Brain glucose uptake is impaired in Alzheimer's disease (AD), and increasing ketone availability to the brain via moderate nutritional ketosis has a modest beneficial effect on cognitive outcomes in mild-to-moderate AD and in mild cognitive impairment. This can be achieved through a high-fat ketogenic diet, supplements providing medium-chain triglycerides, or ketone esters. The development of therapeutic strategies for AD should consider how the underlying problem of deteriorating brain fuel supply can be corrected or delayed.