Most of the research that has been done regarding the relationship between vitamin B12 and Alzheimer's has focused on the apparent impact of B12 deficiencies on cognitive health. In 1990, researchers found that on average, people with Alzheimer's disease had significantly lower concentrations of B12 in their blood than their healthy counterparts. A similar study conducted in 2000 found that "...subjects with low levels of B12 or folate had twice higher risks of developing AD [Alzheimer's]." The authors of the study concluded by recommending that B12 and folic acid levels be monitored in the elderly as a way to better detect, treat, and prevent Alzheimer's and other forms of dementia.
Other studies have looked at the relationship between dementia and levels of B12 and homocysteine. In 1998, researchers reported that those with dementia had unusually high levels of homocysteine and unusually low levels of B12 compared to their healthy counterparts. Four years later, another study found that higher concentrations of homocysteine in the body led to a significantly higher chance of developing Alzheimer's.
There is some debate about how effective vitamin B12 is as a treatment, and some researchers report no clear benefits to adding this vitamin to the diets of those with Alzheimer's.