1.3 Individual Biomarker Analysis

For this profile, the following biomarkers were investigated: homocysteine, serum folate, folate (RBC), vitamin B6, betaine, vitamin B12, methylmalonic acid (MMA), gamma-glutamyl transferase (GGT), ferritin, C-reactive protein (CRP), and methionine.

These biomarkers are grouped into three tiers based on both metabolic role and evidence strength. The first tier focuses on primary folate pathway biomarkers. Biomarkers like homocysteine and folate forms are consistently studied in conjunction with polymorphisms in folate-related genes due to their direct relationship with the MTHFR mechanism (Liew & Gupta, 2015). Due to their strong, closely-linked metabolic pathways, these markers are most often studied, and therefore have some of the most established track records and patterns in current literature (Colson et al., 2017).

The second set focuses on cofactors and compensatory pathway biomarkers, which accounts for conditional or interactive effects. This set will include factors like vitamin B6, which is a cofactor for the transsulfuration pathway, or betaine, which is involved in another major compensatory pathway, the BHMT route, which is further discussed in Part 1.6 (Hustad et al., 2007; Abratte et al., 2008). The third set focuses on downstream and diagnostic markers. These are biomarkers like CRP, which indicates systemic inflammation, or Vitamin B12, which is mechanistically relevant, but effects are seen downstream with weak, null, or conflicting results (Bhatt et al., 2025; Shiran et al., 2015).

1.3.1 Set 1 — Primary Folate Pathway Biomarkers

This section examines homocysteine, serum folate, and RBC folate for trends in conjunction with MTHFR C677T polymorphisms.

1.3.1.1: Homocysteine

Due to its prominent role in the methionine cycle, homocysteine is one of the most investigated and documented biomarkers in relation to MTHFR’s C677T polymorphism (Raghubeer et al., 2021; Liew & Gupta, 2015). As described in section 1.3, homocysteine is a byproduct of methylation that is normally recycled back to methionine via the folate-dependent re-methylation pathway; when MTHFR activity is reduced, like in TT homozygotes, 5-MTHF availability decreases, which impairs this recycling process and allows homocysteine to accumulate in circulation (Raghubeer et al., 2021).

Main Evidence:

This assertion that TT carriers display elevated homocysteine is supported by multiple studies, including a meta-analysis published in 2017 that included 10 folate intervention studies, which reported that the TT genotype was associated with plasma homocysteine concentrations 2.7 μmol/L higher than CT/CC combined groups, and 2.8 μmol/L higher than the CC group alone (Colson et al., 2017). It is worth noting that 20% of the studies included in the meta-analysis demonstrated TT groups with mean homocysteine concentrations >15 μmol/L, which is the clinical threshold for hyperhomocysteinemia (Colson et al., 2017). This concentration only appears in an estimated 5-7% of the general populace, and therefore suggests that the TT genotype effect is clinically meaningful under certain conditions (Qureshi et al., 2019).

A GWAS published in 2018 from a large, healthy Irish population had similar findings, confirming a clear stepwise increase with T allele dosage, with CC (wildtype) individuals averaging 7.8–8.4 μmol/L, CT individuals averaging 8.0–8.9 μmol/L, and TT individuals averaging 9.2–11.0 μmol/L at a confidence of P = 1.26 × 10-19 (Shane et al., 2018). These results are particularly significant when considering the GWAS’s design, which ruled out confounding factors via population stratification, confirming MTHFR locus as a genuine determinant of homocysteine concentration rather than an associated metric (Shane et al., 2018). This consistent stepwise pattern is further supported by a paper published in 2007 that reported median homocysteine of 10.4 μmol/L in CC carriers, 10.9 μmol/L in CT carriers, and 13.3 μmol/L in TT carriers from the Norwegian NORCCAP cohort, which consisted of 10,601 adults (Hustad et al., 2007).

