Variable Energy Availability Exposure and Heterogeneous Physiological Responses to a Prescribed Low Energy Availability Intervention in Free-Living Male Endurance Athletes: A Pilot Case-Series

Low energy availability (LEA) is commonly studied under tightly controlled laboratory conditions. However, its induction and monitoring in free-living athletes remain poorly understood, partly due to challenges in measuring energy intake (EI), exercise energy expenditure (EEE) and daily behavioural variability. This study examined whether a prescribed LEA intervention leads to a consistent energy deficit and described the variability in energy availability (EA) and associated physiological and performance responses. In this pilot case series with a repeated-measures control-intervention structure, trained male endurance athletes (n = 3) completed a 14-day intervention targeting LEA (~20 kcal (~84 kJ)·kg fat-free mass (FFM)−1·day−1) by increasing EEE while maintaining habitual EI. EA was estimated daily using image-based dietary assessment, heart rate-derived EEE and dual-energy X-ray absorptiometry-derived FFM; resting metabolic rate (RMR), performance and well-being were assessed pre- and post-intervention. Despite the target, achieved EA varied substantially within and between participants (day-to-day SD 7–20 kcal·kg FFM−1·day−1), reflecting the magnitude and distribution of EEE rather than a uniform deficiency. Responses were heterogeneous: RMR showed no consistent direction, haemoglobin decreased modestly in all athletes, body-composition changes were small, and performance ranged from decline to improvement. Mean EA interpreted in isolation may obscure intra-individual variability, indicating a need for methods that capture it in free-living research.