MTHFR and depressive disorder: For example, a meta-analysis study including only European Caucasian populations did not find that MTHFR C677T polymorphism was associated with recurrent depression (13), and there was a study suggesting that the C677T genotype may be a protector for MDD (14), yet most evidence from meta-analyses strongly suggested that the T allele or TT genotype tends to be a risk effect to MDD (5, 15).