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The largest study of its kind, which tracked more than 1,658 American seniors for more than five years, has found that low vitamin D levels can more than double the odds of developing Alzheimer’s disease or other forms of dementia. The research, by Columbia University and six other American academic centers, along with British and French researchers, was published in Neurology. Compared with blood levels of vitamin D above 50 nmol/L, levels below 25 nmol/L multiplied risk 2.25 times, and 25-50 nmol/L increased risk 1.53 times. Ask your doctor for a test-it may be covered by insurance.