Energy is defined as the potential to perform work and is required by the animal to perform the ''work of living.'' Some of the more obvious examples of the expenses of living include thermoregulation, voluntary and involuntary muscular activity, ingestion of food, digestion, absorption, excretion, metabolite transport, cell turnover, tissue or product formation (e.g., wool, milk, eggs), etc. There are inefficiencies or waste associated with all of these processes. Energy requirements depend on the additive needs of individual cells, which vary according to physiological needs imposed upon them. Gross dietary requirement is the sum of all cellular needs plus losses. Bioavailability of energy is an expression of the value of an energy source toward meeting the cumulative energy needs of all cells to perform the ''work of living'' of the animal.

Energy is an abstraction that can only be measured in its transformation from one form to another. Bioenerget-ics, the study of energy transformations in biological systems, is based on the fundamental principles stated by the laws of thermodynamics and the law of Hess. Simply stated, these laws assert that: 1) energy can be neither created nor destroyed, but may be converted from one form to another, 2) all forms of energy can be quantitatively converted to heat, and 3) heat generated in a net transformation is independent of the path of conversion. Energy is available to the animal from three sources: diet, body reserves, and to some extent, from its external environment (via radiation, convection, and conduction). This article will concentrate primarily on energy availability from dietary sources and, to a lesser extent, from body tissues.

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