<P> Brain tissue consumes a large amount of energy in proportion to its volume , so large brains place severe metabolic demands on animals . The need to limit body weight in order , for example , to fly , has apparently led to selection for a reduction of brain size in some species , such as bats . Most of the brain 's energy consumption goes into sustaining the electric charge ( membrane potential ) of neurons . Most vertebrate species devote between 2 % and 8 % of basal metabolism to the brain . In primates , however , the percentage is much higher -- in humans it rises to 20 -- 25 % . The energy consumption of the brain does not vary greatly over time , but active regions of the cerebral cortex consume somewhat more energy than inactive regions ; this forms the basis for the functional brain imaging methods PET , fMRI , and NIRS . The brain typically gets most of its energy from oxygen - dependent metabolism of glucose ( i.e. , blood sugar ) , but ketones provide a major alternative source , together with contributions from medium chain fatty acids ( caprylic and heptanoic acids ) , lactate , acetate , and possibly amino acids . </P>
where does the brain get its energy from
most of its energy from oxygen - dependent metabolism of glucose ( i.e. , blood sugar ) , but ketones provide a major alternative source , together with contributions from medium chain fatty acids ( caprylic and heptanoic acids ) , lactate , acetate , and possibly amino acids