Folate-Based Interventions in Major Depressive Disorder: From Folic Acid and L-Methylfolate to MTHFR and Precision Psychiatry
Abstract
Background: Folate metabolism has long been implicated in major depressive disorder (MDD), through its involvement in one-carbon metabolism, methylation reactions, homocysteine regulation, and monoamine synthesis. However, folic acid, folinic acid, and L-methylfolate (5-MTHF) are pharmacologically distinct compounds, and their clinical evidence in depression differs substantially. Interest has also increased in methylenetetrahydrofolate reductase (MTHFR) polymorphisms and metabolic and inflammatory biomarkers as potential predictors of treatment response.
Objective: To critically review the evidence linking folate metabolism to MDD and to evaluate the efficacy, dose-response relationships, biological rationale, and potential predictors of response to folic acid, folinic acid, and L-methylfolate, with particular emphasis on antidepressant augmentation and the clinical relevance of MTHFR polymorphisms.
Methods: A structured narrative review was conducted using MEDLINE/PubMed, Embase, Scopus, Web of Science, PsycINFO, Cochrane Library, Google Scholar, ClinicalTrials.gov, and the WHO International Clinical Trials Registry Platform from inception through September 2026. Evidence was synthesized hierarchically, prioritizing umbrella reviews, meta-analyses, systematic reviews, and randomized controlled trials, followed by prospective, observational, biomarker, genetic, and mechanistic studies. Folate formulations were analyzed separately because of their pharmacological differences.
Results: Higher-level evidence supports a modest overall benefit of folate-based interventions in depression, but substantial heterogeneity exists across formulations and doses. The most consistent formulation-specific evidence was identified for adjunctive L-methylfolate 15 mg/day in patients with inadequate antidepressant response. In contrast, L-methylfolate 7.5 mg/day has not demonstrated consistent efficacy. Evidence for folic acid is mixed; although smaller studies reported beneficial effects, the large FolATED randomized trial failed to demonstrate clinically meaningful augmentation with folic acid 5 mg/day. Evidence for folinic acid remains limited and predominantly uncontrolled. MTHFR C677T and, less consistently, A1298C have been associated with depression susceptibility and possibly treatment resistance; however, current evidence does not validate routine MTHFR genotyping for selecting L-methylfolate treatment. Exploratory analyses suggest that obesity, inflammatory biomarkers, oxidative stress, and abnormalities in one-carbon metabolism may identify patients with greater L-methylfolate response, raising the possibility of a broader folate–metabolic–inflammatory phenotype.
Conclusions: Folate-based interventions in MDD should not be considered a homogeneous therapeutic class. Current evidence most strongly supports L-methylfolate 15 mg/day as an adjunctive strategy in patients with inadequate response to antidepressants, whereas high-dose folic acid cannot presently be considered an evidence-based substitute and data for folinic acid remain insufficient. Although MTHFR is biologically relevant, isolated MTHFR genotyping has not been prospectively validated as a treatment-selection tool. Future biomarker-stratified trials should determine whether integrated genetic, nutritional, metabolic, and inflammatory profiles can identify patients most likely to benefit from L-methylfolate augmentation.
How to Cite This Article
Jozélio Freire de Carvalho (2026). Folate-Based Interventions in Major Depressive Disorder: From Folic Acid and L-Methylfolate to MTHFR and Precision Psychiatry . International Journal of Biological and Biomedical Research (IJBBMR), 2(5), 39-50. DOI: https://doi.org/10.54660/IJBBR.2026.2.5.39-50