To compare plasma vitamin D concentrations among patients with normal cognitive function (control group), mild cognitive impairment (MCI), and Alzheimer disease (AD). In total, 158 patients with AD, 228 patients with MCI, and 603 control subjects were included. Plasma levels of 25-hydroxyvitamin D were measured after comprehensive geriatric assessment and compared among groups. SPSS 15.0 was used for statistical analysis. Mean levels of 25-hydroxyvitamin D were significantly different among the

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... 3 groups of AD patients, MCI patients, and controls (P < 0.001). Post hoc analysis revealed that the levels were significantly lower in the MCI group than the control group (P = 0.002) and significantly lower in the AD group than the control group (P = 0.003). Multivariate analysis showed that age (OR: 1.070, 95% CI: 1.025-1.116, P = 0.002), instrumental activities of daily living score (OR: 0.920, 95% CI: 0.850-0.995, P = 0.037), 25-hydroxyvitamin D level (OR: 0.959, 95% CI: 0.932-0.987, P = 0.004), and diabetes mellitus (OR: 2.476, 95% CI: 1.153-5.319, P = 0.020) were factors independently associated with AD. This study demonstrated that there is a correction between plasma 25-hydroxyvitamin D levels and cognitive functions.