As long as we don't have too much of it, vitamin D is an essential part of a healthy body. When the participants were 39 years of age on average, they had their vitamin D levels analyzed from a blood test. "While these findings are very interesting, they only demonstrate an association between vitamin D and early signs of dementia in the brain," McGrath says. "Further studies, for example, a clinical trial, will be required to determine if vitamin D supplements could prevent dementia." Previous research looking at the effects of vitamin D supplementation on tau levels found no effect based on a one-year nutritional intervention.