Clinical Nutrigenomics: One-Carbon Metabolism, MTHFR/COMT Polymorphisms, and Unmetabolized Folic Acid Toxicity

Background: Synthetic folic acid (pteroylglutamic acid) is among the most widely prescribed micronutrients in clinical practice, mandated periconceptionally for neural tube defect prevention and liberally supplemented across diverse patient populations. However, growing evidence documents that chronic administration of synthetic folic acid, particularly in individuals carrying functional variants in one-carbon metabolism genes, leads to the systemic accumulation of unmetabolized folic acid (UMFA) — a phenomenon with distinct immunological, vascular, and epigenetic consequences that are incompletely recognised in routine clinical settings.

Objectives: This clinical review synthesises the current evidence on:

1. the biochemistry of one-carbon metabolism and its rate-limiting enzymatic steps;
2. the clinical pharmacogenomics of MTHFR C677T, MTHFR A1298C, and COMT Val158Met polymorphisms;
3. the mechanisms, prevalence, and biological sequelae of UMFA accumulation; and
4. evidence-based recommendations for substituting synthetic folic acid with its active metabolite, 5-methyltetrahydrofolate (5-MTHF), in genetically susceptible populations.

Conclusions: UMFA is not a benign analytical curiosity — it exerts measurable immunosuppressive effects on natural killer (NK) cell cytotoxicity, correlates inversely with pro-inflammatory cytokine regulation, and fails to drive homocysteine remethylation in MTHFR TT homozygotes. The routine, undifferentiated prescription of synthetic folic acid requires revisitation in the light of nutrigenomic evidence. Clinically, 5-MTHF bypasses the enzymatic bottleneck imposed by MTHFR polymorphisms and does not generate UMFA, rendering it a pharmacologically superior option for the estimated 30–40% of European individuals carrying at least one T allele.

Keywords: folic acid, UMFA, MTHFR, COMT, one-carbon metabolism, nutrigenomics, 5-methyltetrahydrofolate, homocysteine, NK cells, epigenetics

1. Introduction

Vitamin B9 occupies a central position in cellular metabolism: it functions as an obligate carrier of one-carbon units required for de novo purine and thymidylate synthesis, and for the remethylation of homocysteine to methionine — a reaction that regenerates S-adenosylmethionine (SAM), the universal methyl donor for DNA, histone, and neurotransmitter methylation reactions. The term "folate" encompasses a family of chemically related compounds; synthetic folic acid (FA) — pteroylmonoglutamic acid in its fully oxidised form — is not a physiological molecule. It lacks direct coenzyme activity and must undergo sequential enzymatic reduction by dihydrofolate reductase (DHFR) and methylenetetrahydrofolate reductase (MTHFR) before entering active metabolic pathways. [^1]

The clinical prescription of synthetic FA has been driven by decades of public health evidence demonstrating its efficacy in reducing the incidence of neural tube defects (NTDs) when administered periconceptionally. This evidence base is incontrovertible and constitutes one of the clearest successes of preventive medicine. However, the translation of this evidence into liberal, long-term supplementation across diverse and non-pregnant patient populations — and into mandatory food fortification programmes across more than 80 countries — has created an unprecedented human exposure to doses of a synthetic vitamer that saturate, and sometimes overwhelm, the enzymatic capacity required for its metabolism. [^2]

The emergence of nutrigenomics as a clinical discipline has drawn attention to a hitherto underappreciated consequence of this mass-supplementation paradigm: in a significant proportion of the population carrying functional polymorphisms in one-carbon metabolism genes, synthetic FA accumulates in the circulation in its unmetabolized form. This review addresses the biochemical basis, population genetics, clinical consequences, and therapeutic implications of the UMFA syndrome, with specific attention to MTHFR and COMT polymorphisms.

2. Biochemistry of One-Carbon Metabolism

2.1 The Folate Cycle

Dietary folates — predominantly 5-methyltetrahydrofolate (5-MTHF) in natural foodstuffs — are transported across intestinal epithelium via the proton-coupled folate transporter (PCFT) and the reduced folate carrier (RFC1). Once absorbed, 5-MTHF enters portal circulation in its active, reduced form and is readily available for cellular uptake and metabolic utilisation without further obligate enzymatic transformation.

