Female endurance runners are at high risk for low energy availability (LEA). Preliminary evidence suggests that fasting blood glucose (BG) concentration decreases in response to short-term energy deficiency (low energy availability). BG monitoring tools, such as continuous glucose monitors (CGM), could be used as an early detection device for LEA risk in athletes. PURPOSE: Characterize changes in BG over the course of five days in experimentally-induced LEA in female runners. METHODS: Recreational runners completed a 5-day experimental condition of LEA (15 kcal/kg FFM/d) achieved by a combination of dietary restriction and treadmill running at 65% VO2max. BG concentration was monitored every 15 min across the five experimental days using a CGM (Freestyle Libre Pro, Abbott), which assesses BG from interstitial glucose concentration. Glucose data from the 5-day condition were analyzed to obtain average 24- h BG, fasting BG, BG during sleep, "time in target" BG range (TIT), "time above target" range (TAT), "time below target" range (TBT), as well as glycemic variability using MAGE (mean of amplitude of glucose excursions) and MODD (mean of daily differences). 70 to 120 mg/dL was set as the target range. Data was analyzed using repeated measures analysis of variance; post hoc comparisons were performed using paired t-tests. RESULTS: Contrary to our hypothesis, our results suggest that five days of experimentally-induced LEA in female runners progressively increased 24- h BG and TIT while simultaneously altering patterns of fasting and sleeping BG. Average glucose concentration and TIT significantly increased from day one to day five (P=0.024 and P=0.03, respectively). Fasting and sleeping BG followed the same trends and significantly decreased from day one to day three (P=0.04 and P=0.002, respectively), followed by an increase by day five that was similar to day one. There was not a significant time-effect for MAGE, MODD, TBT, TBT, and glycemic variability (P>0.05). These changes are likely due to alterations in glucose production versus utilization that are driven by decreased insulin and/or increased BG counterregulatory hormones. This study is the first to characterize glycemia during short-term experimentally-induced LEA in female endurance runners using a CGM; emphasizing the potential ability of CGMs to gain insight on BG patterns during conditions of LEA. / Master of Science / Female long-distance runners are at high risk for a lack of proper fueling due to inadequate energy intake compared to the volume they exercise (e.g. low energy availability). Previous studies have found that fasting blood sugar (BS) concentration decreases in response to a short-term bout of low energy availability. BS monitoring tools, such as continuous glucose monitors (CGM), could be used as a tool to detect this risk in athletes. PURPOSE: Characterize changes in BS over the course of five days in experimentally induced low energy availability in female runners. METHODS: Recreational runners completed a 5-d experimental condition of low energy (15 kcal/kg FFM/d) achieved by a combination of dietary restriction and daily treadmill running at 65% of their maximum performance capacity (e.g. VO2max) BS concentration was monitored every 15 min across the five experimental days using a CGM (Freestyle Libre Pro, Abbott). Glucose data from the 5-d condition were analyzed to obtain average 24- h BS, average BS during sleep, fasting BS, "time in target" (TIT), "time above target" (TAT), "time below target" (TBT), the glycemic variability (GV) as percent coefficient of variation, and swings in BS levels (e.g. glycemic variability) using calculations for mean of amplitude of glucose excursions (MAGE), mean of daily differences (MODD). Data was analyzed using repeated measures analysis of variance; post hoc comparisons were performed using paired t-tests. Data was summarized as a mean ± standard deviation. The significance level was set a priori at P<0.05. All statistical analysis was conducted using IBM® SPSS® Statistical software (Version 28.0.2.2, IBM Corporation, NY, USA). RESULTS: Our results suggest that five days of experimentally-induced LEA in female runners progressively increases TIT and 24- h BS while simultaneously altering patterns of fasting and sleeping BS. Average glucose concentration and TIT significantly increased from day one to day five (P=0.025 and P=0.03, respectively). Fasting and sleeping BS significantly decreased from day one to day three (P=0.024 and P=0.002, respectively) and had the same trends. The concentrations of both fell from day one to day three, followed by an increase by day five that were like those of day one. Additionally, there was not a significant time-effect for MAGE, MODD, TBT, TBT, and glycemic variability all had an insignificant time-effect (P>0.05). These changes are likely due to alterations in glucose production compared to glucose use that are driven by changes in the hormones that regulate blood sugar. This study is the first to characterize BS changes during short-term experimentally-induced LEA in female endurance runners using a CGM; emphasizing the potential usefulness of CGMs to gain further insight on BS patterns during conditions of LEA.