Vitamin D deficiency seems to be associated with an increased risk of Alzheimer’s disease,[1] but there’s not enough evidence to say whether supplementing with vitamin D can prevent or treat Alzheimer’s.
Many studies have suggested that vitamin D may help improve some of the factors associated with the development of Alzheimer’s disease, which prompted some scientists to suggest that it may be helpful for treating or preventing the disease.[2] In support of this idea, vitamin D treatment has been shown to have neuroprotective effects in in vivo animal studies,[3][4] and human observational studies have found associations between vitamin D deficiency and an increased risk of Alzheimer’s.[5][1] Additionally, an in vitro study of immune cells cultured from the blood of people with Alzheimer’s found that treatment of these cells with vitamin D can increase their breakdown of amyloid-β protein aggregates, which accumulate in the brains of people with Alzheimer’s disease and may be associated with disease development.[6]
However, the randomized controlled trials (RCTs) to date on vitamin D supplementation and Alzheimer’s have reported mixed results. Although one study found that 800 IU (20 μg) of vitamin D a day for 12 months significantly improved cognitive function in people with Alzheimer’s disease,[7] other trials of vitamin D supplementation in people with Alzheimer’s disease have not found significant effects.[8] Similarly, an RCT that included over 2,000 older adults without known health conditions did not find that vitamin D supplementation affected the incidence of Alzheimer’s disease over 5 years.[9] However, because the incidence of Alzheimer’s is low, it’s possible a larger sample size or longer duration would be needed to observe an effect.
Overall, although vitamin D deficiencies may increase Alzheimer’s disease risk, there is not much evidence that supplemental vitamin D treats or prevents Alzheimer’s