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Observational studies have consistently found a link between low serum 25‐hydroxyvitamin D concentration and higher risk of cognitive impairment. Results from randomized controlled trials have been mixed, and few have been conducted in the general population.

We recruited 21,315 community‐dwelling Australians aged between 60 and 84 years to participate in the D‐Health Trial, a randomized, double‐blind, placebo‐controlled trial. The intervention was monthly oral doses of 60,000 international units of vitamin D or placebo for 5 years. We assessed cognitive function in a randomly sampled group of participants aged ≥70 years using the Telephone Interview for Cognitive Status (TICS) at 2 and 5 years after randomization. The primary outcome for this analysis was TICS score; the secondary outcome was the proportion of people who had cognitive impairment (defined as TICS …