Accuracy of FreeStyle Libre 3 During Moderate-Intensity Continuous Aerobic Exercise at Different Phases of the Menstrual Cycle in Females with Type 1 Diabetes

<b><i>Background:</i></b> The use of continuous glucose monitoring (CGM) devices in managing type 1 diabetes (T1D) has been associated with improved glycemic control in individuals with T1D. A key challenge for CGMs, however, is achieving accuracy, particularly under conditions where glucose levels may fluctuate rapidly, such as during exercise. Another factor contributing to blood glucose variability is the menstrual cycle, during which hormonal fluctuations affect insulin sensitivity, leading to variable glucose levels. This study aimed to assess the accuracy of FreeStyle Libre-3 (FSL3) during continuous moderate-intensity aerobic exercise (CONT) performed in the follicular and luteal phases of the menstrual cycle in females with T1D. <b><i>Methods:</i></b> Participants underwent CONT sessions on a cycle ergometer, one in the follicular phase and one in the luteal phase of the menstrual cycle, at the Research Laboratory of the Faculty of Physiotherapy. Glucose levels were measured every 10 min using FSL3 and the YSI 2500 as a gold standard. Measurements began 20 min before CONT and continued for 20 min after exercise. <b><i>Results:</i></b> A total of 26 females (mean age 32.2 ± 6.1 years and mean duration of diabetes 16.4 ± 8.4 years) participated in this study. FSL3 showed significant differences compared with YSI glucose data for both phases of the menstrual cycle (about 16 mg/dL higher in FSL3). There were no differences in mean absolute relative differences (MARDs) between the follicular (16.06%) and luteal (16.43) phases. Moreover, exercise did not affect MARDs, which were 14.21% pre-exercise and 17.63% postexercise for the follicular phase and 14.95% pre-exercise and 17.71% postexercise for the luteal phase. <b><i>Conclusions:</i></b> The findings suggest that the accuracy of FSL3 is not affected by CONT, showing good accuracy levels in both phases of the menstrual cycle. Thus, this study is the first to examine the influence of the menstrual cycle and exercise on the accuracy of a CGM device. The study was also prospectively registered at clinicaltrials.gov (NCT06086067).