DNA methylation homocysteine DNA DNA methylation processes and health Diet Food S-adenosylmethionine adenosineglutathione autism autistic disorders biomarkers cysteine dietary supplements down syndrome folate folate deficiency folinic acid glutathione growth development and pediatric hepatocarcinogenesis methionine methylcobalamin methylenetetrahydrofolate reductase minerals and phytochemicals mutation nutrition and cancer oxidative stress vitamins