Summary: Background and Aims: Low energy availability (LEA) underpins Relative Energy Deficiency in Sport (REDs) - a multisystemic syndrome impacting athletic performance and overall health. Despite its prevalence, research examining clinical REDs indicators in female runners is limited. This study investigated the relationship between REDs screening, BIA assessment of malnutrition and ovulatory status in female endurance runners using an Energy Availability Questionnaire, Bioelectrical Impedance Assessment (BIA) and female reproductive sex hormone levels. Methods: 70 female runners from London, UK running clubs participated in this study. The median age of the participants was 32 years old (range 18–55 years old), with mean ± SD BMI 21.2±2.4 kg/m2. Participants ≥35 years were classified as master's athletes (MA), and those <35 years were classified as younger athletes (YA). All participants answered the Energy Availability Questionnaire, and a scoring system was used to assess REDs risk. 56 from this group underwent anthropometric measurements using multifrequency BIA, and 20 subsequently underwent finger prick capillary blood test to evaluate oestrogen, progesterone, follicle-stimulating hormone, luteinising hormone, and testosterone levels 7 days before their predicted period. Two-way t-test compared REDs-RS of different fat-free mass index (FFMI) percentiles. Linear regression examined associations between BIA anthropometric measurements, hormones, and REDs scores. One-tailed and two-tailed t-tests compared REDs scores and BIA metrics between ovulatory vs. anovulatory participants, respectively. Results: 64% of YA and 29% of MA were identified as being at significant risk of REDs (REDs-RS ≤ 0). BIA measurements indicated that 19% of YA and 50% of MA had FFMI below the ESPEN clinical malnutrition cut-off point of 15 kg/m2 with a mean FFMI of 15.5± 1.38 kg/m2 for YA and MA. Overall, 82% of the participants had FFMI below the suggested 20th percentile for female athletes in weight-sensitive sports. In YA, REDs risk was significantly increased in FFMI ≤5th percentile and associated with lower fat mass index (5.16±1.57 kg/m2). BIA phase angle (PA) was not significantly different between YA (6.4±0.6°) and MA (6.3±0.6°). Examination of ovulation status revealed that REDs-RS values were significantly greater, thus implicating lower REDs risk, in ovulatory (3.5 ± 3.9) vs anovulatory (-0.7 ± 5.1) participants. Conclusions: REDs is prevalent in female runners, regardless of age. BIA indicators of malnutrition, including FFMI, FMI, PA, may function as useful and novel clinical indicators of REDs in female runners. Low FFMI was highly prevalent in MA runners. REDs risk in female runners is closely related to ovulation status and may be quantified using the Dancers Energy Availability Questionnaire (DEAQ) framework, modified for use for female runners.