Dementia is a leading cause of death in the United States among aging adults. Brain insulin resistance has emerged as a pathologic factor affecting memory, executive function as well as systemic glucose control. Regular aerobic exercise decreases Alzheimer's Disease (AD) risk, in part, through changes in brain structure and function. However, there is limited data available on how exercise impacts brain insulin resistance in aging. This study will test the effect of acute exercise on brain insulin sensitivity in middle-aged to older adults. The study will also examine cognition and cardiometabolic health in relation to brain insulin sensitivity.