Background <p>Iatrogenic peripheral hyperinsulinemia, resulting from peripheral insulin administration in type 1 diabetes, may increase insulin resistance and impair endothelial function. We hypothesized that lowering iatrogenic peripheral hyperinsulinemia via a one-week, reduced-carbohydrate diet (RCD) would improve insulin sensitivity and endothelial function compared with an isocaloric standard carbohydrate diet (SCD).</p> Methods <p>In this randomized, single-blinded, crossover trial, we studied 12 adults with type 1 diabetes. Participants completed both a one-week RCD and a one-week SCD, separated by a three-week washout. After each intervention, we measured insulin sensitivity using a hyperinsulinemic–euglycemic clamp and assessed endothelial function via brachial-artery flow-mediated vasodilation (FMD).</p> Results <p>The RCD reduced total daily insulin doses by 16% compared with the SCD. Despite this reduction, insulin sensitivity did not improve (median glucose infusion rates: RCD 8.1&#xa0;mg/kg FFM/min [IQR 6.7–10.1] vs. SCD 8.6&#xa0;mg/kg FFM/min [7.0–11.0], <i>p</i> = 0.47). Similarly, endothelial function did not differ significantly (FMD after RCD 7.50% [3.25–15.5] vs. SCD 9.81% [4.96–14.3], <i>p</i> = 0.91). Although higher insulin doses correlated with lower insulin sensitivity under both conditions, lowering insulin dose through the RCD alone did not yield measurable improvements.</p> Conclusions <p>Although a one-week RCD significantly lowered insulin requirements, it failed to enhance insulin sensitivity or endothelial function in adults with type 1 diabetes. These findings underscore the complex and dynamic relationship between insulin exposure and cardiometabolic health. Similar basal overnight insulin delivery may have masked potential benefits by the time of testing, highlighting the need for further studies to refine strategies aimed at mitigating hyperinsulinemia’s adverse effects.</p> Trial registration <p>ClinicalTrials.gov NCT04118374.</p> Graphical abstract <p></p>