Energy Availability And Salivary Sex Steroids Across One Menstrual Cycle In Physically Active Females

Energy availability (EA) describes the amount of energy available for metabolic functions after exercise energy expenditure (EEE). When a negative balance emerges between energy intake and EEE over time, an energy conservation state is created where metabolic fuels are focused on life sustaining metabolic processes and are shifted from other systems. Low EA disrupts gonadotropin-releasing hormone pulsatility and the subsequent release of hormones from the pituitary and ovaries, leading to a reduction of estrogen (E2) and progesterone (P). PURPOSE: To examine the causal influence between EA with free E2 and P across two timepoints in one menstrual cycle (MC). METHODS: 35 MC were assessed in naturally cycling females (n = 20; age = 21 ± 3). Seven days of energy intake (EI), exercise, and saliva were collected for one (n = 5) or two MC (n = 15) at two timepoints (i.e., T1, T2) per MC. T1 started 2-4 days after menses began and T2 started 5-8 days post-ovulation or at a predetermined date if no positive ovulation test. Salivary free E2 and P were quantified with 7-day area under the curve. Exercise was recorded with a heart rate monitor and heart rate was used to calculate EEE. EI was self-logged with a phone app. EA = EI – EEE / FFM. A cross-lagged model separately assessed E2 and P with the mean EA. RESULTS: The cross-lagged models with P and EA in T1 and T2 demonstrated excellent model fit, χ2(0) = .00, p = .00, CFI = 1.00, RMSEA = .00 [CI .00, .00], SRMR = .00 and the cross-lagged model improved fit over the baseline model Δχ2(5) = 69.94,p = .00. EA and P both showed high stability across the MC but did not have any significant within-time correlations. The cross-lagged paths revealed that lower EA at T1 was associated with lower P at T2 (β = 0.26, p = .007). No significant relationships emerged between EA and E2. CONCLUSION: These findings provide important context to the relationship between P with EA across the MC. Although our results should be interpreted with caution due to the small sample size, our data suggests that lower EA in the follicular phase is associated with lower P in the latter half of the MC and EA may have potential time-lagged influences on P. The opinions or assertions contained herein are the private views of the author(s) and are not to be construed as official or as reflecting the views of the Army, the Department of Defense, or the U.S. Government.