methylation process regulate the amino acid homocysteine http://www.hsph.harvard.edu/nutritionsource/vitamin-b/ Harvard University School of Public Health http://www.hsph.harvard.edu/nutritionsource/vitamin-b/ Harvard University School of Public Health Harvard University School of Public Health the jury's still out Vitamin B6 and B12 Harvard University School of Public Health http://www.hsph.harvard.edu/nutritionsource/vitamin-b/