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

Randomised controlled trials demonstrating improved longevity are needed to justify high-dose vitamin D supplementation for older populations. To demonstrate the feasibility of a large trial ( ≈ 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. 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