• Change of cognitive function [ Time Frame: from baseline to 16 weeks' follow-up. ]
  Compare the change of Montreal Cognitive Assessment (MoCA, beijing version) from baseline to 16 weeks' follow-up. MoCA score(ranging from 0 to 30)<26 is considered cognitive impairment and when the score is lower, the cognitive impairment is more serious. MoCA score ≥26 is considered normal cognition.
• Proportion of patients whose MoCA<26 scores [ Time Frame: at baseline and at 16 weeks' follow-up. ]
  Compare the proportion of patients whose MoCA<26 scores at baseline and at 16 weeks' follow-up.
• Change of blood glycaemic control [ Time Frame: from baseline to 16 weeks' follow-up ]
  Compare the change of glycosylated hemoglobin(HbA1c) from baseline to 16 weeks' follow-up
• Proportion of patients whose HbA1c<7% [ Time Frame: at 16 weeks' follow-up. ]
  Analyze the proportion of patients whose HbA1c<7% at 16 weeks' follow-up.
• Proportion of patients whose weight loss>3% and >5% [ Time Frame: from baseline to 16 weeks' follow-up. ]
  Analyze the proportion of patients whose weight loss>3% and >5%
• Olfactory threshold test [ Time Frame: from baseline to 16 weeks' follow-up. ]
  The olfactory threshold test (score range 1-13.5) is determined based on a series of binary dilutions of the N-butanol solution in light mineral oil. The higher the score is, the more sensitive the participant is in detecting an odor. Scores of 8-10 were considered normal olfactory sensitivity, whereas scores of 1-3 signified olfactory dysfunction or anosmia, and scores of ≥10 indicate better olfactory sensitivity.

• Type 2 Diabetes Mellitus
• Obesity
• Cognitive Impairment

• Drug: Liraglutid
  Liraglutid will be titrated from 0.6mg/day to 1.8mg/day during the first 2 weeks, if well tolerated. All patients will also continue on their existing dose and regimen of metformin throughout the study.
  Other Name: metformin
• Drug: Dapagliflozin
  Dapagliflozin will be initiated and maintained at 10mg/day every morning until the completion of the study. All patients will also continue on their existing dose and regimen of metformin throughout the study.
  Other Name: metformin
• Drug: Acarbose
  Acarbose will be initiated at 50mg three times daily for the first week, and then titrated to 100mg three times daily if appropriate.All patients will also continue on their existing dose and regimen of metformin throughout the study.
  Other Name: metformin

• Active Comparator: Liraglutid
  Liraglutid will be titrated from 0.6mg/day to a final dose 1.8mg/day during the first 2 weeks, if well tolerated. Meanwhile, All patients will also continue on their existing dose and regimen of metformin throughout the study. Visits at 4-week intervals will be performed to evaluate the safety of drugs. Metformin dose can be reduced in response to hypoglycaemia, but liraglutid could not be adjusted. If the plasma glucose still not achieve the target at the maximum dose, the maximum dose will be maintained until the completion of the study.
  Intervention: Drug: Liraglutid
• Active Comparator: Dapagliflozin
  Dapagliflozin will be initiated and maintained at 10mg/day every morning until the completion of the study. Meanwhile, All patients will also continue on their existing dose and regimen of metformin throughout the study. Visits at 4-week intervals will be performed to evaluate the safety of drugs. Metformin dose can be reduced in response to hypoglycaemia, but dapagliflozin could not be adjusted. If the plasma glucose still not achieve the target at the maximum dose, the maximum dose will be maintained until the completion of the study.
  Intervention: Drug: Dapagliflozin
• Active Comparator: Acarbose
  Acarbose will be initiated at 50mg three times daily for the first week, and then titrated to100mg three times daily if appropriate. Meanwhile, All patients will also continue on their existing dose and regimen of metformin throughout the study. Visits at 4-week intervals will be performed to evaluate the safety of drugs. Metformin dose can be reduced in response to hypoglycaemia, but acarbose could not be adjusted. If the plasma glucose still not achieve the target at the maximum dose, the maximum dose will be maintained until the completion of the study.
  Intervention: Drug: Acarbose

Inclusion Criteria:

• Age ≧ 40 and ≦75 years old
• T2DM patients controlled with metformin monotherapy with stable, maximum tolerated doses (≧1500mg/d, ≧12 weeks)
• HbA1c>7% and ≤9%
• Body mass index(BMI) ≥25kg/m2 and with stable weight during previous 3 months
• Right handedness
• Possessed over 6-year education
• Provision of informed consent prior to any study specific procedures
• Mini-Mental State Examination (MMSE) >24

Exclusion Criteria:

• Allergies to research drugs
• Treated with glucagon-like peptide-1 receptor agonists, dipeptidyl peptidase-4 inhibitors, sodium-glucose cotransporter 2 inhibitors, insulins, and glycosidase inhibitors in the previous 6 months
• Moderate to severe renal dysfunction defined as estimated glomerular filtration rate(eGFR)<60ml/min/1.73m2 ( eGFR was estimated by CKD-EPI creatinine equation using an online calculator).
• Hepatic insufficiency
• A history of neurological and psychiatric disorders, nasal pathologies, abnormal thyroid, pancreatitis, repeated urinary tract infection, chronic gastrointestinal dysfunction, any disease that may worsen by intestinal flatulence, alcohol or substance abuse, steroid treatment
• Any acute disease
• Inability to undergo tests or MRI scanning
• Pregnant or lactating women
• Participating in other clinical trials at the same time or within 6 months prior to the start of the trial