The real-world evidence on the association between vitamin D supplementation and cognitive outcomes has been scant and controversial. We aimed to investigate the longitudinal association between vitamin D supplement use and subsequent cognitive functioning among US older adults.

Prospective cohort study.

A nationally representative ageing cohort in USA: the Health and Retirement Study (HRS).

Participants were drawn from the HRS wave 12 and included respondents who had complete data on dietary supplement use and cognitive assessment. A total of 5065 participants (mean age: 67.5±10.2 years, 61.6% female, 76.6% White ethnicity) were included, of whom 2004 (39.6%) participants were vitamin D supplement users.

Change in cognitive function scores over 6 years of follow-up (from HRS waves 12–15), estimated by linear mixed model adjusted for multiple covariates.

Compared with non-users, vitamin D users had an accelerated decline in global cognitive function (difference in the rate of change: –0.052 points/year; 95% CI –0.092 to –0.013, p=0.010) and in executive function score (difference: –0.021 points/year; 95% CI –0.037 to –0.005, p=0.010). Sensitivity analysis suggested that accelerated cognitive decline was only observed among supplement users with normal baseline serum 25(OH)D level (p=0.004), but not the group with insufficient/deficient levels (p=0.826).

Our findings do not support vitamin D supplementation as a means of preventing or slowing cognitive decline in older people with adequate vitamin D status. While healthcare providers should encourage adequate vitamin D intake from dietary sources and moderate sun exposure, caution should be taken when recommending such supplements to older adults without a clear indication for it.