Synthetic folic acid follows a fundamentally different route. Following intestinal absorption, FA must first be reduced by DHFR to dihydrofolate (DHF) and subsequently to tetrahydrofolate (THF), the parent compound for one-carbon unit metabolism. THF accepts one-carbon groups to form 5,10-methylene-THF (CH₂-THF), which occupies a critical bifurcation point: it can be directed toward thymidylate synthesis (converted to dTMP by thymidylate synthase) or committed irreversibly to homocysteine remethylation by MTHFR, which catalyses the reduction of CH₂-THF to 5-MTHF. The latter then serves as the methyl donor for methionine synthase (MTR), converting homocysteine to methionine in a vitamin B12-dependent reaction. [^3]

2.2 The Methionine Cycle and Methylation

Methionine generated from homocysteine remethylation is activated by ATP to form SAM, which serves as the universal methyl donor in more than 200 enzymatic methylation reactions, including: CpG DNA methylation (epigenetic silencing and gene expression regulation), histone methylation (chromatin remodelling), neurotransmitter methylation (catecholamine inactivation by COMT), and RNA methylation. Following methyl transfer, SAM is converted to S-adenosylhomocysteine (SAH), which is hydrolysed back to homocysteine, completing the cycle. The ratio SAM:SAH serves as the principal intracellular index of methylation capacity.

2.3 The DHFR Bottleneck

Human hepatic DHFR has a markedly lower activity compared to its bacterial and murine counterparts — a characteristic that severely limits the capacity to reduce supraphysiological doses of synthetic FA. At oral doses exceeding 200–400 µg, the DHFR-dependent reduction pathway becomes saturated, and unmetabolized FA appears in portal and systemic circulation. [^4] Critically, DHFR activity is highly variable between individuals, ranging more than four-fold in human liver samples, and is subject to genetic regulation and induction. This enzymatic heterogeneity is the proximate biochemical basis for UMFA accumulation.

3. MTHFR Polymorphisms: Prevalence, Mechanism, and Clinical Impact

3.1 MTHFR C677T (rs1801133)

The C677T single nucleotide polymorphism in exon 4 of the MTHFR gene substitutes cytosine for thymine at nucleotide position 677, resulting in an alanine-to-valine amino acid change at codon 222 of the encoded protein. This substitution renders the enzyme thermolabile and reduces its catalytic activity — by approximately 35% in heterozygotes (CT genotype) and by 70% or more in homozygotes (TT genotype) compared with the wild-type CC genotype. The reduced activity impairs the conversion of CH₂-THF to 5-MTHF, leading to redistribution of folate species toward nucleotide synthesis and away from homocysteine remethylation. [^5][^6]

The epidemiology of MTHFR C677T is strongly geographically patterned. In European populations, the T allele frequency is approximately 30–40%, with homozygosity (TT) found in 8–15% of individuals depending on the country of origin. Mediterranean populations consistently demonstrate higher TT prevalence, with rates reaching 18–20% in Italy. Among Northern Europeans, prevalence is typically lower (TT: 8–10%). Sub-Saharan African populations have markedly lower T allele frequencies. [^5]

The principal biochemical consequence of MTHFR C677T in the TT genotype is hyperhomocysteinaemia — particularly under conditions of relative folate insufficiency. A meta-analysis pooling individual participant data from 40 case-control studies (11,162 cases, 12,758 controls) found that TT homozygotes had a 16% higher odds of coronary heart disease compared with CC homozygotes (OR 1.16, 95% CI 1.05–1.28), with heterogeneity driven by folate status: in European populations with lower background folate, the risk was more pronounced (OR 1.14, 95% CI 1.01–1.28) than in North American populations with mandatory food fortification (OR 0.87, 95% CI 0.73–1.05). [^7] This gene-nutrient interaction is perhaps the most elegant demonstration in nutrigenomics that genotype effect is conditional on nutritional context.

