 On the ketogenic diet , carbohydrates are restricted and so cannot provide for all the metabolic needs of the body . Instead , fatty acids are used as the major source of fuel . These are used through fatty @-@ acid oxidation in the cell 's mitochondria ( the energy @-@ producing parts of the cell ) . Humans can convert some amino acids into glucose by a process called gluconeogenesis , but cannot do this for fatty acids . Since amino acids are needed to make proteins , which are essential for growth and repair of body tissues , these cannot be used only to produce glucose . This could pose a problem for the brain , since it is normally fuelled solely by glucose , and most fatty acids do not cross the blood – brain barrier . Fortunately , the liver can use long @-@ chain fatty acids to synthesise the three ketone bodies β @-@ hydroxybutyrate , acetoacetate and acetone . These ketone bodies enter the brain and substitute for glucose . Medium @-@ chain fatty acids <unk> and <unk> acids can cross the barrier and be used by the brain . 
