Background For athletes, it is imperative that energy intake (EI) adequately addresses the total energy requirements inherent to their specific sport. Low energy availability (LEA), which is the failure to provide the body with adequate energy to sustain homeostasis, may potentially result in Relative Energy Deficiency in Sport (REDs). This is a syndrome that can have detrimental health consequences, including altered menstrual, bone and endocrine functions. There is limited understanding of the energy requirements, as well as the prevalence and health consequences of LEA and REDs among female football players. Further, several methodological limitations in measuring EA impede the validity of findings. Considering this, the objective of this thesis was to explore the concepts of energetics, energy availability (EA) and REDs among female football players, with additional focus on methodological approaches. Methods All four papers presented in this thesis are conducted on professional female football players from the Norwegian premier league (Toppserien) and first division. In paper I we assessed the measurement accuracy of commonly used tracking devices to estimate the energetic cost of exercise, a key factor in the calculation of EA. Paper II quantified the EI, energy expenditure, and EA. Dietary recalls, doubly labeled water and GPS measurements were used to provide a wide range of measures related to the energetics of female players. Paper III assessed the suitability of the commonly used Low Energy Availability in Females Questionnaire (LEAF-Q) by comparing the questionnaire scores against indicators associated with the female athlete triad (Triad) and LEA. In Paper IV, we assessed the prevalence of indicators associated with REDs and formulated a cumulative risk index, based on their strength of association with REDs, providing better risk information regarding the development of the syndrome. Results Paper I demonstrated that all examined tracking devices significantly underestimated the energetic cost of intermittent exercise, ranging between 10.7 - 20.6%. All devices revealed a systematic inverse relationship with increasing exercise intensity. Omitting excess post-exercise oxygen consumption from the calculation significantly improved the accuracy with underestimation between 3.1 - 7.2%. In paper II, the average total daily energy expenditure was 2918 ± 322 kcal (physical activity level of 2.0 ± 0.3). The average EI was 2274 ± 450 kcal. A significant difference was observed between EI on match vs rest days (p < 0.05), but not between match versus training or training vs rest days. Carbohydrate intake on both match and training days was below the recommended guidelines for large parts of the cohort. The prevalence of LEA was 36% and 23% on match and training days, respectively. Paper III examined the suitability of the LEAF-Q as an indicator of the Triad and LEA; however, the area under the curve (AUC) for the LEAF-Q showed poor performance (0.44 - 0.53) for all clinical markers, apart from detection of amenorrhea (AUC = 0.86). The questionnaire did not statistically differentiate players classified as at risk or not at risk based on broad indicators of LEA. In paper IV, the absence of any primary, secondary, or associated indicators ranged between 33-55%. However, 22% presented with clustered primary and secondary indicators, which was unrelated to player position. Amenorrhea was reported by 30% of the non-contraceptive users (n = 27). Amenorrheic players displayed a significantly greater number of cumulative indicators compared to eumenorrheic players (p < 0.05). Conclusion Current generation of tracking devices did not provide accurate estimates of the energetic cost of intermittent exercise, consequently undermining the validity of EA assessment using these devices. While the energetic requirements of female football players appear to be moderate, a substantial portion of the cohort did not meet the nutritional requirements, notably pertinent to carbohydrates. Furthermore, there was limited evidence of nutritional periodization. Despite a marked prevalence of LEA, the LEAF-Q was not able to reliably identify players at risk for developing of health detriments. Continued usage of the LEAF-Q to detecting LEA among female football players can not be recommended. Rather, methods to evaluate player health outcomes, such as identifying the presence of clinical indicators associated with REDs should be prioritized to provide better risk assessments for the individual athlete, as sole assessments EA seems to insufficiently reflect health risk among female football players.
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