Impact of methyl-donor micronutrient supplementation on DNA methylation patterns: A systematic review and meta-analysis of in vitro, animal and human studies

Background: DNA methylation patterns are directly associated with diverse metabolic disorders. The status of methyl-donor micronutrients has been associated with DNA methylation levels, and altered ingestion of folate, choline, betaine, B vitamins and methionine may impact genes both globally and at the level of promoter regions. Despite this, the role of methyl-donor micronutrient supplementation on DNA methylation profiles is currently unclear. Objectives: The aims of this systematic review and meta-analysis were to identify and synthesize the evidence about methyl-donor nutrients supplementation on DNA methylation profile. Methods: A systematic literature search was performed in MEDLINE, EMBASE, SCOPUS and Web of Sciences databases with a combination of terms related to DNA methylation assessment, supplementation and methyl-donor nutrients. Studies (in vitro, animal models or human clinical trials) were included if DNA methylation levels after any kind of methyl-donor micronutrient supplementation or treatment was investigated. Studies were assessed for bias using Revised Cochrane risk-of-bias tool for randomized trials, Risk Of Bias in Non-randomized Studies of Interventions or Systematic Review Centre for Laboratory Animal Experimentation tools. Data was extracted from studies measuring DNA methylation level in any sample or tissue, following any kind of methyl-donor micronutrient supplementation or treatment. Separate random-effects meta-analyses were performed for animal model studies and human clinical trials, which examined the effects of folic acid supplementation on DNA methylation. Results: Fifty-seven studies were included in the systematic review: 18 human clinical trials, 35 in animal model and 4 in vitro studies. Concerning overall risk of bias, most of the studies were classified as "high risk" or "some concerns". Meta-analysis with meta-regression from studies in animal models showed that folic acid dose significantly affected DNA methylation and that high and very high dose showed increases in DNA methylation when compared to low doses. However, meta-analysis from human clinical trials showed that folic acid supplementation did not promote significant changes in DNA methylation when compared to placebo. Conclusion: Folic acid supplementation may change global DNA methylation levels in animals supplemented with high, as compared to low, doses. Heterogeneity in studies and supplementation protocols make it difficult to establish clinical recommendations. However, these effects, even if small, might be of clinical importance in the management of patients with diseases related to DNA hypomethylation.