Background: Inflammation has been implicated in the pathogenesis of diabetes. This study investigated the randomised treatment effect of low-dose aspirin on incident type 2 diabetes and fasting plasma glucose (FPG) concentrations among older adults.

Methods: ASPREE was a double-blind, placebo-controlled trial of daily oral low-dose aspirin. The study population included community-dwelling individuals aged 70 years or older (≥65 years for US minority ethnic groups) in the USA and Australia who were free of cardiovascular disease, independence-limiting physical disability, or dementia. For the post-hoc analysis, we excluded participants with diabetes at baseline or with incomplete or missing incident diabetes data during follow-up. Participants were randomly assigned 1:1 to oral 100 mg daily enteric-coated aspirin or placebo. Incident diabetes was defined as self-reported diabetes, commencement of glucose-lowering medication, or a FPG concentration of 7·0 mmol/L or more assessed at annual follow-up visits among participants with no diabetes at baseline. We used Cox proportional hazards models and mixed-model repeated measures to assess the effect of aspirin on incident diabetes and FPG concentrations in the intention-to-treat population. We assessed major bleeding in participants who had taken at least one dose of study medication.

Findings: Between March 10, 2010, and Dec 24, 2014, a total of 16 209 participants were included (8086 [49·9%] randomly assigned to aspirin and 8123 [50·1%] randomly assigned to placebo). During a median follow-up of 4·7 years (IQR 3·6-5·7), 995 (in 6·1% individuals) incident cases of type 2 diabetes were recorded (459 in the aspirin group and 536 in the placebo group). Compared with placebo, the aspirin group had a 15% reduction in risk of incident diabetes (hazard ratio 0·85 [95% CI 0·75 to 0·97]; p=0·013) and a slower rate of increase in FPG concentration at year 5 (between-group difference estimate -0·048 mmol/L [95% CI -0·079 to -0·018]; p=0·0017). Major bleeding (major gastrointestinal bleeding, intracranial bleeding, and clinically significant bleeding at other sites) occurred in 510 (3·2%) of 16 104 participants (300 [3·7%] in the aspirin group and 210 [2·6%] in the placebo group). Compared with placebo, the aspirin group had a 44% increase in risk of major bleeding (hazard ratio 1·44 [95% CI 1·21 to 1·72]; p<0·0001).

Interpretation: Aspirin treatment reduced the incidence of type 2 diabetes and slowed the increase in FPG concentration but increased major bleeding among community-dwelling older adults. Given the increasing prevalence of type 2 diabetes among older adults, the potential for anti-inflammatory agents such as aspirin to prevent type 2 diabetes or improve glucose levels warrants further study with a comprehensive assessment of all potential safety events of interest.