People with Alzheimer's have high blood levels of a compound called homocysteine, and people with elevated levels of the compound have been shown to be at higher risk for Alzheimer's. 

 

It's known that folic acid (vitamin B-9) and vitamin B-12 lower homocysteine levels, so it was believed that taking B vitamins may lower a person's risk of Alzheimer's. However, this review, published recently in the American Journal of Clinical Nutrition, revealed different results. Taking B vitamins does not slow age-related mental decline or prevent Alzheimer's disease, a new review says.