A Longitudinal Study to Explore the Impact of Gut Microbiome on Brain Health in Alzheimer's Disease

Inclusion Criteria: * Cognitively normal group: 1. normal performance on standardized cognitive tests; 2. with no cognitive complaints or any concerns (worries). * SCD group: 1. self-experienced persistent decline in memory, rather than other domains of cognition; 2. normal performance on standardized cognitive tests; 3. failure to meet the criteria for MCI or dementia; 4. age at onset of SCD ≥ 60 years old; 5. onset of SCD within the last 5 years; 6. concerns (worries) associated with SCD; 7. feeling of worse performance than others of the same age group. * MCI group: 1. having impaired scores on both measures in at least one cognitive domain (memory, language, or speed/executive function); 2. having impaired scores in each of the three cognitive domains (memory, language, or speed/executive function); 3. the Functional Activities Questionnaire (FAQ)≥9. * AD dementia group: 1. meet the criteria for dementia and have impaired daily functional activities; 2. episodic memory deficit; 3) Clinical Dementia Rating (CDR) ≥ 1. Exclusion Criteria: * a history of stroke; * major depression and anxiety; * other central nervous system disorders that may cause cognitive impairment, such as Parkinson's disease, tumors, encephalitis, and epilepsy; * cognitive impairment caused by traumatic brain injury; * systemic diseases, such as thyroid dysfunction, syphilis and HIV; * psychosis or congenital mental developmental delay; * a history of using antibiotics, probiotics, prebiotics, or synbiotics within 3 months; * the use of corticosteroid, immune stimulating medications, and immunosuppressive agents; * major gastrointestinal tract surgery in past 5 years; * severe gastrointestinal diseases, such as irritable bowel syndrome, infammatory bowel disease, severe gastritis, other dysfunction in digestion and absorption, which has been reported to infuence gut microbiota