Background: Past studies suggest that inadequate energy intake (EI) is directly related to menstrual dysfunction (MD) in active females. Inadequate EI causes activation of the hypothalamic-pituitary-adrenal axis, disrupting the normal hormonal signaling of reproductive cycling and resulting in MD. However, studies have also demonstrated similar EI in athletes, but with different menstrual function. Traditionally, energy balance has been evaluated in 24-hour time periods. Recent research suggests there is benefit to analyzing energy balance at smaller intervals to better address physiologic response in real time. It is possible that women who meet their daily EI needs could still, therefore, experience MD if the majority of the day is spent in energy deficit. Objective: The purpose of this study is to determine whether within-day energy balance is a factor in menstrual status in active adult females. Methods: Twenty active females tracked hourly EI and energy expenditure over three days. A two-page survey was utilized to obtain information on training, health, and menstrual status. Participants were also asked to respond to several questions regarding eating habits and attitude towards food. Within-day energy balance was calculated and evaluated using NutriTimingTM software. Collected energy balance data were analyzed for associations with menstrual status. Results: Participants spent more hours in a catabolic state (energy balance less than zero) than in an anabolic state (20.5 hrs vs 3.5 hrs) and averaged a caloric deficit of -504 kcal over 24-hours. Nine subjects (45%) had experienced loss of menses for greater than 3 months (LoM\u3e3mos), indicative of amenorrhea. Hours spent in energy surplus \u3e400 kcal was inversely correlated with LoM\u3e3mos (r = -0.463; P = 0.04). The impact of number of miles run per week and menstrual dysfunction, based on a quartile stratification of miles run, were analyzed using an ANOVA with Tamhane non-parametric post hoc test. Significant differences were found between the upper two quartiles of distance run per week and LoM\u3e3mos (P = 0.048). There was no relationship between end-of-day energy balance and Lom\u3e3mos. Conclusion: Researchers and healthcare professionals would do well to examine energy balance in an hour-by-hour manner as it is has implications for MD. This would help to clarify whether within-day energy balance is a factor in MD, and allow for the development of appropriate intervention strategies to improve health and athletic outcomes for active women
