APOE and Alzheimer disease: For example, APOE4, which is associated with increased LDL-cholesterol and AD risk, relative to APOE3, is associated with a gut microbiome profile with reduced SCFA-producing bacteria, such as Ruminococcaceae (Maldonado Weng et al., 2019; Tran et al., 2019; Parikh et al., 2020; Zajac et al., 2022).