3.2 MTHFR A1298C (rs1801131)

The A1298C polymorphism in exon 7 results in a glutamate-to-alanine substitution at position 429, which reduces MTHFR activity by approximately 20–40% in CC homozygotes and has a weaker independent effect on plasma homocysteine than C677T. Its primary clinical relevance emerges in the context of compound heterozygosity (C677T/A1298C), which confers a level of enzyme impairment functionally intermediate between CT and TT for C677T alone, with corresponding elevations in homocysteine and reductions in 5-MTHF bioavailability.

3.3 The Paradox of Synthetic FA Supplementation in TT Homozygotes

A clinical intervention study in cardiovascular disease patients supplemented with 5 mg FA daily for 8 weeks demonstrated genotype-dependent homocysteine responses: TT homozygotes achieved the greatest fractional reduction in plasma homocysteine (approximately 40%), followed by CT heterozygotes (23%) and CC wild-types (10%). [^8] However, this apparent benefit must be contextualised by the concurrent generation of UMFA at such doses in a significant proportion of participants — UMFA that, unlike 5-MTHF, cannot directly participate in homocysteine remethylation and simultaneously occupies folate-binding proteins and folate receptors, potentially inhibiting the cellular uptake and utilisation of endogenous 5-MTHF. The paradox is that high-dose FA supplementation in TT individuals may partially lower homocysteine via mass-action effects while simultaneously generating UMFA that impairs immune function and receptor-mediated folate transport.

4. The UMFA Syndrome: Definition, Prevalence, and Mechanisms

4.1 Definition and Measurement

Unmetabolized folic acid is operationally defined as the presence of synthetic pteroylmonoglutamic acid in serum or plasma in its unreduced form — a species not detected in populations unexposed to synthetic FA supplements or fortified foods. Detection requires HPLC-tandem mass spectrometry rather than conventional microbiological assays, which measure total folate activity and cannot distinguish FA from reduced folate species.

A clinically relevant threshold for UMFA is generally taken as >1 nmol/L in the fasting state (>8 hours post-prandial), as concentrations below this level are substantially attributable to recent dietary exposure. Concentrations above this threshold in the fasting state represent persistent systemic accumulation indicative of saturated or impaired first-pass reduction capacity.

4.2 Prevalence in Supplemented and Fortified Populations

Cross-sectional NHANES data from 2007–2008 (n = 2,707 individuals aged ≥1 year) demonstrated detectable UMFA (>0.3 nmol/L) in over 95% of both supplement users and non-users — a direct consequence of pervasive exposure to FA-fortified flour and foodstuffs. [^9] Concentrations exceeding 1 nmol/L were found in 33.2% overall and in 21.0% of fasting adults. Among supplement users, the geometric mean UMFA was approximately twice that of non-users (1.54 vs. 0.794 nmol/L). In an earlier NHANES analysis of adults aged ≥60 years, UMFA was detected in 38% of the population, with a mean concentration of 4.4 nmol/L in those affected. [^10]

Data from Brazilian populations exposed to mandatory flour fortification revealed UMFA detectability in 68–81% of adults not using supplements. [^11] A prospective controlled trial administering 5 mg FA daily to 30 healthy adults documented an 11.9-fold increase in UMFA concentrations after 45 days, with UMFA exceeding the 1.12 nmol/L threshold in 96.6% of participants. [^12] These findings establish that UMFA accumulation is both ubiquitous at population level under fortification conditions and highly predictable and pronounced with supplemental FA at doses routinely prescribed in clinical practice.

