This study aims to understand how vitamin D (VD) affects human health. Typically, prisoners are low on vitamin D, as it is difficult to receive through diet, and is mostly obtained via exposure to the sun. The investigators predict that VD supplements could help improve overall mental well-being, as well as improve bone health. The investigators aim to recruit two groups of participants from a United Kingdom (UK) Prison, all of whom will participate via an informed consent process. The first group of prisoners will have chosen to take VD supplements, the second group will have chosen not to take VD supplements. At the start of the study, prisoners will have their bone density and blood VD levels tested. The investigators will also ask participants to complete a series of questionnaires to understand the state of mental well-being at the start of the study. Participants will be asked to complete a food diary to track dietary intake over the following week. Additionally, the investigators are interested in identifying what proportion of participants have a specific genetic makeup relating to their ability to metabolise VD, and participants will be asked to provide a saliva sample to test this. Every month following the start of the study, participants will be asked to complete the same questionnaires and food diary again. On the 3rd month, the investigators will again test the participants' bone density and blood levels of VD, to see whether supplementation has improved participant VD status. This study will run for a minimum of 3 months, up to a maximum of 6.