HCAR2 and early-onset autosomal dominant Alzheimer disease: HCAR2 ligands can generate alleviating effects on Parkinson’s disease, Alzheimer’s disease, impaired learning, memory and motor functions, as well as amyotrophic lateral sclerosis via anti-inflammatory effects [43,50,57,258], suggesting that EKSs-evoked ketosis (βHB) exerts its alleviating effects on learning, memory, as well as age and age-related diseases through βHB/HCAR2-evoked downstream signaling (Figure 2).