Investigation of glucose and energy metabolism in eumenorrheic female ultra-runners across the menstrual cycle

Abstract

Female athletes, ultra-running, and the utilization of continuous glucose sensors in endurance sports are each individually underrepresented in current literature. When these areas are examined collectively, a substantial gap in knowledge becomes evident. Therefore, the purpose of this PhD study was to collect glucose and energy metabolism data in regularly menstruating female athletes within the complex sporting environment of ultra-running. Initially, a narrative review of the current study methodologies in female performance research was completed. The narrative review aided in the identification of gaps in study design and validation methodologies within female performance research, put forth a suggestive framework for standardizing terminology, and provides a comprehensive list of options for a combined methods approach to menstrual tracking. Subsequently, a randomized, cross-over experimental design was performed with twelve experienced female ultra-runners (age 39 ± 6 y) acting as their own control across menstrual cycle phases during an ultra-marathon simulation. Participants completed a 3-hour fasted outdoor run (FASTED) followed by a one-hour treadmill run (TREAD) where three standardized oral glucose doses were provided, in the mid-follicular (FOL) and mid-luteal phases (LUT). Using a mixed linear model, the menstrual phase was identified as being statistically significant for differences in glucose measurements between CGM and capillary glucose during TREAD (p = 0.02) but not FASTED. Also, the CGM reported glucose levels an average of -0.43 mmol·L-1 (95% CI -0.86, -0.005) and -1.02 mmol·L-1 (95% CI -1.63, -0.42) lower in fasted and fed scenarios respectively when compared to capillary glucose. Median glucose was higher in the LUT (Capillary 0.25-0.29 mmol·L-1, CGM 0.38-0.4 mmol·L-1) both when fasted or supplemented with carbohydrate. Of note, this is the first study to show that glucose is on average higher in the mid-luteal phase as well as observe the avoidance of clinically significant non-diabetic hypoglycemia (< 3.0 mmol·L-1) during 3-hours of fasted running following an 8-hour fast among female ultra-runners. Additionally, higher individual responses of low blood sugar were observed in the follicular phase. Further, there were no significant differences in the respiratory exchange ratio (RER) between the FOL (0.805 ± 0.028) and LUT phases (0.809 ± .035); t (10) -0.401, p = 0.697 during comparable submaximal running intensity (57% ± 7.3% and 57.3% ±7.9% VO2MAX respectively). Post-hoc analysis revealed no significant main effect of RER on glucose levels.