Despite female athlete health becoming a growing area of interest in sport science reasearch, there are still gaps to be filled in the literature. Low energy availabilty (LEA) and iron deficiency are two prevalent issues in women (or female athletes) that are under high loads of exercise stress from training. Thus, it is critical that measurement and identification of these parameters is accurate. The first part of this thesis reviews the literature surrounding LEA and iron deficiency with a focus on current identification techniques used in research and clinical settings. The second part of this thesis includes two original investigations. The first study, Chapter 2, had two aims; 1) to investigate the test-retest reliabilty of an indirect calorimetry analyser (Parvo Medics TrueOne), and 2) compare measured RMR with three RMR-predictive (pRMR) equations in female athletes. To address the first aim, 12 recreationally-exercising women (mean ± SD; age 27.5 ± 12.3 y) performed two RMR assessments, on separate days, utilising the Parvo Medics TrueOne analyser. To address the second aim, 25 females (mean ± SD; age 30.1 ± 10.2 y) underwent an RMR assessment using the Parvo Medics TrueOne analyser, which was compared to three calculated pRMR equations (Harris-Benedict [H-B], Mifflin-St Jeor [M], World Health Organisation [WHO]). Test-retest reliability for the TrueOne analyser was deemed acceptable (CV = 5.3%, ICC = 0.92). The validity of pRMR when compared to measured RMR showed low levels of agreement in all 3 predictive equations (M: CV = 21.4%, TEE = 269 kcal•day⁻¹, r = 0.16, WHO: CV = 21.5%, TEE = 270 kcal•day⁻¹, r = 0.13 H-B: CV = 21.6%, TEE = 270 kcal•day⁻¹, r = 0.13). The second study, Chapter 3, was designed to investigate specific health parameters related to iron, resting metabolic rate and energy balance in a cohort of highly-trained women. Thirteen highly-trained female endurance athletes (mean ± SD; age 32 ± 7 y, training volume 18.5 ± 4.1 hrs) provided a blood test to assess iron markers (haemoglobin [Hb], haematocrit [Hct], C-reactive protein [Crp], serum iron, serum ferritin and transferrin). Additional measures included resting metabolic rate (RMR) assessment, dual-energy x-ray absorptiometry (DXA) scans, diet and training diaries and menstrual cycle tracking to identify any additional risk factors which may be associated with relative energy deficiency in sport. Analysis revealed that 46% (6/13) of this population had iron levels below optimal. Serum iron, serum ferritin and transferrin were all significantly decreased in the iron deficient (ID) group (p < 0.05). When compared to the non-iron deficient (non-ID) group haemoglobin (Hb) and haematocrit (Hct) were significantly lower in the ID group compared with the non-ID group (p < 0.05). The relationship between Crp and serum iron revealed a large negative correlation (r = -0.66), Crp and serum ferritin a very large negative correlation (r = -0.72) and transferrin presented a large correlation with Crp (r = 0.70). In conclusion, the outcomes of these studies demonstrate the need for greater stratification of risk thresholds for this (emerging) group of highly-trained, non-professional female athletes.