This randomized, double-blind, placebo-controlled pilot trial will evaluate the effects of alpha-linolenic acid (ALA) supplementation on cognitive function, blood-brain barrier integrity, and brain vascular health in older adults with mild cognitive impairment and APOE4 genotype. By targeting the endogenous synthesis of docosahexaenoic acid (DHA) through ALA supplementation, the investigators aim to overcome the limitations of direct DHA supplementation, particularly in APOE4 carriers who exhibit low brain DHA levels and impaired blood-brain barrier function. This innovative approach offers a safe, cost-effective, and easily implementable therapeutic strategy for older adults at high risk for Alzheimer's dementia, especially APOE4 carriers, addressing a critical need given the limited cognitive benefits and significant adverse events of current amyloid-clearing drugs in this population.