The main goal of this study is to examine how sex hormones and low energy availability affect the concentrations of bone remodelling markers at rest and after a running protocol. The research questions to be addressed are:

1: Does the ovarian hormone profile influence serum bone marker concentrations in women exercising resting and in response to to exercise?

2. Are there differences in response to low energy availability in bone marker concentrations between eumenorrhoeic female athletes and oral contraceptive users?

3: Is the response of bone markers affected by different exercise stimuli: endurance or aerobic endurance?

4: Do bone markers return to baseline levels after 24 hours of high energy availability diet after 6 days of low energy availability?

To achieve the aim of the study, there will be two experimental groups - e.g. eumenorrhoeic females and oral contraceptive users - who, after screening, will be randomly assigned to a low- or high-energy diet. After 6 days on this diet, the participant will undergo an intervallic running protocol and blood samples will be taken before and after the test. The following month the participant will do the other type of diet.

Inclusion Criteria:

• Be healthy and between 18 and 30 years old.
• Engage in endurance sports that involve running (such as athletics, running, triathlon, trail running, etc.) for 3 to 12 hours per week..
• Eumenorrheic females with regular menstruation at least 6 months prior to the study.
• Monophasic oral contraceptive users at least 6 months prior to the study.

Exclusion Criteria:

• Amenorrheic females.
• Pregnant females.
• Affected by any disorder related to the hypothalamo-pituitary-ovarian axis.
• Having suffered a bone fracture in the year before the study.
• Taking any medication or supplements that affect bone metabolism.