4.3 Immunological Consequences: NK Cell Cytotoxicity

The most extensively documented biological effect of UMFA accumulation is a reduction in natural killer (NK) cell number and cytotoxic activity. In the landmark study by Troen et al. (2006), postmenopausal women (n = 105) with plasma UMFA above the detectable threshold had NK cytotoxicity approximately 23% lower than women without detectable UMFA (p = 0.04), with a dose-response relationship of increasing effect at higher UMFA concentrations (p-trend = 0.002). Women aged ≥60 years demonstrated a more pronounced effect. [^13]

The prospective intervention study by Paniz et al. (2017) confirmed these immunological observations under controlled experimental conditions: 90 days of 5 mg FA supplementation was associated with significant reductions in both the number (p < 0.001) and cytotoxic function (p = 0.003) of NK cells, alongside upregulation of IL-8 and TNF-α mRNA expression in mononuclear leukocytes at 45 and 90 days (p = 0.001 for both). [^12] The plausible mechanism involves the capacity of UMFA to competitively occupy folate receptors on NK cells, impairing folate-dependent biosynthesis of nucleotides required for lymphocyte proliferation and effector function. Functional DHFR mRNA upregulation observed at 90 days likely represents a compensatory cellular response to UMFA-mediated receptor occupancy.

In sickle cell disease patients receiving FA supplementation, more than 50% had detectable UMFA, with median UMFA levels significantly elevated in patients in crisis (131.8 ng/mL) compared with those at steady state (36.31 ng/mL), suggesting a potential immunological link between UMFA burden and disease exacerbation. [^14]

4.4 Inflammatory Signalling

Cross-sectional data from São Paulo (n = 302) found that individuals in the highest tertile of UMFA concentrations had significantly lower odds of elevated TNF-α (OR 0.44, 95% CI 0.24–0.81), IL-1β (OR 0.45, 95% CI 0.25–0.83), and IL-12 (OR 0.49, 95% CI 0.27–0.89), compared with the lowest tertile. [^15] The interpretation of these findings requires caution: the inverse association does not imply that UMFA is anti-inflammatory. Rather, impaired NK cell activity associated with UMFA may lead to reduced cytokine output from innate immune effectors, which in some contexts could be mechanistically downstream of immunosuppression rather than therapeutic anti-inflammation. Prospective interventional data are needed to resolve causality and directionality in this cytokine relationship.

4.5 Homocysteine: The Vascular Toxicity Axis

Central to the clinical rationale for folate supplementation is homocysteine reduction. Elevated total plasma homocysteine is an independent cardiovascular risk factor, strongly associated with endothelial dysfunction, prothrombotic vascular changes, and oxidative stress. In the MTHFR TT homozygote, the principal biochemical deficit is the reduced conversion of CH₂-THF to 5-MTHF — the direct methyl donor for homocysteine remethylation. Administering synthetic FA to such individuals replenishes the folate pool as a mass-action substrate, but the FA must first be reduced to active species via the very pathway that is functionally compromised. Consequently, the homocysteine-lowering efficacy of FA is attenuated in TT individuals relative to CC wild-types, and the efficiency gap is most apparent when comparing equimolar doses of FA with pre-formed 5-MTHF.

In a randomised crossover pharmacokinetic study in MTHFR C677T TT homozygotes with coronary artery disease, a single oral dose of 5 mg 5-MTHF achieved peak plasma concentrations approximately seven-fold higher than an equivalent dose of folic acid, demonstrating markedly superior bioavailability. A prospective RCT by Venn et al. (2003) in 167 healthy volunteers found that 24-week supplementation with low-dose L-5-MTHF (113 µg/day) reduced plasma total homocysteine by 14.6% more than placebo, compared with 9.3% for equimolar FA, with L-5-MTHF demonstrating significantly greater homocysteine-lowering efficacy (p < 0.05) without generating detectable UMFA. [^16]

5. COMT Polymorphisms and the Methylation Nexus

5.1 COMT Biochemistry and the Val158Met Polymorphism

Catechol-O-methyltransferase (COMT) catalyses the O-methylation of catecholamine neurotransmitters (dopamine, noradrenaline, adrenaline), catechol oestrogen metabolites, and xenobiotic catechols, using SAM as the obligate methyl donor. The resulting product is SAH, which is hydrolysed to homocysteine. COMT thus represents a direct biochemical link between methylation capacity and both catecholaminergic neurotransmission and oestrogen metabolism.

