Vitamin D and Alzheimer's/Parkinson's: what 60 studies do and do not show - review Nov 2026

The Impact of Vitamin D Deficiency on the Development and Progression of Alzheimer's and Parkinson's Diseases: A Narrative Review

Cureus 18(11): e119125, November 4, 2026, https://doi.org/10.7759/cureus.119125

Aleksandra Katarzyna Klosowicz, Aleksandra Olga Pakulska, Marcin Marek Dziubiela, Aleksandra Maria Bloch, Natalia Rzadzinska, Sara Paulina Szczepaniak

Summary by Claude Opus 5.5 - November 2026

Takeaway: People with Alzheimer's (AD) or Parkinson's (PD) usually have lower vitamin D, and low vitamin D predicts later dementia. But randomized trials so far have not shown that supplements prevent or slow either disease.

This is a narrative review of 60 studies (PubMed only, 2010–2026, no quality grading) by six physicians in Bydgoszcz, Poland. It adds no new data.

Alzheimer's

  • Two UK Biobank cohorts: supplement users had lower AD risk; dementia risk was highest below 25 nmol/L (10 ng/mL) vs. 50–75.9 nmol/L (20–30 ng/mL); raising everyone to 50 nmol/L was estimated to prevent 17% of dementia cases
  • RCTs found no cognitive benefit: VITAL (3,400+ adults aged ≥60, 2,000 IU/day D3, 2.8 years), VitaMIND (620 adults >50 with mild-to-moderate deficiency, 24 months), and a 5-year Finnish trial in people with normal baseline levels
  • The review rightly notes that the 2022 paper claiming vitamin D "worsens Alzheimer's progression" actually studied calcitriol prescriptions in Taiwanese patients with kidney or parathyroid disease, as Vieth 2022 and Gombart 2023 pointed out

Parkinson's

  • Meta-analysis (63 studies, ~10,700 people): PD patients had significantly lower vitamin D; supplementation gave only a small, non-significant motor improvement
  • Small, short trials (3 months of D3 in deficient patients; 12 weeks of BMI-based D3) reported better motor function or physical performance
  • Mendelian randomization found no causal link between genetically lower vitamin D and PD

The authors call for trials limited to people with confirmed deficiency, warning that mixing up D3 and its metabolites "may lead to misinterpretation of clinical findings."

What this does NOT show

  • Whether correcting deficiency helps: VITAL did not select for deficiency, the Finnish trial enrolled people with normal levels, and VitaMIND's dose is not given
  • Achieved 25(OH)D levels are not reported for any trial
  • The review cites Navale 2022 only as an association, omitting that its Mendelian randomization supported a causal effect of deficiency on dementia
  • Reverse causation (less sun and activity after diagnosis) remains possible for both diseases

Sources:

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