Background The Vitamin D Assessment (ViDA) study is a randomised, double-blind, placebo-controlled trial to evaluate the efficacy of monthly vitamin D supplementation in reducing the incidence of a range of acute and chronic diseases and intermediate outcomes. Methods The study was carried out in Auckland, New Zealand, among 5110 adults, aged 50-84 years, who were followed for a median 3.3 years. The intervention was vitamin D3 (2.5 mg or 100,000 IU) or placebo softgel oral capsules, mailed monthly to participants' homes, with two capsules sent in the first mail-out post-randomisation (i.e. 200,000 IU bolus, or placebo), followed 1 month later (and thereafter monthly) with 100,000 IU vitamin D3 or placebo capsules. Outcomes were monitored through routinely collected health data and self-completed questionnaires. Results The results showed no beneficial effect of vitamin D supplementation on incidence of cardiovascular disease, falls, non-vertebral fractures and all cancer. However, beneficial effects from vitamin D supplementation were seen: for persistence with taking statins in participants on long-term statin therapy; and also in bone mineral density and arterial function in participants with low 25-hydroxyvitamin D levels, and in lung function among ever smokers (especially if vitamin D deficient). The latter findings are consistent with several previous studies, Conclusion Monthly high-dose vitamin D supplementation does not prevent a range of diseases, but may be beneficial for some intermediate outcomes in people who are vitamin D deficient.
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