Inclusion Criteria:  Healthy endurance trained runners. Total weekly running distance of greater than 50km/week and perform endurance exercise for more than 5 hours/week. Age 18-35 years old Eumenorrheic over last 3 months (one cycle no longer than 42 days and no shorter than 20 days). Not using hormonal contraceptives (the pill, implants and injections, hormonal intrauterine devices (IUDs), vaginal ring, skin patch). Do not have polycystic ovarian syndrome (PCOS) or other conditions effecting ovarian hormone regularity. Not pregnant within the past year. Must meet the classification of 'superior' aerobic power for their respective age category based on the ACSM criteria; (women age 20-29 years VO2max >50ml/kg/min and women age 30-35 years VO2max >46ml/kg/min) (men age 20-29 years VO2max >56ml/kg/min and men age 30-35 years VO2max >54ml/kg/min).  Exclusion Criteria:  Inability to meet health and physical activity guidelines according to the CSEP 'Get Active' Questionnaire (Appendix G). Inability to adhere to study protocol requirements relating to diet and exercise (i.e. alcohol and caffeine consumption, training/exercise prior to beginning a study testing day). Regular use of tobacco and/or other recreational drugs/banned substances (i.e. growth hormone, testosterone, marijuana). Self-reported intentional or unintentional weight loss (past 6 months) or plan to lose weight (>5-10kg). Amenorrhea (self-reported) in the past 3 months. Fewer than 6 menstrual cycles per year. Answering yes to any questions asked within the COVID-19 screening questionnaire until 14 days of being symptom free and/or having a negative COVID-19 test