The Brain Energy for Amyloid Transformation in AD (Alzheimer's disease) or BEAT-AD study will compare the effects of a ketogenic low-carbohydrate diet and a low-fat diet in adults with mild cognitive impairment. The data collected will help determine whether diet interventions induce changes in cognitive function, cerebral blood flow, and levels of certain proteins and hormones in body fluids. The study will include volunteers who have mild cognitive impairment, who will be randomly assigned to receive either a ketogenic low-carbohydrate diet or a low-fat diet for 16-weeks, with follow-up assessment 8 weeks after diet completion. Study measures, clinic visits and phone sessions will occur at baseline and throughout the 24-week study. Participant will follow either a low-carbohydrate or low-fat diet that will be individually planned with help from a study dietitian. After completing the study diet for 16 weeks, participants will resume their normal diet. The final visits will occur at week 24 (8 weeks after the completing the diet). At the end of the 24-week study, participants will be given the opportunity to meet with the study dietitian for education and assistance with planning a healthy diet.