Unmetabolized Folic Acid: Biology, Epidemiology, and Clinical Consequences: A Systematic Review.

Overview

Authors: Richard E. Frye^1,2, Daniel A. Rossignol^1,3

ORCID iDs: Richard E. Frye, Daniel A. Rossignol

Autism Discovery and Treatment Foundation, Phoenix, AZ 85050, USA
Rossignol Medical Center, Phoenix, AZ 85050, USA
Rossignol Medical Center, Aliso Viejo, CA 92656, USA

Journal: Nutrients, volume 18, issue 17, article 2887

Dates: received 4 August 2026; accepted 26 August 2026; published online 3 September 2026

Type: Review · Language: English

License: CC BY

Identifiers: DOI 10.3390/nu18172887 · PMID 42739064 · PMCID PMC13567015 · OpenAlex W7207618373

Open access: gold, a free copy (OpenAlex)

Status: code on request

Categories: human (organism), clinical / translational (subfield)

Methods: Connectivity

Keywords: unmetabolized folic acid, folic acid, 5-methyltetrahydrofolate, folate fortification, dihydrofolate reductase, one-carbon metabolism, folate receptor alpha, systematic review, meta-analysis

MeSH: Folic Acid*, Erythrocytes, Food, Fortified, Homocysteine, Humans, Neural Tube Defects, Tetrahydrofolate Dehydrogenase, Tetrahydrofolates (* major topic)

Topic: Folate and B Vitamins Research (Rheumatology, Medicine), according to OpenAlex

Citations: not cited yet (Europe PMC); 158 references in the paper

Abstract

Background: Mandatory folic acid (FA) fortification prevents neural tube defects but generates unmetabolized folic acid (UMFA) in blood, because hepatic dihydrofolate reductase (DHFR) reduces FA only slowly. Whether UMFA itself, as distinct from high total folate, carries biological consequences is unresolved. Methods: We systematically searched six databases (January 1995–May 2026; PROSPERO CRD420261407616) and included 65 publications describing 61 unique primary studies, yielding 71 analytic contributions (15 mechanistic, 24 observational, 32 interventional). The question of whether equimolar (6S)-5-MTHF substitution alters UMFA, erythrocyte folate, or total homocysteine (tHcy) versus FA was evaluated using random-effects meta-analysis with a DerSimonian–Laird estimator for τ2 and Hartung–Knapp–Sidik–Jonkman small-sample confidence intervals and was rated with GRADE. Mechanistic and observational streams were synthesized narratively and classified by exposure metric (UMFA, total folate, FA intake, fortification). Results: 5-MTHF substitution reduced plasma UMFA (SMD +0.99; 95% CI +0.20 to +1.79; I2 = 30%; k = 3; GRADE low). The two trials reporting erythrocyte folate were directionally discordant (Hedges’ g +0.40; +0.08 to +0.73 and −0.38; −0.92 to +0.16; I2 = 83%) and were not pooled. For tHcy, no pooled estimate could be derived; it is reported qualitatively. Observational UMFA–outcome associations were inconsistent, confounded by total folate, and unreplicated. Conclusions: Substitution of (6S)-5-MTHF for FA appears to reduce circulating UMFA. For erythrocyte folate, the two available trials were directionally discordant but insufficient to establish equivalence or non-inferiority. For tHcy, no between-form difference was detected. With two to three trials per outcome, no prespecified margins, and no trial designed for that purpose, the evidence cannot support an equivalence or non-inferiority claim for either outcome and clinical benefit remains unestablished. Current evidence does not establish that UMFA independently causes harm.

Reproduced under the paper's license (CC BY), from the paper cited above.

Code

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Data

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Data Availability Statement

All data underlying this Review are distributed as Supplementary Materials. The analytic code (meta_analyses.py; Python 3.11, numpy 1.26, scipy 1.12) is described in Supplementary Methods Section S8 and is available from the corresponding author on reasonable request. The review protocol is publicly registered at PROSPERO (CRD420261407616).

Reproduced under the paper's license (CC BY), from the paper cited above.

Versions

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Version 1, 27 September 2026: the first record

Recorded: type, language, journal, volume, issue, pages, dates, 2 authors, 9 keywords, 8 MeSH terms, 153 references.

Cite

This paper

Frye, R. E., & Rossignol, D. A. (2026). Unmetabolized Folic Acid: Biology, Epidemiology, and Clinical Consequences: A Systematic Review. Nutrients, 18(17), 2887. https://doi.org/10.3390/nu18172887

BibTeX

@article{frye2026unmetabolized, author = {Frye, Richard E. and Rossignol, Daniel A.}, title = {{Unmetabolized Folic Acid: Biology, Epidemiology, and Clinical Consequences: A Systematic Review}}, journal = {Nutrients}, year = {2026}, month = sep, volume = {18}, number = {17}, pages = {2887}, publisher = {Multidisciplinary Digital Publishing Institute (MDPI)}, issn = {2072-6643}, doi = {10.3390/nu18172887}, url = {https://doi.org/10.3390/nu18172887}, pmid = {42739064}, pmcid = {PMC13567015} }

RIS

TY - JOUR AU - Frye, Richard E. AU - Rossignol, Daniel A. TI - Unmetabolized Folic Acid: Biology, Epidemiology, and Clinical Consequences: A Systematic Review T2 - Nutrients J2 - Nutrients PY - 2026 DA - 2026/09/03 VL - 18 IS - 17 SP - 2887 SN - 2072-6643 PB - Multidisciplinary Digital Publishing Institute (MDPI) DO - 10.3390/nu18172887 UR - https://doi.org/10.3390/nu18172887 LA - en ER -

CSL-JSON

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