Timing of Energy Availability on Menstrual Cycle Function

The primary aim of this clinical trial is to evaluate how fasted prolonged exercise may influence circulating reproductive hormones (i.e., estradiol, progesterone, luteinizing hormone) and menstrual cycle length when energy availability is maintained at or above 30 kcal/kg lean body mass/ day among women who regularly do running or cycling exercise. A secondary aim is to determine the effects of fasted prolonged exercise on serum leptin and cortisol.

Participants will be asked to do the following over a \~3 month enrollment period:

* attend a laboratory visit at the beginning of the study to have their resting metabolic rate, aerobic fitness, and body composition tested * monitor their menstrual cycle length, urine hormones, perceived stress levels, and diet for \~3 months * complete 3, 90-minute exercise sessions on a stationary bike or treadmill either fed (consuming a carbohydrate meal 1 h prior and 0.7 g carbohydrate/ kg body mass/ h during exercise) or fasted (consuming no meal prior and no carbohydrates during exercise) during month \~3 * saliva samples will be taken prior to and after 90-minute exercise sessions for quantification of leptin and cortisol