COVID-19 study status being reviewed on an ongoing basis.

The vitamin D status of older adults in residential care facilities in Northern Ireland (NI) is currently unknown. An observation study is currently being carried out by Ulster University and the Western Health & Social Care Trust (WHSCT) to provide data on the prevalence of vitamin-D deficiency among older adults within residential care facilities in NI. It is expected that some 40% of the cohort will be deficient in vitamin D and will require immediate replenishment of vitamin D status. Therefore, as a follow up to the observational study, an intervention study will be conducted, where participants identified as vitamin D deficient (25(OH)D) status <30nmol/L (cut-off currently used by the WHSCT will be re-contacted and provided with a vitamin D supplement. The aim of this study is to investigate the dose and duration of vitamin D supplementation that is required to replenish and maintain vitamin D status in older adults with vitamin D deficiency within residential care facilities. Participants will be randomly assigned to receive one of two supplementation regimes: 3200IU/day vitamin D3 or 20,000IU/week vitamin D3. Blood samples (no more than 20ml per appointment) will be taken at 4 different time points: week 0, week 2, week 4 and week 8. These two doses are routinely used in clinical practice for remedial supplementation of vitamin D and have been chosen based upon their availability from manufacturers of vitamin D supplements. Also, albeit these doses are currently routinely used to replenish vitamin D status little is known on how effective they are and how soon vitamin D status is optimised. Clinicians can only administer medication for the management of osteoporosis (e.g. bisphosphonates) once vitamin D status is optimised. Therefore, measurements will be taken every two weeks to establish response to the remedial supplementation. This project provides a unique opportunity to monitor the replenishment and maintenance of vitamin D status following remedial supplementation more regularly (at weeks 0,2,4,8) than would be done under standard National Health Service care. This does and duration of vitamin D required to replenish vitamin D deficiency in older adults is currently ambiguous within the literature and clinical care.

Inclusion Criteria:

• Currently residing (≥1 month) within residential care facilities in Northern Ireland and who have previously undertaken the observation study (full title: A cross-sectional analysis of vitamin D status and bone health of older adults within residential care facilities in Northern Ireland) and have been identified as vitamin D deficient.

Exclusion Criteria:

• Participants will be excluded from the study if by the judgement of the consultant rheumatologist that they have very abnormal bone or vitamin D metabolism due to conditions such as end stage renal failure or bone metastases.