One-carbon metabolism that includes vitamin B12, folic acid, and homocysteine has been investigated in psychiatric disorders like depression. In this study, serum levels of folate, vitamin B12, and homocysteine related to one-carbon metabolism and vitamin D were investigated in children and adolescents with depression and to assess possible roles in depression pathogenesis (1). The results of the studied showed that there was no significant difference between the groups in terms of folate levels. In the patient group, the vitamin B12 and vitamin D levels were clearly low, while homocysteine levels were found to be remarkably high. In addition, there was a negative correlation between depression severity and vitamin B12 and vitamin D, while a positive correlation was found with homocysteine. The authors conclude that vitamin B12 deficiency or insufficiency and elevated homocysteine may contribute to the etiopathogenesis of depression. Additionally, it was shown that lower vitamin D levels may be associated with depression.