In type 2 diabetes patients, higher levels of serum 25-hydroxyvitamin D (25[OH]D) were found to be significantly linked with a lower risk of all-cause dementia, Alzheimer disease (AD), and vascular dementia (VD).

• According to various epidemiological studies, vitamin D status is linked with risk of dementia in general populations.

• Using data from the UK Biobank study, experts assessed the links of circulating vitamin D levels with risks of all-cause dementia, AD, and VD among 13,486 patients with type 2 diabetes who were ≥60 years old.

• Of participants (mean age, 64.6 years; men, 64.3%), 38.3% had vitamin D ≥ 50 nmol/L and only 9.1% had vitamin D ≥ 75 nmol/L.

• Higher serum levels of 25(OH)D were found to be significantly linked with a lower risk of all-cause dementia, AD, and VD.

• A nonlinear association between serum 25(OH)D and risk of all-cause dementia and VD was found, and the nonlinear link reached borderline significance for AD, with a threshold at around a serum 25(OH)D value of 50 nmol/L for all the outcomes.

• For participants who had serum 25(OH)D ≥ 50 nmol/L, vs those who were severely deficient (25[OH]D < 25 nmol/L), the multivariate hazard ratios were 0.41, 0.50, and 0.41 for all-cause dementia, AD, and VD, respectively.