High-dose oral vitamin D supplementation and mortality in people aged 65–84 years: the VIDAL cluster feasibility RCT of open versus double-blind individual randomisation

Abstract

Background: Randomised controlled trials demonstrating improved longevity are needed to justify high-dose vitamin D supplementation for older populations. Objectives: To demonstrate the feasibility of a large trial (n ≈ 20,000) of high-dose vitamin D in people aged 65–84 years through general practitioner (GP) practices, and to cluster randomise participating practices between open-label and double-blind randomisation to compare effects on recruitment, compliance and contamination. Design: Twenty GP practices were randomised in matched pairs between open-label and double-blind allocation. Within each practice, patients were individually randomised to vitamin D or control (i.e. no treatment or placebo). Participants were invited to attend their GP practice to provide a blood sample and complete a lifestyle questionnaire at recruitment and again at 2 years. Randomisation by telephone followed receipt of a serum corrected calcium assay confirming eligibility ( 400 IU vitamin D per day at 2 years was 5.0% in open practices and 4.8% in double-blind practices. Mean serum 25(OH)D concentration was 51.5 nmol/l [95% confidence interval (CI) 50.2 to 52.8 nmol/l] with 82.6% of participants < 75 nmol/l at baseline. At 2 years, this increased to 109.6 nmol/l (95% CI 107.1 to 112.1 nmol/l) with 12.0% < 75 nmol/l in those allocated to vitamin D and was unaltered at 51.8 nmol/l (95% CI 49.8 to 53.8 nmol/l) in those allocated to no vitamin D (no treatment or placebo). Conclusions: A trial could recruit 20,000 participants aged 65–84 years through 200 GP practices over 2 years. Approximately 80% would be expected to adhere to allocated treatment (vitamin D or placebo) for 5 years. The trial could be conducted entirely by e-mail in participants aged < 80 years, but some participants aged 80–84 years would require postal follow-up. Recruitment and treatment compliance would be similar and contamination (self-administration of vitamin D) would be minimal, whether control participants are randomised openly to no treatment with no contact during the trial or randomised double-blind to placebo with monthly reminders. Trial registration: Current Controlled Trials ISRCTN46328341 and EudraCT database 2011-003699-34. Funding: This project was funded by the National Institute for Health Research Health Technology Assessment programme and will be published in full in Health Technology Assessment; Vol. 24, No. 10. See the NIHR Journals Library website for further project information.

Keywords

• vitamin d
• gp practice
• feasibility
• cluster randomisation
• randomised trial