The biochemical pathways of energy metabolism in the brain are in most respects like those of other tissues, but special conditions peculiar to the central nervous system in vivo limit full expression of its biochemical potentialities. In no tissue are the discrepancies between in vivo and in vitro properties greater or the extrapolations from in vitro data to conclusions about in vivo metabolic functions more hazardous. Valid identification of normally used substrates and products of cerebral energy metabolism, as well as reliable estimations of their rates of utilization and production, can be obtained only in the intact animal; in vitro studies identify pathways of intermediary metabolism, mechanisms and potential rather than actual performance.

Although the brain is said to be unique among tissues in its high rate of oxidative metabolism, the overall cerebral metabolic rate for O₂ (CMRO₂) is of the same order as the unstressed heart and renal cortex [1]. Regional fluxes in the brain may greatly exceed CMRO₂, however, and these are closely coupled to changes in metabolic demand.