Low Energy Availability, Disordered Eating Behaviours, Menstrual Function and Exercise in Endurance-trained Women
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The concept of energy availability (EA) recognises that dietary energy expended in one physiological process is not available for supporting others. When EA is low, adaptive mechanisms occur to draw energy away from non-essential physiological processes and towards those imperative for sustaining life. In exercising women, low EA directly suppresses the hypothalamic-pituitary-ovarian axis and menstrual disturbances develop. The combination of low EA and the hypoestrogenic state associated with energy-related menstrual disturbances pose significant risks to bone and cardiovascular health. Disordered eating behaviours (DEB) are a core feature in the development of low EA and, DEB, low EA and menstrual disturbances are commonly observed in endurance-trained women. However, little is known about the diversity and scale of DEB in endurance-trained New Zealand women. Furthermore, individual susceptibility to the response of low EA on menstrual function in exercising women also appears to exist. This is confusing as currently a specific threshold of EA >30kcal·kgFFM·d-1 is recommended to prevent the occurrence of menstrual disturbances in exercising women who aspire to lose weight. In addition, despite the high prevalence and known health considerations, a paucity of literature exists on the direct effect of low EA on exercise training and sport performance in endurance-trained women. Therefore, this thesis sought to extend our understanding of low EA in endurance-trained women via three different investigations which aimed to: 1) Classify clusters of variables associated with DEB in endurance-trained New Zealand women in order to determine the nature and complexity of DEB, and thus risk for low EA, specific to this cohort; 2a) Compare the effect of EA equal to 25kcal·kgFFM·d-1 for a full menstrual cycle on metabolic and menstrual function in endurance-trained women to ascertain if the current recommendation that a threshold EA >30kcal·kgFFM·d-1 is necessary to prevent menstrual disturbances remains appropriate; 2b) Determine recovery of metabolic and menstrual function by ad libitum dietary intake following EA equal to 25kcal·kgFFM·d-1 for a full menstrual cycle in endurance-trained women and; 3) Determine the effect of EA equal to 15kcal·kgFFM·d-1 for 7-days and ad libitum re-feeding on markers of metabolism, exercise training and menstrual function in an elite female endurance athlete to determine risks and benefits of acute and severe periods of low EA on endurance training, adaptations and health. Usual a novel methodological approach, Study 1 (Chapter 3) classified clusters of variables associated with DEB in a large sample (N=698) of endurance-trained New Zealand women. Four diverse clusters of DEB, independent of age and race pace, emerged. The key findings that endurance-trained women report body dissatisfaction and dietary restraint, both with and without a high drive for thinness, may be used to facilitate screening procedures and identify all types of DEB and subsequent risk for low EA in this cohort. Study 2 (Chapter 4) compared the effect of EA equal to 25kcal·kgFFM·d-1 for a full menstrual cycle, and resumption of ad libitum dietary intake, on metabolic and menstrual function in eumenorrheic endurance-trained women (N=3) in a field setting. Individual responses were apparent, as a direct result of the intervention, with subtle menstrual disturbances developing in two participants and apparent maintenance of a eumenorrheic cycle in the remaining participant. These findings provide evidence for a revision of the current guidelines for exercising women to maintain EA >30kcal·kgFFM·d-1 during weight-loss to ensure menstrual function is not compromised. Variability in restoration of menstrual function also occurred following the intervention with a subtle menstrual disturbance persisting in one participant while eumenorrheic menstrual cycles were observed the remaining two participants. Study 3 (Chapter 5) determined the effect of 7-days EA equal to 15kcal·kgFFM·d-1 and ad libitum re-feeding on markers of metabolism, exercise training and menstrual function in an elite female endurance athlete. The intervention compromised measures of cycling training quality, metabolism and menstrual function to the point that a severe menstrual disturbance developed. Conversely, 4-10 days of ad libitum feeding following the intervention was associated with an improvement in peak aerobic power and peak and mean repetition threshold interval training power at levels higher than their CV%. Possible mechanisms by which EA influenced exercise training in this study include changes in body and fat mass, power-weight ratio and thyroid hormone effects on cardiovascular and neuromuscular function. The observed risks and benefits of this short and severe period of low EA require careful consideration with further studies required in relation to quantifying effects on training adaptation, sport performance and athlete health. In summary, this thesis suggests that: 1) DEB, and subsequent risk for low EA, have diverse and complex presentations that should be acknowledged in screening procedures, 2) Variability in the direct effect of, and recovery from, EA equal to 25kcal·kgFFM·d-1 on menstrual function in endurance-trained women exists and, 3) EA equal to 15kcal·kgFFM·d-1 negatively affected exercise training and severely compromised menstrual function in an elite female endurance athlete but a short period of ad libitum re-feeding improved exercise measures above baseline levels.