Aims/hypothesis <p>We aimed to evaluate whether prandial insulin timing affects vascular function in people with type 1 diabetes. Our hypothesis was that premeal insulin administration would lead to greater myocardial microvascular blood flow (MBF) via blunting postprandial hyperglycaemia.</p> Methods <p>People with type 1 diabetes between 18 and 35 years of age with BMI &lt;30 kg/m² underwent two protocols with a 1:1 randomised crossover design wherein prandial insulin was injected either 15 min before or 15 min after meal intake began. To provide a physiological comparison, age-, sex- and BMI-matched control participants completed one study where they consumed the same meal but received no exogenous insulin. Glucose, insulin, vascular function (including ultrasound measures of myocardial and skeletal muscle microvascular perfusion, aortic stiffness, brachial artery endothelial function) and biomarkers of systemic inflammation and endothelial dysfunction were assessed at baseline and then 2 h after meal ingestion within each protocol. The primary outcome was change in myocardial MBF within each protocol. Study personnel assessing outcomes were masked to group assignment.</p> Results <p>Eighteen people with type 1 diabetes and 18 matched control participants were analysed within each protocol. Glucose area under the curve was significantly greater (<i>p</i>=0.015) in the postmeal insulin study compared with the premeal insulin study in participants with type 1 diabetes. Myocardial microvascular flow velocity significantly increased (<i>p</i>=0.031) with premeal insulin administration in people with type 1 diabetes and this consequently led to greater myocardial MBF (<i>p</i>=0.044). There were no changes in myocardial MBF within the other protocols. Changes in vital signs were similar between all protocols.</p> Conclusions/interpretation <p>Appropriately timed premeal insulin led to lower postprandial blood glucose along with increased myocardial MBF in people with type 1 diabetes. Further work is needed to determine the underlying aetiology of these changes.</p> Trial registration <p>ClinicalTrials.gov NCT04730882</p> Graphical Abstract <p></p>