The Val158Met SNP (rs4680) at codon 158 produces a valine-to-methionine substitution that reduces COMT thermostability and enzymatic activity by approximately four-fold in the homozygous Met/Met genotype compared with Val/Val. The low-activity Met allele is present in approximately 50% of Caucasians, with Met/Met homozygosity in roughly 25% of the population. [^17]

5.2 COMT, SAM, and the Methylation Dependency

Because COMT requires SAM as its methyl donor, its catalytic efficiency is directly dependent on the cellular availability of SAM — itself a product of the homocysteine remethylation pathway. In individuals with concomitant MTHFR TT genotype and reduced 5-MTHF generation, SAM synthesis is attenuated, and COMT-dependent methylation reactions are correspondingly impaired. This creates a compound genetic vulnerability: reduced MTHFR activity constrains SAM supply; reduced COMT activity due to Val158Met further diminishes the efficiency of available methyl groups for catecholamine and oestrogen detoxification. The consequences include:

• Neurotransmitter dysregulation: Prefrontal dopamine availability is elevated in Met/Met carriers due to slower catabolism, associated with altered working memory, stress reactivity, and predisposition to affective disorders. Val/Val carriers have lower prefrontal dopamine and poorer cognitive flexibility under low-stress conditions.
• Oestrogen toxicity: Catechol oestrogen metabolites (4-hydroxyoestradiol, 2-hydroxyoestradiol) are substrates for COMT-dependent inactivation. Impaired methylation allows accumulation of genotoxic quinone intermediates, associated with oxidative DNA damage and elevated breast cancer risk in low-COMT individuals. [^18]
• Epigenetic vulnerability: Reduced methylation flux downstream of compromised one-carbon metabolism leads to global and locus-specific DNA hypomethylation, with effects on tumour suppressor gene silencing and chromatin architecture. [^19]

6. Folate Receptor Saturation and the Competitive Inhibition Hypothesis

One mechanistically plausible but incompletely characterised consequence of UMFA accumulation is competitive occupancy of folate-binding proteins and folate receptors (particularly folate receptor alpha, FRα, which is highly expressed in the kidney, choroids plexus, and various epithelial tissues). Synthetic folic acid binds FRα with higher affinity than 5-MTHF, creating the paradox that high FA intake could competitively displace the physiologically active folate form from cellular receptors, thereby impairing functional folate uptake despite apparently adequate serum folate concentrations. This mechanism is particularly concerning for tissues relying on receptor-mediated folate transport, including the developing neural tube and the blood-brain barrier.

Unmetabolized FA in plasma does not engage the one-carbon transfer cycle directly and cannot substitute for 5-MTHF in homocysteine remethylation, thymidylate synthesis, or SAM regeneration. Its presence at the receptor level is biologically inert in metabolic terms but potentially inhibitory in transport terms — a state of "functional folate insufficiency despite serum folate sufficiency" that standard total folate assays fail to detect.

7. Clinical Pharmacology of Active Folate Forms

7.1 5-Methyltetrahydrofolate (5-MTHF, L-methylfolate)

5-MTHF, commercially available as its calcium salt (Metafolin® or generic L-methylfolate), is the predominant circulating and cellular form of folate in humans. It requires no enzymatic activation prior to entering the folate cycle, bypassing both DHFR and the rate-limiting MTHFR step. Its key clinical advantages over synthetic FA include:

• No UMFA generation. Pharmacokinetic studies confirm that UMFA rarely appears in plasma following 5-MTHF administration, even at supraphysiological doses.
• Bioavailability independent of MTHFR genotype. 5-MTHF achieves markedly higher peak plasma concentrations than equimolar FA in MTHFR TT homozygotes and CC wild-types alike, with area-under-the-curve and Cmax up to seven-fold higher in pharmacokinetic studies.
• Superior homocysteine reduction. In randomised trials, 5-MTHF achieves comparable or superior homocysteine lowering to equimolar FA with a cleaner pharmacological profile. [^16]
• No masking of B12 deficiency. Unlike high-dose FA — which can correct the macrocytosis of B12 deficiency while leaving the neurological sequelae untreated — 5-MTHF does not correct B12 deficiency-associated anaemia and thus does not conceal B12 deficiency from routine haematological screening. [^20]
• Penetration of the blood-brain barrier. 5-MTHF efficiently crosses the blood-brain barrier via RFC1 and PCFT, supporting central nervous system methylation reactions that are relevant to psychiatric pharmacology and neuroprotection. [^21]

