Study flow chart. MTHFR, methylenetetrahydrofolate reductase; [6S]-5-MTHF, [6S]-5-methyltetrahydrofolate; FA, folic acid; TT, TT genotype of the 677C→T mutation of the 5,10-methylenetetrahydrofolate reductase; CC, CC genotype of the 677C→T mutation of the 5,10-methylenetetrahydrofolate reductase.