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Abstract Background Preserved cognitive health with ageing is a public health imperative. Vitamin D deficiency is associated with poor cognition, but it is unclear whether supplementation would provide benefit, particularly in individuals with mild/moderate deficiencies which do not have other clinical risks. The objective of this study was to establish the impact of daily vitamin D supplementation on cognition in older adults with mild to moderate vitamin D deficiency. Methods and Findings Two-arm parallel 24-month randomised controlled trial, with Vitamin D supplementation compared with a placebo. This was a remote trial, completed from home involving 620 adults 50 years or older with mild to moderate vitamin D deficiency and early cognitive impairment. The primary outcome was executive function measured through Trail making B and other secondary measures of cognition, function and wellbeing. Vitamin D supplementation conferred no significant benefit to executive function compared to placebo at follow-up on the primary outcome (between-group difference: 5770, 95% CI: -2189 to 13730) or cognition, function, or wellbeing. Secondary analyses in defined subgroups and a per-protocol analysis also showed no significant impact on any outcome measures. Conclusions Vitamin D supplementation produced no measurable improvement in cognitive outcomes in older adults with mild to moderate vitamin D deficiency. The remote trial methodology provides an innovative approach to large-scale trials. Trial Registration ISRCTN79265514 https://www.isrctn.com/ISRCTN79265514
Corbett et al. (Fri,) studied this question.