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