Additionally, two smaller studies further corroborated these findings. One reported concentrations of 9.3 ± 2.5 μmol/L in CC carriers and 10.4 ± 3.0 μmol/L in TT carriers (P = 0.043) in 115 healthy adults, which is directionally consistent with other results despite limited sample size (Rooney et al., 2020). The second study reported similar levels, but they incorporated sex-stratification, finding TT men displayed particularly elevated levels, as shown in the following report: men (TT: 19 ± 7 vs CC: 12 ± 8 μmol/L, P < 0.001) and women (TT: 14 ± 7 vs CC: 9 ± 7 μmol/L, P = 0.01) (Dedoussis et al., 2005). It is important to note that these values are derived from a log-transformed analysis, which means they cannot be directly quantitatively compared to other included studies. However, an independent European cohort that used a different assay platform and significant confounder adjustment supported the directional findings, which strengthened overall confidence in this source’s contribution to the homocysteine biomarker synthesis. This is strengthened further by the fact that the proportionally larger TT elevation observed in men is consistent with known sex-based differences in homocysteine metabolism reported across the broader literature (Dedoussis et al., 2005).

Directional Trend and Confidence Assessment:

Across all five reviewed studies, the directional trend is unanimous: increased T allele dosage is directionally associated with increased homocysteine levels (TT > CT > CC). This trend was replicated across GWAS, meta-analyses, large observational studies and smaller, independent cohort-based designs. Together, they support the assertion that elevated homocysteine in TT carriers is the most robustly established downstream consequence of reduced MTHFR C677T enzyme activity in the current literature, and it serves as the mechanistic anchor for the biomarker profile that is developed in the following sections (Colson et al., 2017; Shane et al., 2018; Hustad et al., 2007; Rooney et al., 2020; Dedoussis et al., 2005).

1.3.1.2: Serum Folate

Serum folate is one of the primary substrates depleted by MTHFR dysfunction, and like homocysteine, is a frequently examined biomarker in literature on MTHFR C677T (Liew & Gupta, 2015; Raghubeer et al., 2021; Tsang et al., 2015). However, unlike homocysteine, which reflects downstream metabolic consequences of impaired folate systems, serum folate reflects the folate status more directly; this makes it a mechanistically distinct but complementary indicator in the TT biomarker profile (Colson et al., 2017; Raghubeer et al., 2021).

Main Evidence:

A meta-analysis published in European Journal of Nutrition in 2017 found that TT carriers exhibited an average decrease in serum folate levels at 2.5 nmol/L when compared to CC/CT carriers, and decreased by 3.6 nmol/L when compared to CC carriers alone (Colson et al., 2017). This supports the assertion that there is a directional effect of the C677T genotype on serum folate, with T allele dosage correlating to a decrease in serum folate (Colson et al., 2017). Additionally, while this investigation is not focused on treatment-based improvement, it is worth noting that this publication included an intervention study that provided folic acid supplementation of 1,670 μg DFEs for at least 4 weeks that displayed some relevant results (Colson et al., 2017). After supplementation, all genotypes displayed non-significant levels of homocysteine, but TT individuals showed a persistently low serum folate response, with a difference of 7.2 nmol/L versus CT/CC and 8.7 nmol/L versus CC (Colson et al., 2017). Since TT group serum folate levels remained lower even after intervention, it indicates there may be a weaker biochemical response or persistently low folate status despite folic acid exposure, and supports serum folate as a mechanistically distinct biomarker from homocysteine (Colson et al., 2017).

Another peer-reviewed study supported this directionality by identifying genotype-stratified serum folate thresholds of 7.5, 10.2, and 11.1 nmol/L for CC, CT, and TT genotypes, respectively (Girelli et al., 2003). The values represent the lowest folate levels needed to maintain mean homocysteine levels within normal range per genotype, which indicates that increasing T allele dosage requires progressively increasing folate levels to maintain normal metabolic function (Girelli et al., 2003). These findings are supported by meta-analytic evidence indicating that serum folate concentrations are 10-20% lower in TT carriers than CC carriers, with CT carriers showing intermediate levels (Tsang et al., 2015). This is consistent with the co-dominant model in which each T allele present contributes incrementally to reduce serum folate status (Tsang et al., 2015). In corroboration with this, a smaller study that uses LC-MS/MS (n=115) found mean serum folate of 6.7 ± 4.0 nmol/L in TT carriers versus 12.2 ± 8.0 nmol/L in CC carriers at P < 0.001 confidence (Rooney et al., 2020). This ~45% difference exceeds the meta-analytic estimates and likely reflects considerable in-group variability, though it still fully supports the directionality aspect consistently (Rooney et al., 2020).

