Comparative Effects of a2 Skim Milk on Mild Cognitive Impairment in Older Adults

For inclusion in the study, participants must meet all of the following criteria: 1. Adults aged 65-75 years; 2. Complaint of memory loss for more than 6 months; 3. Base on the Chinese version of Mini Mental State Examination (MMSE), cognitive ability score is lower than the standard cut-off value according to age and education levels: MMSE score≤ 17 points if years of education is 0 year; ≤ 20 points if 6 years or less of education; ≤ 24 points if more than 6 years of education 4. Living and social functions are reduced: activities of daily living (ADL) score ≤18; 5. Not meeting the diagnostic criteria for dementia (Diagnostic and Statistical Manual of Mental Disorders - Fourth Edition), Alzheimer Disease (National Stroke Institute for Neuropathic Speech Disorders and Association for Alzheimer Disease and Related Disorders); no mental disorder, brain damage, or other physical disorders that can lead to cognitive impairment. 6. Agreed not to participate in another interventional clinical research study during the present study; 7. Signed informed consent Participants must be excluded from the study if they meet any of the following criteria: 1. \<65 years; or \>75 years 2. Neurological examination showed focal signs of central nervous system disorder such as hemiplegia, dysesthesia, aphasia; history of cerebrovascular diseases (including hemorrhagic and ischemic types), internal brain trauma or fracture; 3. Asthmatic bronchitis, severe hypertension, angina and severe infection; 4. Mental disorders such as depression and anxiety; endocrine system diseases (such as hyperthyroidism, hypothyroidism, systemic lupus erythematosus, rheumatoid arthritis); 5. Newly diagnosed, progressing or advanced tumors; 6. Visual, reading, hearing impairment or language communication difficulties that significantly affect cognitive function tests; 7. History of alcohol dependence and abuse of psychoactive substances (e.g., antipsychotics, benzodiazepines, cholinesterase inhibitors, sedatives), or use of drugs that affect cognitive function; 8. Neurological diseases (e.g. Parkinson's disease, epilepsy); 9. Antibiotic treatment in previous 2 weeks; 10. Administered immunosuppressive drugs in the 4-weeks preceding screening; 11. Allergy to milk or dairy; 12. Diagnosed lactose intolerance; 13. Other diseases that investigators judged as unsuitable to participate the study.