This randomised trial will investigate the effect of calcium supplementation on bone and calcium metabolism in women during acute load carriage exercise. This study will test the hypothesis that calcium supplementation before load carriage exercise will attenuate the decline in serum ionised calcium and increase in parathyroid hormone and bone resorption.

Endurance exercise decreases serum ionised calcium and increases parathyroid hormone and bone resorption. These disturbances to calcium homeostasis can be attenuated by calcium supplementation before exercise. Military field exercises are characterised by high exercising energy expenditures, restricted dietary intake, and prolonged periods of load carriage. These exercise and nutritional challenges can result in impaired bone turnover, a negative calcium balance, and an increased risk of stress fracture. It is unknown if military load carriage disturbs calcium homeostasis and whether calcium supplementation before exercise can attenuate this disturbance. This crossover randomised controlled trial will investigate the effect of acute calcium supplementation on bone and calcium metabolism in women during acute load carriage exercise. Each participant will complete two 2 h load carriage trials. The trials will be performed after either consuming 1000 mg of calcium one hour before or with no treatment. During each experimental trial blood and urine will be sampled for markers of bone and calcium metabolism.

Primary Outcome: The absolute change in urine calcium balance (Ca44:Ca42) between pre- and post-load carriage will be compared between the non-supplemented (Control) and calcium supplemented trial (Supplement) using one-way ANCOVAs with pre-load carriage calcium balance (either as a ratio, or both the numerator and denominator) as the covariate, or a linear mixed model with the restricted maximum likelihood estimation to allow incorporation of incomplete data.

Secondary Outcomes: Circulating measures of bone turnover and calcium metabolism during load carriage will be compared between Control and Supplement using linear mixed models with the restricted maximum likelihood estimation to allow incorporation of incomplete data.

Inclusion Criteria:

1. Female sex;
2. Aged 18 to 36 years old;
3. Maximal rate of oxygen uptake of ≥ 35 ml∙kg-1∙min-1;
4. Weight stable (no change in self-reported body mass ≥ 5% over the previous 3 months);
5. BMI between 18 and 30 kg∙m2;
6. Not pregnant.

Exclusion Criteria:

1. Evidence of disordered eating (≥ 20 on the EAT-26);
2. Self-reported change in body mass of ≥ 5% over the previous 3 months;
3. Vitamin D deficient (total 25(OH)D < 30 nmol∙L-1);
4. Maximal rate of oxygen uptake of < 35 ml∙kg-1∙min-1;
5. Total body BMD T-score of < -1;
6. Evidence of menstrual disturbance (oligomenorrhoea: < 9 menstrual cycles in previous 12 months or amenorrhoea: ≤ 3 menstrual cycles in the previous 12 months);
7. Pregnant;
8. Current smoker, or stopped smoking within the last three months;
9. Taking any medications known to affect bone or calcium metabolism (e.g. treatment for thyroid disorders);
10. Self-declared history of heart, liver or kidney disease, diabetes or thyroid disorder;
11. Self-reported stress fracture or any other bone injuries in the previous 12 months;
12. Anaemia (haemoglobin <12 g∙/dL-1).
13. Currently using the implant, injection, or the progesterone-only pill ('mini pill').

Data will be owned by UK Ministry of Defence, not the Principal Investigator. Data will be made available where possible by the Principal Investigator assuming the appropriate approvals are granted by the UK Ministry of Defence.

Calcium supplements will be provided open-label.