The number of Americans diagnosed with Alzheimer's disease (AD) is expected to triple by 2050. Compared to the general population, Veterans have a greater risk of AD, likely in part due to their increased incidence of traumatic brain injury, post-traumatic stress disorder, depression, and other vascular-related health issues. Based on available data, 423,000 new cases of AD are anticipated in Veterans by 2020. Thus, the discovery of effective therapies to prevent or delay the onset of AD in Veterans is critical. The goal of this study is to evaluate the efficacy of a purified form of the omega-3 fatty acid eicosapentaenoic acid (EPA) called icosapent ethyl (IPE), on improving brain blood flow, spinal fluid markers of AD pathology, and cognitive performance in middle-aged, cognitively-healthy Veterans with increased risk of AD. If IPE delays the onset of AD by even 5 years, the incidence of AD would be reduced by 50% in this population and could have a profound effect on Veteran quality of life and healthcare costs.