Relative Energy Deficiency in Sport (RED-S) is a complex condition that has a range of health consequences involving, but not limited to, metabolic, immune, and reproductive function, and psychological health. Low energy availability (LEA) is the underlying cause of RED-S. LEA can be defined as limited dietary energy available for normal physiological and metabolic functions, after accounting for energy expended from exercise training. Quantitative and qualitative methodological approaches have been used to investigate LEA (Chapter 2). However, most research methods and findings on LEA are typically conducted and interpreted in isolation. LEA is a complex and integrated syndrome, yet the research that brings together the dialogue between quantitative and qualitative findings is limited. Therefore, the purpose of the thesis was to expand on RED-S by using a mixed-method approach and gain further understandings of the complexity of LEA (Chapter 3). The thesis is divided into seven chapters that incorporate published and unpublished manuscripts. A mixed-method approach was used within an elite athletic cohort to understand the nuances and the relationship between the quantitative physiological and qualitative socio-psychological aspects of LEA (Chapter 4). Data were collected at two time points (T₁ and T₂, respectively), from 15 (10 female, 5 male) members of the Cycling New Zealand (CNZ) elite endurance track cycling high-performance programme. Physiological data were captured from blood (hormonal and metabolic markers), metabolic testing, energy availability status, and bone health at both time points. Socio-psychological data were captured via semi-structured interviews from a sub-set of the cyclists, following the physiological data at T₁. The interviews covered topics in the context of RED-S such as menstruation, body image, nutrition, and injury/illness. These topics were chosen as they had been previously identified in the literature as key markers of LEA. Also, the interview covered topics related to the high-performance environment, such that relationships with team-mates and coaches, and performance pressures, could be explored. The thesis showcases results, firstly, using quantitative methodological approaches and presents two manuscripts (Chapter 5; 5i and 5ii). The first manuscript investigates the energy availability status and bone health in female (n=7) and male (n=4) elite track cyclists two time points (T₁ and T₂). It was found that the cohort had optimal bone health; having said that, 64% had LEA (<30 kcal/kg fat-free mass [FFM]/day) at least once during the cycling season. Improvement in energy availability (EA) status was associated with an increase in lumbar spine bone mineral density, yet it was clinically unmeaningful and requires further investigation. The second manuscript (5ii) is a case-series that investigated EA, resting metabolic rate, dietary protein, and testosterone concentration in four elite male track cyclists at T₁. The male athletes demonstrated having mid-range testosterone, lowered resting metabolic rate ratio, varied luteinizing hormone and sub-optimal EA (16.9 - 19.8 nmol/L, 0.76 – 0.98, 4-10 U/L, 26 - 41 kcal/kg FFM/day, range; respectively). It is suggested the male athletes may have within-day energy deficiency, putting them in a catabolic state. It is unknown if the observable higher dietary intakes of protein consumed may have prevented a further reduction in testosterone. Although presented second, the most important section of this thesis builds upon transcending methods; providing evidence for the advantages in implementing mixed-methods to gain greater insights of LEA in elite athletes. The evidence is presented by three manuscripts in Chapter 6 (6i, 6ii, and 6iii). The first manuscript (6i) brings quantitative and qualitative data sets together to explore the socio-cultural dimensions of eight female athletes’ experiences of LEA. In utilizing both data sets, the categorization of individuals with LEA and the relationships (or lack of) with body image, menstruation and nutritional practices, and the athletes’ experiences of LEA, were observed. This manuscript also revealed the challenges of interview athletes on sensitive topics such as LEA and the silences and deflection strategies among those with more severe cases of the condition. The second manuscript (6ii) continues the mixed-method theme by investigating the socio-psychological contributions to the classification of LEA in 15 (10 females, 5 males) elite track cycling athletes. The physiological data demonstrated that the athletes categorized with LEA had lower energy intakes s compared to those categorized with higher EA (>30 kcal/kg FFM/day). The reduced energy intake stemmed from a reduction in carbohydrate and fat. No other physiological differences were observed. The complexity of LEA continues to be apparent as the interviews revealed that nutritional practices of reducing carbohydrate and athlete body image perceptions were not dependent on the category of EA, i.e., regardless of EA severity, both LEA and HEA groups experienced similar pressures, perceptions of body image and nutritional practices. To extend on 6ii, the third manuscript (6iii) draws on examples from data that were collected as part of the thesis, as well as two multidisciplinary projects that followed similar mixed-methodologies however, involved elite female Ironman triathletes and elite female rugby players. The manuscript highlights the integrative and complex nature of LEA by using interview data that help to explain the results of the physiological data. Although the athletes within each sport were categorized by EA status, the difference between those categorized with LEA and higher EA, and their physiological data were not as clear. While each of the sports had a different sporting culture, the interviews revealed a range of socio-psychological factors that were impacting the player's risks of LEA. 6iii also demonstrates the differences between team and individual sports and how teammates, especially those in power positions, can mould and influence others within the team. In summary, the work included in this thesis recognizes the value of implementing a mixed-method approach to uncover the interactions and complexities of LEA in athletes, with track cyclists used as the primary example. The evidence presented firmly identifies the weaknesses in the current RED-S model and the limitations in the dominant methodological approaches. The findings allowed for a greater understanding of the nuances in complex socio-psycho-physiological conditions such as LEA. The thesis adds to the current body of knowledge by demonstrating the value of incorporating the dialogue between the quantitative and qualitative data sets. Furthermore, the thesis demonstrates the challenges in implementing mixed-methods. To further understand the interconnectedness and complexity of LEA and RED-S, future research should aim to incorporate mixed-methods.