In December 2017 researchers from the USA published their review of the scientific literature which assessed whether individuals with depression had lower levels of folate than individuals without depression. Studies evaluating folate levels in individuals with and without depression via red blood cell folate, serum folate, or dietary intake of folate methods were identified and an analysis revealed that individuals with depression appeared to have lower folate and dietary folate levels than those without depression.
Bender A et al. The association of folate and depression: A meta-analysis. J Psychiatr Res. 2017 Dec;95:9-18