After consuming a chilled 4 ounce shake which contains various dietary fats, cognitive testing and dual-tracer Positron Emission Tomography scan will be completed. At Visit 1, each participant will be randomized to Challenge A (low calorie Ensure®) or Challenge B (Ensure® plus ketone esters). At Visit 2, each participant will undergo the remaining challenge (A or B) prior to the second imaging session. Each visit is designed to be identical to the other and only vary the dose of ketone esters. Randomization to either Challenge at Visit 1 will ensure that test/re-test learning effects are equally distributed between the challenge groups. This study consists of two visits that will take place over a two week period.