Energy balance parameters (dietary intake and energy expenditure) are influenced by gonadal hormones, such as estradiol and progesterone that fluctuate across the menstrual cycle in pre-menopausal women. Dietary intake varies across the menstrual cycle, with higher self-reported energy intake in the luteal phase. However, it is unclear how physiological and behavioral parameters such as appetite, resting metabolic rate, body composition, physical activity, or premenstrual symptoms relate to energy intake fluctuations. Furthermore, only two studies have measured free-living objective dietary intake across the menstrual cycle, neither of which have measured other aspects of energy balance in detail. This study will help address these knowledge gaps by characterizing several aspects of energy balance in both laboratory and free-living settings in healthy pre-menstrual women across the menstrual cycle. Twenty-three women (for complete data on n=18) aged 18-35 without major comorbidities and with normal menstrual cycles will be enrolled in an observational trial. In addition to a baseline visit to measure body composition, participants will undergo two 4.5-hour study visits, one of which will occur in the follicular phase and the other in the luteal phase (as confirmed with multiple low-burden methods including self-reported menstrual cycle history and urinary luteinizing hormone). Participants will be given a two-day run-in diet prior to each study day to ensure energy balance. On each study day visit, participants will undergo a resting metabolic rate test followed by a series of fasting saliva samples quantitatively pooled to measure estradiol and progesterone. Following collection of fasted ratings of appetite, participants will be provided with a standard breakfast meal. Appetite ratings will be repeated 30, 60, 90, 120, 150, and 180 minutes after the meal. Questionnaires related to eating behavior will also be completed during this time. After the 180-minute timepoint, participants will be provided with a buffet-like lunch meal with instructions to eat as much or as little as they would like; this will allow measurement of ad libitum dietary intake at a single meal. To assess free-living ad libitum dietary intake, participants will receive three days of food boxes tailored to their preferences, with uneaten food returned at the end of the three-day period. After each study visit, participants will be asked to record their dietary intake and appetite (three days) and wear and activity monitor (seven days). Differences in outcomes between visits will be assessed using paired samples t-tests and relationships among variables will be assessed via Pearson correlations. This study will provide preliminary evidence of the factors associated with dietary intake fluctuations across the menstrual cycle and provide essential preliminary data that could inform future weight management and health-promotion strategies.