Directional Trend and Confidence Assessment:

Reduced folate status is a stable component of the TT biomarker profile across meta-analytic, GWAS, and individual studies (Colson et al., 2017; Shane et al., 2018; Tsang et al., 2015; Rooney et al., 2020). At the GWAS level, the 677T allele has been confirmed to be a significant determinant of lowered serum folate at genome-wide significance (P = 2.82 × 10⁻¹¹), though genotype-stratified values are not available from this source (Shane et al., 2018). The results of these studies help to distinguish folate as a separate indicator biomarker from homocysteine (Colson et al., 2017). Homocysteine differences can disappear after supplementation, but serum folate differences indicate persistence despite supplementation, which suggests that serum folate may be the more durable genotype-sensitive indicator of the two (Colson et al., 2017). This is an important distinction to note for analysis and future study since homocysteine can function as a modifiable marker while serum folate appears to reflect the underlying enzymatic constraint of the TT genotype consistently.

1.3.1.3: RBC Folate

RBC folate, like serum folate, is a folate metric used frequently to assess disease states, but unlike serum folate, RBC folate reflects longer-term status and is less susceptible to short-term dietary fluctuations (Raghubeer et al., 2021). This stability makes it a more mechanistically informative complement to serum folate while still demonstrating a consistent genotype-stratified pattern across C677T literature.

Main Evidence:

The strongest and most rigorous evidence comes from a Bayesian meta-analysis that restricted studies based on microbiological assay methodology, which is the gold standard for population folate assessment (Tsang et al., 2015). The six studies that were included showed RBC folate followed a clear CC > CT > TT stepwise pattern, with TT carriers averaging 16% lower than CC (95% CrI: 12%, 20%), CT carriers 8% lower than CC (95% CrI: 4%, 12%), and TT carriers 9% lower than CT (95% CrI: 5%, 13%); in this study’s TT carriers, the RBC folate deficiency exceeded the serum folate deficiency (16% and 13%, respectively) (Tsang et al., 2015). This discrepancy suggests that RBC folate may be a more sensitive genotype-specific marker, which is mechanistically consistent with established pathways given that RBC folate reflects longer-term status instead of circulating concentration (Raghubeer et al., 2021).

These findings are corroborated by another large population-based RCT that provided full genotype-stratified baseline metrics: TT at 552.2 nmol/L, CT at 616.3 nmol/L, and CC at 669.0 nmol/L (Crider et al., 2011). This shows TT RBC folate as significantly lower than both other genotypes (CT/CC), both P < 0.001, though CC vs. CT fell just short of significance at P = 0.06 (Crider et al., 2011). These results are consistent with the asymmetric co-dominant effect suggested by the Tsang estimates, especially given that the TT < CC trend was maintained across all doses in the trial (Tsang et al., 2015; Crider et al., 2011). However, throughout the trial, even at the highest supplementation doses (4000 μg/d), TT carriers never achieved CC-level RBC folate concentrations, which reinforces the emerging pattern that the genotype effect is not recovered with supplementation (Crider et al., 2011). Supporting this, a smaller LC-MS/MS study (n = 115) found RBC folate approximately 23% lower in TT versus CC carriers at P = 0.045, which is directionally consistent, though it’s important to note this study is limited by absent CT data and displays within-group variability, though that feature consistent with other studies as well (Rooney et al., 2020).

Additionally, a GWAS across 2,232 healthy Irish adults was able to confirm MTHFR C677T as the dominant genetic modifier of RBC folate at a genome-wide significance (P = 1.37 × 10⁻17); this was done by mapping all 32 SNPs of significance onto the MTHFR region of chromosome 1, and C677T displayed the strongest signal by a margin of over 4 orders to the next strongest variant (Shane et al., 2018). When the experiment was repeated with homozygotes (CC) alone, none of the MTHFR-region SNPs remained significant, which confirmed that the signal is driven specifically by T allele dosage rather than linkage with other variants (Shane et al., 2018). Additionally, this study was able to determine that the C677T polymorphism accounts for 7% of total variance in RBC folate concentrations (Shane et al., 2018). This is analytically significant since a single SNP explaining 7% of total variance in a continuously distributed biomarker is a large effect in the context of complex genetic traits.

