Sufficient amounts of energy are stored in the animal body to support its needs during periods when dietary intake does not meet energy requirements. To be utilized, these energy reserves must be converted to compounds appropriate for oxidation at the cellular level. In addition, energy supplies must be transferred from storage sites to sites of utilization. The biochemical pathways of oxidation provide not only a means of deriving energy from carbon compounds, but also a means of transferring carbons from one type of energy source to another. The transfer of carbons between carbohydrates, lipids, ketone bodies, and proteins is regulated by endocrine and substrate effects. Regulation of this activity constitutes fuel homeostasis. Breakdowns in these homeostatic mechanisms result in metabolic disease.