APP and Alzheimer disease: Given the variations in the timing of the clinical diagnosis of AD patients (even if a patient is clinically diagnosed with AD, it can be difficult to determine the patient’s initial pathological starting point) and the susceptibility of the human gut microbiota to environmental factors such as diet and lifestyle [16], APP/PS1 transgenic mice, one of the most widely used AD animal models [17], were selected as the subjects of this study.