Folate (vitamin B9) and cobalamin (vitamin B12) are essential for the normal 
development and function of the central nervous system. The metabolism of these 
vitamins is intimately linked and supports the synthesis of 
S-adenosylmethionine (SAMe), the major methyl group donor in methylation 
reactions. This article reviews the metabolic and clinical importance of folate, 
vitamin B12, and SAMe, as well as clinical trials in relation to depression and 
dementia.