Vitamin D and Cognitive Decline
Vitamin D receptors are dense in the hippocampus and other memory-critical brain regions. Observational studies consistently link low 25(OH)D to accelerated cognitive decline and higher dementia risk. Randomised supplementation trials for cognition have been smaller and less definitive.
Cohort evidence
The largest analysis comes from the UK Biobank (~294,000 participants, Navale 2022): adults with 25(OH)D < 25 nmol/L (10 ng/mL) had 79% higher dementia risk than those with 50 nmol/L (20 ng/mL) or greater, after adjusting for age, sex, and lifestyle. A 2014 Neurology study (Littlejohns) similarly found deficient older adults developed Alzheimer's roughly 2× more often than those with sufficient levels.
Why the association
Vitamin D promotes amyloid-β clearance by macrophages, modulates neuroinflammation, supports hippocampal neurogenesis, and protects against oxidative stress. All of these pathways are implicated in Alzheimer's and vascular dementia. Whether supplementation can move the needle at scale remains uncertain.
Supplementation trials
RCTs in cognitively normal older adults have generally not shown improvement in global cognition or dementia incidence with 800–4,000 IU/day cholecalciferol over 1–5 years. The trials are limited by short duration (dementia develops over decades) and by including participants who were already sufficient at baseline. Trials in established mild cognitive impairment have shown small improvements in executive function but not memory.
Why the interpretation is difficult
Cognitive decline develops over 20+ years before symptoms. A 3-year RCT in older adults may be too short and too late. The cohort associations may reflect vitamin D as a marker of overall lifestyle (outdoor activity, physical fitness, better diet) rather than a causal factor. But because correcting deficiency is safe and inexpensive, most geriatricians favour keeping 25(OH)D ≥ 30 ng/mL in older adults.
Practical recommendation
- For adults ≥ 65: test 25(OH)D at least once, then supplement to maintain 30–50 ng/mL.
- Typical dose 800–2,000 IU/day cholecalciferol (see our elderly page for context).
- Don't rely on vitamin D as a dementia-prevention strategy — physical activity, social engagement, hearing correction, and controlling vascular risk factors have far stronger evidence.
- If diagnosed with cognitive impairment, correcting deficiency is a low-risk part of overall care.