Vitamin D and Dementia
Cohort studies consistently link low serum 25(OH)D with increased dementia risk over follow-up. Mendelian randomisation studies partly support causation. Large randomised supplementation trials have not clearly shown prevention. The evidence justifies correcting deficiency in older adults but not high-dose supplementation for cognitive protection.
Observational data
Littlejohns 2014 (Neurology) followed 1,658 older adults for 6 years — severely deficient participants (25(OH)D < 25 nmol/L) had ~120% higher risk of any dementia and ~120% higher Alzheimer's risk than sufficient participants. Multiple similar cohort findings across Europe and North America.
Mendelian randomisation
Larsson 2018 and 2023 MR studies find genetically-lowered 25(OH)D is associated with higher dementia risk — supporting causal interpretation. Effect sizes are modest.
Supplementation trials
- VITAL cognitive substudy (2020) — 3,424 older adults; 2,000 IU/day for 3 years; no cognitive benefit vs placebo overall.
- D-Health cognitive substudy — pending final results but interim analyses show no meaningful cognitive impact of 60,000 IU monthly in older Australian adults.
- Ghahremani 2023 — retrospective analysis of NACC dataset: vitamin D users showed lower dementia incidence, but observational confounds are hard to rule out.
Reconciling the discrepancy
Observational and MR studies suggest chronic lifelong vitamin D adequacy may reduce dementia risk. Short-term supplementation trials in late life may miss a benefit that acts across decades. Alternatively, the observational association may reflect residual confounding (health-conscious individuals have both higher vitamin D and lower dementia risk from many other factors).
Practical guide
- Older adults (65+): 1,000-2,000 IU/day cholecalciferol maintenance is safe and reasonable — the general benefits (bones, falls, muscle) are established.
- Don't expect dementia prevention from supplementation started late in life on top of existing sufficiency.
- Evidence-based dementia prevention: cardiovascular risk management, physical activity, cognitive/social engagement, adequate sleep, hearing-aid use if hearing loss.
- If you have concerns about cognitive decline: prompt medical evaluation, not just supplement adjustment.