Directional Trend and Confidence Assessment:

Decreased RBC folate in the stepwise pattern of TT < CT < CC is highly directionally consistent and supported across meta-analytic, RCT, and GWAS studies (Tsang et al., 2015; Crider et al., 2011; Shane et al., 2018). For RBC folate specifically, the CC versus CT distinction is less statistically robust than TT versus either group in the other biomarker syntheses, so caution is warranted surrounding heterozygote-specific assertions. Additionally, since the RBC folate deficit in TT carriers appeared to exceed the folate serum deficit even with supplementation, the assertion that RBC folate is a stable, genotype-sensitive biomarker is strengthened (Tsang et al., 2015; Crider et al., 2011). The only primary limitation across the evidence is the sex-based restriction to female populations in both the Tsang meta-analysis and Crider RCT, though generalizability to mixed-sex adult populations was directionally supported by the Irish GWAS (Tsang et al., 2015; Crider et al., 2011; Shane et al., 2018).

1.3.2 Set 2 — Cofactor and Compensatory Pathway Biomarkers

This section examines vitamin B6/PLP, betaine, vitamin B12, and methionine for trends in conjunction with MTHFR C677T polymorphisms.

1.3.2.1: Vitamin B6

Unlike the previously reviewed biomarkers, vitamin B6, which is measured in its actively circulating form, pyridoxal-5’-phosphate (PLP), does not show a simple, genotype-stratified effect in current C677T literature (Midttun et al., 2007; Hustad et al., 2007). Rather, the relationship between C677T and PLP appears to function more akin to a modifier effect: the TT genotype seems to amplify existing metabolic consequences of low B6 status rather than producing lower B6 levels independently, which is not only mechanistically distinct, but also is supported across available research (Midttun et al., 2007; Jarrett et al., 2022).

Main Evidence:

The strongest evidence comes from the largest available dataset, the NORCCAP cohort, which measured PLP via LC-MS/MS with full MTHFR genotyping of 10,601 individuals. The results showed median concentrations of 47.9 nmol/L for CC carriers, 49.0 nmol/L for CT carriers, and 44.4 nmol/L for TT carriers, which while directionally consistent with the TT disadvantage established in other biomarkers, was not statistically significant, at P = 0.34 (Midttun et al., 2007). However, the same dataset was able to establish a significant genotype interaction with B6 on homocysteine at P = 0.006, when stratifying CC/CT versus TT. CC and CT carriers showed a 0.5 μmol/L homocysteine difference in the highest and lowest PLP, whereas TT carriers showed a 2.1 μmol/L difference (Hustad et al., 2007). This difference indicates that TT carriers are substantially more dependent on sufficient B6 status in order to properly regulate homocysteine (Hustad et al., 2007; Midttun et al., 2007).

This conditional status is further supported by a large multi-cohort study (n = 5,612) that examined PLP across MTHFR genotype and riboflavin (B2) status (Jarrett et al., 2022). The results showed that when B2 levels were sufficient, TT carriers showed no significant difference in PLP concentrations compared to non-TT carriers at P = 0.704, which indicates that sufficient B2 can maintain PLP regardless of genotype; however, under B2-deficient conditions, TT carriers showed PLP levels of 52.1 ± 2.87 nmol/L compared to approximately 60 nmol/L in CC/CT deficient individuals, at a confidence of P = 0.016 (Jarrett et al., 2022). This further indicates additional vulnerability to PLP depletion specifically in B2-deficient TT carriers (Jarrett et al., 2022). This is mechanistically coherent, since B2 is a required cofactor for pyridoxine-5’-phosphate oxidase, the enzyme that produces PLP, which would explain why B2-deficient TT carriers would be further exposed to PLP depletion than other cohorts (Jarrett et al., 2022). Additionally, a smaller LC-MS/MS study (N = 115; CC n = 68, TT n = 47) found mean PLP approximately 35% lower in TT versus CC carriers with 47.5 ± 22.2 vs. 72.0 ± 38.3 nmol/L, respectively, at P < 0.001 (Rooney et al., 2020). While this does provide significant and direct genotype comparison, the large standard deviations and lack of heterozygosity consideration limit its contribution to the directionality assertion.

Directional Trend and Confidence Assessment:

In summary, the directional genotype-stratified trend of PLP is not statistically supported, with the strongest study returning P = 0.34 (Midttun et al., 2007). This is likely reflective of biological realities and not a gap in existing literature. Even with the three-way genotype stratification from the NORCCAP cohort displaying a directionality trend across CC, CT, and TT, it did not reach statistical significance, which limits the strength of the directional claim for PLP as a direct genotype indicator (Midttun et al., 2007). However, the evidence does establish a high confidence that TT carriers demonstrate significantly amplified sensitivity to B6 and B2 deficiencies, which can produce meaningfully lower PLP in those specific conditions (Midttun et al., 2007; Jarrett et al., 2022). This suggests PLP functions as a conditional vulnerability biomarker rather than a stable depletion or inflation signal, which is the mechanistically distinct finding.

