The main objective of this study is to find whether supplemental vitamin d and calcium can reduce the incidence of stress fractures in recruits. These high-risk recruits undergo intensive training which elevated bone turnover, which requires adequate level of vitamin D and calcium in order to support bone health. It is hypothesized that supplemental vitamin d and calcium will decrease the occurrence of stress fractures.

Inclusion Criteria:

• Recruits newly enlisted at a training base in September 2021.
• Agree to accept relevant inspections and serological research, and sign an informed consent form.

Exclusion Criteria:

• Suffer from diabetes, osteomyelitis and other diseases.
• Suffer from claustrophobia and other MRI examination contraindications.
• Long-term use of glucocorticoids, non-steroidal anti-inflammatory drugs or calcium, vitamin D.
• A history of kidney disease.
• Allergic to any ingredient of the medicine.