BACKGROUND: Vitamin D deficiency has been implicated as a risk factor for dementia in several cross-sectional studies. We tested the hypothesis that reduced plasma 25-hydroxyvitamin D (25[OH]D) is associated with increased risk of Alzheimer's disease (AD) and vascular dementia in the general population.

METHODS: We measured baseline plasma 25(OH)D in 10,186 white individuals from the Danish general population.

RESULTS: During 30 years of follow-up, 418 participants developed AD and 92 developed vascular dementia. Multivariable adjusted hazard ratios for AD were 1.25 (95% confidence interval [CI], 0.95-1.64) for 25(OH)D less than 25 nmol/L vs. greater than or equal to 50 nmol/L, and 1.29 (95% CI, 1.01-1.66) for less than the 25th seasonally adjusted 25(OH)D percentile vs. more than the 50th seasonally adjusted 25(OH)D percentile. Multivariable adjusted hazard ratios for vascular dementia were 1.22 (95% CI, 0.77-1.91) for 25(OH)D less than 50 nmol/L vs. greater than or equal to 50 nmol/L, and 1.22 (95% CI, 0.79-1.87) for less than or equal to the 50th vs. more than the 50th seasonally adjusted 25(OH)D percentile. Last, multivariable adjusted hazard ratios for the combined end point were 1.28 (95% CI, 1.00-1.64) for 25(OH)D less than 25 nmol/L vs. greater than or equal to 50 nmol/L, and 1.27 (95% CI, 1.01-1.60) for less than the 25th vs. more than the 50th seasonally adjusted 25(OH)D.

CONCLUSIONS: We observed an association of reduced plasma 25(OH)D with increased risk of the combined end point of AD and vascular dementia in this prospective cohort study of the general population.