HCAR2 and Alzheimer disease: Preclinical studies have demonstrated that GPR109A agonists, such as niacin and R-β-hydroxybutyric acid (BHB), attenuate disease progression and the accumulation of Aβ into senile plaques and improve passive avoidance behaviors and responses to the Morris water maze test in the 5XFAD mouse model of Alzheimer’s disease [8,36].