1.3.2.2: Betaine

As mentioned above in section 1.1.6, betaine functions as an alternative methyl donor through the betaine-homocysteine methyltransferase (BHMT) pathway, unlike primary folate pathway metabolites, which allows betaine to operate independently of folate availability (Raghubeer et al., 2021). Current literature supports the importance of the BHMT pathway, particularly for individuals suffering from folate pathway dysfunction, like TT carriers, since methylation dysfunction causes an increased reliance on BHMT for homocysteine recycling (Holm et al., 2007).

Main Evidence:

The strongest evidence of this pattern comes from the NORCCAP cohort study that also supported the PLP profile investigation, which demonstrated that betaine functioned as a strong determinant of plasma homocysteine in TT individuals with low serum folate concentrations, especially those with low B vitamin status; specifically, for TT carriers with low serum folate and B vitamin deficiencies, the difference in homocysteine across betaine quartiles was 8.8 μmol/L (95% CI: 1.3–16.2), indicating a substantial metabolic dependence on the betaine pathway function (Holm et al., 2007).

A recent supplementation study provided further direct evidence of genotype-stratified altered betaine metabolism, with plasma betaine base concentrations occurring at 4.52 ± 1.20 μg/mL for CC carriers and 5.31 ± 1.88 μg/mL for CT/TT carriers, showing numerically elevated betaine concentrations for individuals carrying the T allele (Zawieja et al., 2024). Additionally, after betaine supplementation, T-allele carriers demonstrated significantly greater responses than their CC counterparts to a statistically significant degree of P = 0.027; this difference in response pattern supports the assertion that betaine is used in enhanced capacity for TT carriers (Zawieja et al., 2024). A smaller LC-MS/MS study confirmed no significant genotype-stratified concentration differences with CC individuals displaying concentrations of 53.1 ± 13.7 μmol/L vs TT individuals showing concentrations of 50.5 ± 15.8 μmol/L at P = 0.194, further indicating that betaine’s role in the TT biomarker profile is primarily functional rather than concentration-based (Rooney et al., 2020). In addition, a smaller isotopic flux study further confirmed that while baseline betaine concentrations were similar across homozygous genotypes (CC: 56 ± 7.9 μmol/L vs. TT: 51 ± 3.5 μmol/L, at a P = 0.418), TT carriers demonstrated significantly higher betaine pathway usage through multiple flux indicators, including elevated betaine:choline enrichment ratios (P = 0.041) and increased urinary sarcosine enrichment (P = 0.041) (Yan et al., 2011). This reflects compensatory pathway upregulation since elevated betaine:choline enrichment ratios measure the conversion rate of choline to betaine, meaning these values demonstrate greater demand for betaine synthesis in TT carriers (Yan et al., 2011). Additionally, urinary sarcosine enrichment (USE) is a metric for betaine oxidation; since sarcosine is a downstream metabolite of the BHMT pathway, increased USE levels in this context further support the idea that TT carriers show increased active BHMT pathway utilization as a compensatory method against impaired folate-dependent methylation (Yan et al., 2011).

Directional Trend and Confidence Assessment:

Overall, the literature indicates that betaine demonstrates a pattern of enhanced metabolic importance and utilization for TT carriers specifically, rather than standard genotype-stratified concentration differences. The directional signaling present reflects compensatory upregulation, with TT carriers showing similar baseline concentrations with greater metabolic responsiveness to betaine availability (Zawieja et al., 2024; Holm et al., 2007). These findings are particularly relevant for liver biology, since betaine metabolism is involved in hepatic lipid regulation through the choline-betaine-phosphatidylcholine axis, which could provide a potential mechanistic bridge between MTHFR C677T genotype and fatty liver pathophysiology (Kitagawa et al., 2017; Christensen et al., 2025). This relationship will be explored more in depth in the integrated discussion portion of the project.

1.3.2.3: Vitamin B12

While betaine’s role in one-carbon metabolism is primarily within the alternative BHMT pathway, vitamin B12 is an essential cofactor for the primary folate re-methylation cycle (Leclerc et al., 2013). Population-level B12 concentrations show no significant association with C677T genotypes under normal conditions, though emerging evidence suggests TT carriers exhibit increased vulnerability to B12 deficiency, and experience conditionally amplified metabolic consequences when deficient (Shane et al., 2018; Shiran et al., 2015).

