Unravelling Energy Issues Underpinning Low Energy Availability in High Performance Athletes

The goal of this clinical trial is to understand how the body responds to short-term, severe low energy availability (LEA) in healthy, weight-bearing endurance athletes aged 18-45 years old. LEA describes a mismatch between an individual's dietary energy intake and the energy cost of their commitments for training and competition. The main questions this trial aims to answer are: 1. What effect does short-term, severe LEA have on sleeping metabolic rate? 2. What effect does short-term, severe LEA have on other body systems identified within the Relative Energy Deficiency in Sport (REDs) Health and Performance Conceptual models? Researchers will compare a control trial with both a LEA trial achieved through diet restriction and a LEA trial achieved through increased exercise to see if there are differences in the body's response. Participants will complete three 6-day trials, a minimum of 3-weeks apart, involving: * Prescribed diet (all food provided) * Prescribed running and/or cycling exercise * Two visits to ACU Fitzroy campus for blood tests and exercise testing * 50 hour stay (two nights and two days) in the ACU metabolic chamber

Inclusion Criteria: * 18-45 years old * Well trained weight-bearing athlete (tier 2-4 1) * Perform \~45km per week of weight-bearing endurance training, and able to undertake the prescribed exercise in each trial condition * Pass the ESSA pre-exercise screening tool and/or obtain GP clearance to exercise Exclusion Criteria: * Assessment of red status on the REDs Clinical Assessment Tool * Unable to attend ACU in Fitzroy, Victoria, for the 6 study visits for completion of the study protocol * Pregnancy * Use of hormonal contraceptive with the previous 3 months * Onset of peri/menopause