The aim of this study is to evaluate the effect of a 3-month walking program on brain energy metabolism in patient with mild Alzheimer's disease (AD). Two groups of sedentary patients with mild AD are followed and compared over a 3-month period of time: Control (non-active) and walking (from 15 to 45 minutes of exercise on a treadmill, 3 times a week for 12 weeks) groups. All the participants are evaluated on their cognition, brain volumes (MRI) and brain fuel consumption (PET scan with 18-FDG and 11C-AcAc) at the beginning and at the end of the study.