Main Evidence:

Large population studies in healthy adults consistently show a lack of significant differences in MTHFR genotype-stratified B12 concentrations; a comprehensive analysis of 2,232 Irish adults reported mean B12 concentrations of approximately 319 and 350 pmol/L for women and men, respectively (Shane et al., 2018). This study measured B12 as a covariant and found no significant association with the C677T genotype, which establishes that in healthy individuals with adequate nutrition, MTHFR genotype does not impact circulating B12 levels (Shane et al., 2018).

However, when B12 status drops to marginal or deficient levels, a conditional vulnerability emerges. One case-control study of 360 adults showed that TT carriers had significantly increased rates of B12 deficiency compared to CC/CT individuals, with 29.8% and 9.2% deficient rates, respectively at P < 0.0001 (Zittan et al., 2007). This meant that at a 95% confidence interval, the authors concluded TT individuals had a 4.2 times increased risk of B12 deficiency compared to CC/CT individuals (95% CI: 2.1–8.3), which is a significant difference that indicates substantial genotype-stratified susceptibility (Zittan et al., 2007). It is important to note that TT carriers who were also B12-deficient exhibited markedly elevated homocysteine concentrations (20.6 ± 18.8 μmol/L) compared to non-TT B12-deficient carriers (9.4 ± 3.2 μmol/L, at P < 0.0001), which is a notable demonstration of the conditional, compounding, and downstream metabolic effects of nutritional status on specific genotypes, like C677T polymorphisms (Zittan et al., 2007). These trends were independently confirmed by another case-control study that showed similar statistically significant findings of B12 deficiency rates (TT: 28% vs CC/CT: 15%, at P = 0.005) and elevated homocysteine levels under B12-deficient conditions (TT: 21.2 ± 16 vs. CC/CT: 12.3 ± 5.6 μmol/L, at P = 0.008), confirming the trend that TT genotype is at higher risk of B12 deficiency and exacerbates the metabolic impact of this deficiency downstream (Shiran et al., 2015).

Directional Trend and Confidence Assessment:

Large-scale population studies, cross-sectional examinations, and targeted genotype analyses all support the conclusion that a conditional B12 vulnerability pattern exists for TT carriers, and it is characterized by normal baseline concentrations with an increased susceptibility to deficiency that can amplify downstream metabolic consequences when B12 status is compromised (Shane et al., 2018; Zittan et al., 2007; Shiran et al., 2015). This conditional gene-nutrient interaction indicates that while MTHFR C677T does not influence baseline B12 levels, it can alter susceptibility to metabolic dysfunction. This aligns with established mechanistic literature; since B12 is a required cofactor of methionine synthase, reduced B12 availability would logically amplify any downstream accumulation of homocysteine, which for individuals with impaired folate pathways, like TT carriers, could substantially worsen an already existing homocysteine:methionine imbalance (Leclerc et al., 2013; Shane et al., 2018).

1.3.2.4: Methionine

Methionine is the direct product of folate-dependent remethylation of homocysteine and functions as a precursor to S-adenosylmethionine (SAM), which acts as the universal methyl donor in one-carbon metabolism (Leclerc et al., 2013). Since impaired MTHFR activity in T allele carriers reduces remethylation efficiency, lower circulating methionine levels could be plausibly anticipated in this genotype profile (Leclerc et al., 2013).

Main Evidence:

