Inclusion Criteria:  Female sex; Aged 18 to 36 years old; Maximal rate of oxygen uptake of ≥ 35 ml∙kg-1∙min-1; Currently either taking no hormonal contraceptives or the combined oral contraceptive pill; Weight stable (no change in self-reported body mass ≥ 5% over the previous 3 months); BMI between 18 and 30 kg∙m2; Not pregnant.  Exclusion Criteria:  Evidence of disordered eating (≥ 20 on the EAT-26); Habitual energy intake of < 35 kcal∙kg FFM∙d-1; Self-reported change in body mass of ≥ 5% over the previous 3 months; Vitamin D deficient (total 25(OH)D < 30 nmol∙L-1); Maximal rate of oxygen uptake of < 35 ml∙kg-1∙min-1; Total body BMD T-score of < -1; Evidence of menstrual disturbance (oligomenorrhoea: < 9 menstrual cycles in previous 12 months or amenorrhoea: ≤ 3 menstrual cycles in the previous 12 months); Pregnant; Current smoker, or stopped smoking within the last three months; Taking any medications known to affect bone or calcium metabolism (e.g. treatment for thyroid disorders); Self-declared history of heart, liver or kidney disease, diabetes or thyroid disorder; Self-reported stress fracture or any other bone injuries in the previous 12 months; Anaemia (haemoglobin <12 g∙/dL-1).