A 2025 comparative analysis across folate forms in clinical practice confirmed that 5-MTHF and folinic acid (CHO-THF) demonstrated key advantages over synthetic FA with respect to UMFA avoidance, genetic variant compatibility, and metabolic support, while acknowledging that synthetic FA remains the only form with proven efficacy in large RCTs for NTD prevention. [^21]

7.2 Evidence from RCTs

The evidence landscape for 5-MTHF superiority over FA is growing but not yet definitive. A 2024 narrative review evaluating supplementation forms for NTD prevention concluded that 5-MTHF can effectively improve folate biomarkers in early pregnancy, but clinical RCT data specifically powered for NTD prevention outcomes remain absent, and FA retains its regulatory status as the primary recommended supplement for this indication. [^22] In the context of homocysteine reduction and non-NTD indications (MTHFR carrier management, psychiatric comorbidity, cardiovascular risk attenuation), the pharmacological case for preferring 5-MTHF is substantially stronger and is supported by multiple controlled trials and pharmacokinetic studies. [^16]

8. Clinical Implications and Proposed Management Framework

8.1 Identifying Patients at Risk

Clinicians should consider the possibility of MTHFR-related FA malmetabolism in patients presenting with:

• Persistently elevated plasma homocysteine despite FA supplementation
• Unexplained subfertility or recurrent miscarriage with MTHFR TT genotype confirmed
• Personal or family history of cardiovascular disease with hyperhomocysteinaemia
• Psychiatric comorbidities (particularly treatment-resistant depression or bipolar spectrum disorder) — where methylation capacity and COMT Val158Met status modulate antidepressant response
• Autoimmune conditions with evidence of NK cell dysfunction
• Women of reproductive age in European populations (TT prevalence 8–15%)

8.2 Diagnostic Approach

Genotyping for MTHFR C677T, MTHFR A1298C, and COMT Val158Met is available through validated clinical molecular genetic assays and can be included in nutrigenomic panels. Where genotyping is not immediately available or accessible, a functional biochemical approach can be adopted: measurement of fasting plasma total homocysteine, serum folate speciation by HPLC-MS (including UMFA), red blood cell folate, and vitamin B12 provides a functional read-out of one-carbon metabolism integrity.

8.3 Therapeutic Recommendations

Based on the reviewed evidence, the following principles can guide clinical nutrigenomic practice:

1. MTHFR C677T TT homozygotes should be preferentially supplemented with 5-MTHF rather than synthetic FA. Doses equivalent to standard FA recommendations (400–800 µg dietary folate equivalents per day) are appropriate for periconceptional use; higher doses for specific indications should be individualised.
2. MTHFR C677T CT heterozygotes with evidence of functional impairment (elevated homocysteine, documented UMFA accumulation, or symptomatic presentation) represent a second-tier population who may benefit from 5-MTHF over FA, particularly when high-dose supplementation is contemplated.
3. COMT Val158Met Met/Met homozygotes, particularly women, warrant optimisation of upstream one-carbon metabolism (adequate B12, riboflavin, and folate as 5-MTHF) to support SAM availability for COMT-mediated catecholamine and oestrogen detoxification.
4. Concurrent B12 supplementation (as methylcobalamin or hydroxocobalamin) should accompany 5-MTHF prescription in all patients with documented or suspected functional B12 insufficiency, given the methyltransferase dependency of homocysteine remethylation.
5. Monitoring: Plasma total homocysteine and serum folate speciation (including UMFA where available) provide the most clinically actionable biochemical targets for follow-up in nutrigenomic prescribing.