However, the limited available literature does not consistently support this expectation. The strongest data comes from the NORCCAP cohort study (n=10,601), which reported mean plasma methionine of 23.5, 23.6, and 23.4 μmol/L for CC, CT, and TT carriers respectively, at P = 0.186, showing no significant difference in concentrations across genotypes (Midttun et al., 2007). This study also included a partial Spearman correlation analysis between homocysteine and methionine, finding r = -0.07 (Midttun et al., 2007). This very weak, negative association further suggests that methionine does not track in conjunction with genotype-driven elevated homocysteine pattern despite close pathway associations and interactions. Two small, independent studies corroborated these null findings. One of these was a smaller LC-MS/MS study that reported plasma methionine concentrations of 29.5 ± 7.2 μmol/L for CC and 30.3 ± 6.7 μmol/L for TT individuals at P = 0.450, which, while statistically insignificant, still counters the established pattern expectation given that the CC cohort displayed numerically lower methionine concentrations than the TT individuals (Rooney et al., 2020). Another study further supported these null findings through urinary methionine data reports that showed a similar absence of genotypic effect, though it does mirror the unexpected pattern previously observed of TT carriers displaying higher average concentrations than their CC counterparts (TT: 9.5 ± 1.5 vs CC: 7.4 ± 1.2 μmol/L, at P = 0.356) (Yan et al., 2011). This consistent directional pattern spans three independent studies that use three distinct measurement approaches, supporting the plausibility of a genuine mechanistic phenomenon rather than measurement artifact (Midttun et al., 2007; Rooney et al., 2020; Yan et al., 2011). One plausible contributor to the maintained circulating methionine levels despite impaired folate methylation pathways is compensatory upregulation of the BHMT pathway, which remethylates homocysteine independently of the folate pathway (Holm et al., 2007). That would be mechanistically consistent with the established upregulation of BHMT observed in TT carriers, which is discussed more in depth in section 1.3.2.2 (Holm et al., 2007; Yan et al., 2011). However, there are likely other methylation pathways and cellular mechanisms contributing to this specific phenomenon, warranting further and expanded investigation (Martinez et al., 2017).

Importantly, the LC-MS/MS study data revealed that while circulating methionine is preserved regardless of genotype, downstream methylation capacity is not, which could provide a mechanistic explanation for the observed directional pattern. The study showed that TT carriers displayed significantly lower plasma SAM concentrations (TT: 74.7 ± 21.0 vs CC: 85.2 ± 22.6 nmol/L, at P = 0.013) compared to CC individuals (Rooney et al., 2020). As the universal methyl donor, SAM concentration reflects active methylation capacity; therefore, a decrease in SAM in this context suggests that despite normal methionine supply, TT carriers experience a compromised conversion rate between methionine and usable methyl groups (Martinez et al., 2017). This is further supported by the SAM:SAH ratio results which demonstrate the same directional trend (TT: 1.66 ± 0.55 vs CC: 1.85 ± 0.51, at P = 0.043) (Rooney et al., 2020). S-adenosylhomocysteine (SAH) is the direct byproduct of SAM after methyl group donation, and the SAM:SAH ratio is used to directly reflect active methylation potential (Martinez et al., 2017; Rooney et al., 2020). Together, a decrease in both SAM and SAM:SAH levels indicates that TT carriers exhibit methylation dysfunction driven not only by methyl group supply deficits, but also active inhibition of methylation potential, even when methionine concentrations appear normal (Rooney et al., 2020).

Directional Trend and Confidence Assessment:

Overall, the literature indicates that circulating methionine levels are not significantly impacted by the C677T genotype, but T allele dosage is still associated with a consistent pattern of downstream methylation dysfunction regardless; confidence in this null finding is high given the statistical power of the NORCCAP cohort studies and the cross-methodological consistency in trends across all three independent studies (Midttun et al., 2007; Rooney et al., 2020; Yan et al., 2011). The SAM and SAM:SAH evidence further indicates that preserved methionine should not be interpreted as preserved methylation capacity, which is a distinction with meaningful implications that will be discussed in the integrated profile (Rooney et al., 2020).

1.3.3 Set 3 — Downstream and Diagnostic Biomarkers

This section examines methylmalonic acid (MMA), C-reactive protein (CRP), ferritin, and gamma-glutamyl transferase (GGT) for trends in conjunction with MTHFR C677T polymorphisms.

1.3.3.1: MMA

Methylmalonic acid (MMA) is a byproduct created when a vitamin B12-dependent enzyme, methylmalonyl-CoA mutase (MCM), converts methylmalonyl-CoA (MM-CoA) to succinyl-CoA in fat and protein metabolism processes (Molloy et al., 2016). This means circulating MMA levels are dependent on, and therefore indicative of, vitamin B12 enzymatic activity, which is indirectly linked to folate-dependent remethylation pathway function through their shared B12 pathway reliance (Leclerc et al., 2013; Molloy et al., 2016). Elevated homocysteine is associated with both folate and B12 deficiencies, which makes pathway-specific (folate vs. B12) attributions difficult to determine (Liew & Gupta, 2015). However, since MMA is not directly involved in the folate cycle and only increases when B12-specific pathways are impaired, MMA biomarker synthesis can help isolate that determination, given we would expect MMA levels to increase with homocysteine if B12-deficiency was the predominant driving pathway of the profile (Molloy et al., 2016; Fredriksen et al., 2007). That indirect link is why MMA’s inclusion in the TT MTHFR profile is valuable: it functions as a diagnostic metric of mechanism to determine if B12 deficiency is a confounding or legitimate contributor of elevated homocysteine.

