The ε4 allele of apolipoprotein E (APOE4) is the strongest genetic risk factor for Alzheimer's disease (AD), increasing AD risk about fourfold in ~ 34 million American and ~ 75 million European females. APOE4 carriers exhibit cerebral metabolic deficits decades before clinical onset. We previously demonstrated that ketogenic diet (KD), a low-carbohydrate, high-fat diet promoting ketone metabolism, confers cognitive benefits in aged C57BL/6 mice, and in the PS1/APP mouse model of early-onset AD. ...