9. Limitations and Research Gaps

Several important caveats should inform the clinical translation of this evidence. First, while the immunological consequences of UMFA are biologically plausible and observationally documented, the prospective clinical trial evidence linking UMFA accumulation to hard clinical outcomes (infection incidence, cancer progression, cardiovascular events) remains limited and is principally from cross-sectional and short-term intervention designs. Second, the NTD prevention evidence base for 5-MTHF as a direct substitute for FA is currently insufficient for guideline-level recommendation, and FA retains regulatory primacy for this indication. Third, the clinical utility of routine MTHFR genotyping as a population screening tool is contested, with some health technology assessment bodies having downgraded its clinical utility in non-specialist settings — in part because the cardiovascular risk attributable to TT genotype in populations with adequate folate status is modest and the evidence base for genotype-guided supplementation remains incompletely developed from RCT data. Fourth, folate pharmacogenomics extends beyond MTHFR and COMT to include RFC1, DHFR, methionine synthase (MTR), methionine synthase reductase (MTRR), and TYMS variants that interact with individual folate response — underscoring that clinical nutrigenomics requires a pathway-level rather than single-gene perspective.

10. Conclusion

The ubiquitous clinical prescription of synthetic folic acid — a molecule absent from natural food matrices and dependent on a limited, genetically variable enzymatic system for activation — represents an instructive case study in the gap between population-level pharmacological efficacy and individual-level biochemical safety. In persons carrying MTHFR C677T variant alleles, which affect an estimated 30–40% of European populations to varying degrees, the routine administration of synthetic FA generates measurable systemic UMFA accumulation. This accumulation is associated with quantifiable impairment of NK cell cytotoxic function, failure to optimally remethylate homocysteine, and — through its interaction with COMT-mediated methylation — indirect consequences for catecholamine regulation, oestrogen detoxification, and epigenetic maintenance.

The active metabolite 5-MTHF circumvents the enzymatic constraints imposed by MTHFR polymorphisms, achieves superior bioavailability independent of genotype, does not generate UMFA, and does not mask haematological indicators of cobalamin deficiency. The body of evidence reviewed here, while not yet sufficient to mandate universal guideline revision, is substantial enough to justify a clinical posture of precision over uniformity in folate supplementation — a posture that begins with awareness of genotype, continues with measurement of functional metabolic status, and proceeds with pharmacological selection calibrated to the individual biochemistry of the patient.

The dictum primum non nocere applies as much to vitamins prescribed for their benefits as to drugs prescribed for their dangers. For the one-carbon network, the form of the molecule matters as much as its dose.

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4. Liew SC, Gupta ED. Methylenetetrahydrofolate reductase (MTHFR) C677T polymorphism: epidemiology, metabolism and the associated diseases. Eur J Med Genet. 2015;58(1):1–10. [^5]

5. Zarembska E, Ślusarczyk K, Wrzosek M. The Implication of a Polymorphism in the Methylenetetrahydrofolate Reductase Gene in Homocysteine Metabolism and Related Civilisation Diseases. Int J Mol Sci. 2024;25(1):193. [^6]

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7. Liu CS, et al. Methylenetetrahydrofolate reductase polymorphism determines the plasma homocysteine-lowering effect of large-dose folic acid supplementation in patients with cardiovascular disease. Nutrition. 2004;20(11-12):1050–1055. [^8]

8. Pfeiffer CM, et al. Unmetabolized folic acid is detected in nearly all serum samples from US children, adolescents, and adults. J Nutr. 2015;145(3):520–531. [^9]

9. Bailey RL, et al. Unmetabolized serum folic acid and its relation to folic acid intake from diet and supplements in a nationally representative sample of adults aged ≥60 y in the United States. Am J Clin Nutr. 2010;92(2):383–389. [^10]

10. Palchetti C, et al. Association between Serum Unmetabolized Folic Acid Concentrations and Folic Acid from Fortified Foods. J Am Coll Nutr. 2017;36(7):525–533. [^11]

