Low levels of folate and vitamin B12 and high levels of a substance called homocysteine have been found to be associated with depression in a number of studies although not all studies have found this link. A Korean study of 521 people aged 65 and over measured participants' levels of these substances and followed them over 2-3 years to see who developed depression. The researchers found that lower levels of folate and vitamin B12 and higher homocysteine levels at baseline were associated with a higher risk of incident depression at follow up. Incident depression was associated with a decline in vitamin B12 and an increase in homocysteine levels over the follow-up period.
Kim, Jae-Min ... [et al] - Predictive value of folate, vitamin B12 and homocysteine levels in late-life depression British Journal of Psychiatry April 2008, 192(4), 268-274