Patients will be asked to take part in this study if they are aged 16 years or older and come to the Royal London Hospital with a broken bone in the arm or leg or collar bone (clavicle). They will be asked to give a 20ml (about 4 teaspoons) blood sample on three occasions: at the start of the study, at 5-14 days after the fracture; and at 4-10 weeks after the fracture. The rest of their normal care would not be affected. Follow-up blood samples will be timed to coincide with routine clinic visits which are scheduled as part of normal clinical care. Patients' vitamin D level will be measured as part of the study, and those who have a low vitamin D level will be informed about this, and advised about appropriate vitamin D supplementation. The investigators will let the patient or their GP know if any blood tests are abnormal