11. Paniz C, et al. A Daily Dose of 5 mg Folic Acid for 90 Days Is Associated with Increased Serum Unmetabolized Folic Acid and Reduced Natural Killer Cell Cytotoxicity in Healthy Brazilian Adults. J Nutr. 2017;147(9):1677–1685. [^12]

12. Troen AM, et al. Unmetabolized folic acid in plasma is associated with reduced natural killer cell cytotoxicity among postmenopausal women. J Nutr. 2006;136(1):189–194. [^13]

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14. Steluti J, et al. Unmetabolized folic acid is associated with TNF-α, IL-1β and IL-12 concentrations in a population exposed to mandatory food fortification with folic acid: a cross-sectional population-based study in Sao Paulo, Brazil. Eur J Nutr. 2020. [^15]

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19. Kapiszewska M, et al. THE COMT-MEDIATED METABOLISM OF FLAVONOIDS AND ESTROGEN AND ITS RELEVANCE TO CANCER RISK. Pol J Food Nutr Sci. 2003. [^18]

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21. Prinz-Langenohl R, et al. A study of plasma folate under the influence of [6S]-5-MTHF in women with 677C→T polymorphism of MTHFR with different types of inheritance. Reprod Endocrinol. 2017.

22. Obeid R, Holzgreve W, Pietrzik K. Is 5-methyltetrahydrofolate an alternative to folic acid for the prevention of neural tube defects? J Perinat Med. 2013;41(5):469–483. [^4]

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Submitted for peer review. The author declares no conflicts of interest. No funding was received for this work. All evidence cited derives from peer-reviewed publications identified through structured literature search.

[^1]: Pietrzik et al., 2010. Folic Acid and L-5-Methyltetrahydrofolate. Clinical Pharmacokinetics.

[^2]: Obeid et al., 2013. Is 5-methyltetrahydrofolate an alternative to folic acid for the prevention of neural tube defects?. Journal of Perinatal Medicine.

[^3]: Raghubeer & Matsha, 2021. Methylenetetrahydrofolate (MTHFR), the One-Carbon Cycle, and Cardiovascular Risks. Nutrients.

[^4]: Liew & Gupta, 2015. Methylenetetrahydrofolate reductase (MTHFR) C677T polymorphism: epidemiology, metabolism and the associated diseases. European Journal of Medical Genetics.

[^5]: Zarembska et al., 2023. The Implication of a Polymorphism in the Methylenetetrahydrofolate Reductase Gene in Homocysteine Metabolism and Related Civilisation Diseases. International Journal of Molecular Sciences.

[^6]: Klerk et al., 2002. MTHFR 677C-->T polymorphism and risk of coronary heart disease: a meta-analysis. Journal of the American Medical Association (JAMA).

[^7]: Liu et al., 2004. Methylenetetrahydrofolate reductase polymorphism determines the plasma homocysteine-lowering effect of large-dose folic acid supplementation in patients with cardiovascular disease. Nutrition (Burbank, Los Angeles County, Calif.).

[^8]: Pfeiffer et al., 2015. Unmetabolized folic acid is detected in nearly all serum samples from US children, adolescents, and adults. Journal of NutriLife.

[^9]: Bailey et al., 2010. Unmetabolized serum folic acid and its relation to folic acid intake from diet and supplements in a nationally representative sample of adults aged > or =60 y in the United States. American Journal of Clinical Nutrition.

[^10]: Palchetti et al., 2017. Association between Serum Unmetabolized Folic Acid Concentrations and Folic Acid from Fortified Foods. Journal of the American College of Nutrition.

[^11]: Paniz et al., 2017. A Daily Dose of 5 mg Folic Acid for 90 Days Is Associated with Increased Serum Unmetabolized Folic Acid and Reduced Natural Killer Cell Cytotoxicity in Healthy Brazilian Adults. Journal of NutriLife.

[^12]: Troen et al., 2006. Unmetabolized folic acid in plasma is associated with reduced natural killer cell cytotoxicity among postmenopausal women. Journal of NutriLife.

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