy lost in the fecal matter and that lost via the urine and combustible gases. In this Animal Diet Reference Guide, metabolizable energy is reported by two procedures. Physiological fuel values are an estimate of metabolizable energy and are calculated by assigning 4 kcal/gm of metabolizable energy for the protein fraction of the feed, 4 kcal/gm for the carbohydrate fraction (as measured by the nitrogen-free extract), and 9 kcal/gm for the fat fraction. A value is also given for metabolizable energy based upon values obtained from swine, poultry or ruminants, whichever is appropriate.
Net energy is the energy remaining for production uses after deducting from the gross energy those energies lost in the feces, urine, combustible gases and body heat losses.
When nourishment is withheld from the body, energy is derived from the carbohydrate glycogen in the liver, fat stores and protein in the body tissues.
The major function of a food is to supply energy for body processes and to form non-nitrogenous, organic matter of tissues and secretions. Without adequate energy, other important organic nutrients are not used for the normal needs such as tissue maintenance, growth processes, reproduction and lactation, nor for work and heat.
Without sufficient energy, the young animal's body cannot grow. Because of the additional energy requirements of the young animal and the breeding female, they obviously need more energy-producing nutrients than other animals.