Effects of Menstrual Cycle on Insulin Sensitivity in Type 1 Diabetes: An Observational Study of Individuals Using an Automated Insulin Delivery System

OBJECTIVE: In menstruating individuals with type 1 diabetes, hormonal fluctuations across the menstrual cycle may alter insulin sensitivity and glycemia, despite advances in automated insulin delivery (AID) systems. This study aimed to characterize glycemic patterns, insulin requirements, and insulin sensitivity across menstrual cycle phases. RESEARCH DESIGN AND METHODS: We conducted a decentralized, observational study using real-world data donated by menstruating adults reporting regular menstrual cycles and using an AID system. Participants self-enrolled and contributed anonymized continuous glucose monitoring (CGM), insulin delivery, carbohydrate intake, and menstrual cycle data. Glycemic outcomes and insulin requirements were compared across cycle phases. Cycle phase-specific variation in insulin sensitivity was quantified using a hierarchical Bayesian state-space model that estimates a latent insulin sensitivity parameter from CGM time series data. RESULTS: Seventy-seven individuals contributed 380 menstrual cycles. Insulin requirements, carbohydrate intake, and glycemic outcomes showed systematic variation across cycle phases. Total daily insulin dose and carbohydrate intake peaked during the luteal phase, accompanied by higher mean glucose and reduced time in range. At the population level, model-derived estimates indicated greater insulin sensitivity (+2.6%, credible interval [CrI] 0.3, 5.0) in the early follicular compared with the midluteal phase (-2.6%, CrI -5.2, -0.1). Although 84.7% of participants showed patterns consistent with the population-level trend, substantial interindividual heterogeneity was observed. CONCLUSIONS: Our findings confirm phase-dependent variation in insulin needs, glycemic outcomes, and insulin sensitivity across the menstrual cycle and highlight that sex-specific physiology is an important factor in glycemic regulation that is not yet addressed by current guidelines and AID systems.