Effect of metformin on neurodegenerative disease among elderly adult US veterans with type 2 diabetes mellitus

OBJECTIVE: This study aimed to evaluate the association between metformin treatment and the risk of neurodegenerative disease (ND) among elderly adults with type 2 diabetes mellitus (T2DM). DESIGN/SETTING/PARTICIPANTS: This retrospective longitudinal cohort study examined the effects of the length of metformin exposure on ND among elderly US veterans with T2DM and insulin treatment using the Veterans Affairs electronic medical record database. PRIMARY AND SECONDARY OUTCOME MEASURES: The primary clinical outcome was defined as diagnosis of ND including dementia, Alzheimer’s disease (AD), Parkinson’s disease (PD), Huntington’s disease (HD) and mild cognitive impairment during the follow-up period. The secondary clinical outcomes were separately measured by AD, PD, HD, dementia and mild cognitive impairment. RESULT: Adjusted by propensity score weight, a total of 5528 patients (mean age, 63.2±10.9 years; male, 98%; white, 60%) with a median follow-up of 5.2 years were selected. Those with ND or other mental disorders at baseline or who were on insulin for less than two-thirds of the study period were excluded. The incidence rate of ND was 11.48 per 1000 person-years among patients with metformin treatment, compared with 25.45 per 1000 person-years for those without metformin. Compared with no metformin use, 2–4 years and >4 years of metformin exposure were significantly associated with lower risk of ND (adjusted HR (aHR)=0.62, 95% CI 0.45 to 0.85; aHR=0.19, 95% CI 0.12 to 0.31, respectively), while metformin exposure in the first 2 years showed no significant influence. CONCLUSION: We conclude that long-term metformin therapy (>2 years) was associated with lower incidence of ND among elderly veterans with T2DM. We need to conduct a study with more representative population and more robust method for causal inferences. Further investigation into the mechanism involved is needed along with randomised trials to confirm a potential neuroprotective effect of metformin.