The APOE ε4 allele — carried by approximately 25% of the general population — is the strongest known common genetic risk factor for late-onset Alzheimer's disease, conferring a 3–4× increased risk in ε3/ε4 heterozygotes and an 8–12× increased risk in ε4/ε4 homozygotes relative to the ε3/ε3 reference genotype. Despite this substantial genotype-conferred risk, penetrance is not deterministic. Large longitudinal cohort studies — including the UK Biobank, the Framingham Heart Study, and the WHICAP cohort — consistently demonstrate that modifiable cardiometabolic, dietary, sleep, and lifestyle variables significantly attenuate, and in some subgroups nearly normalize, the excess hazard associated with APOE ε4 carriage. The mechanistic basis for this modifiability lies in the isoform-specific lipoprotein biology of ApoE4, which impairs glymphatic and perivascular amyloid-β clearance, dysregulates neuronal lipid homeostasis, promotes tau hyperphosphorylation, and amplifies microglial-mediated neuroinflammation — all pathways directly responsive to metabolic and behavioral intervention. The pre-symptomatic window represents the highest-yield intervention period, and the emerging evidence base justifies a structured, genotype-aware risk reduction protocol.

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