Main Evidence:

All reviewed literature indicates that elevated MMA levels are not associated with genotype-specific responses for MTHFR C677T (Molloy et al., 2016; Fredriksen et al., 2007; Barbosa et al., 2008). The strongest evidence comes from a GWAS (n=2,210) that was specifically designed to identify genetic determinants of elevated MMA, screening 758,443 candidate SNPs for association and measuring plasma MMA at a cohort-wide median of 0.17 μmol/L (IQR: 0.14–0.21) (Molloy et al., 2016). It determined the top dominant genetic determinants of MMA were HIBCH (rs291466) and ACSF3 (~12% of MMA), which are related to the valine catabolism and MM-CoA synthesis, respectively, and are both entirely unrelated to both folate and B12 remethylation cycles; it also directly stated that MTHFR C677T produced no genome-wide significant signal for MMA in both the discovery cohort and in an independent replication cohort of 1,481 older Irish adults, further confirming the null findings (Molloy et al., 2016). It is important to note the power scale of this study: it is designed and executed to detect effects at genome-wide level, and its determination of a notable absence of MTHFR related signaling avoids the inconclusiveness often associated with null results given their frequent partial attribution to the power-limitation of their studies. A small Brazilian study (n=102) that used the validated GC-MS assay method for MMA quantification and included C677T genotyping provided additional null corroboration; it determined that serum cobalamin and creatinine were the sole examined determinants of MMA concentration and incidentally reported MMA experienced no genotypic effect during their investigation into cobalamin deficiency pathways (Barbosa et al., 2008).

The null findings are further backed by another study that used the same NORCCAP cohort (n=10,601) included in some of the earlier biomarker profile syntheses, which measured MMA and 12 additional one-carbon metabolites across genotyped individuals, looking specifically at 13 one-carbon-related polymorphisms, including MTHFR C677T (Fredriksen et al., 2007). It confirmed MTHFR as the highest predictor of folate and homocysteine out of all measured polymorphisms, which aligns with this analysis’s findings but returned no significant association with MMA in the same individuals; however, MMA was significantly associated with TCN2 C776G at P < 0.001, which is a polymorphism of the transcobalamin-II gene and is involved in B12 transport pathways, unrelated to folate pathways (Fredriksen et al., 2007). This internal contrast of strong homocysteine and folate detection with no MMA detection suggests the two pathways are metabolically and genetically distinct, at both genomic and population levels.

Directional Trend and Confidence Assessment:

All three reviewed studies, which included a GWAS, a large-scale population-based metabolic phenotyping, and an independent observational cohort, unanimously found that MTHFR C677T genotype does not predict MMA concentration, and no TT > CT > CC gradient appears in any dataset (Molloy et al., 2016; Fredriksen et al., 2007; Barbosa et al., 2008). The methodological rigor and population coverage across these studies indicates a high confidence in these consistently null results, which are mechanistically significant for the TT profile. Because B12 deficiency would increase both homocysteine and MMA levels, the absence of elevated MMA indicates that, under baseline conditions, elevated homocysteine associated with the TT profile is primarily folate-driven, which is mechanistically coherent with earlier biomarker syntheses and strengthens the profile (Leclerc et al., 2013; Molloy et al., 2016).

1.3.3.2: CRP

C-reactive protein (CRP) is a hepatic protein that is used commonly as a low-grade, chronic inflammation marker for various disease states, which is measured frequently in studies as high-sensitivity CRP (hs-CRP) (Yeniova et al., 2014). While CRP is not a one-carbon metabolite, it is implicated in the MTHFR C677T profile since the polymorphism is strongly associated with elevated homocysteine levels, which is, in turn, a known trigger for systemic inflammation (Matté et al., 2009). While literature investigating the relationship between MTHFR C677T and CRP is sparse, existing sources largely indicate a conditional, modest, directional relationship (Dedoussis et al., 2005; Bhatt et al., 2025).

Main Evidence:

The strongest evidence for this relationship comes from the ATTICA study, which is a cross-sectional population analysis of Greek adults (n=574) that stratified their data by age, sex, and genotype to investigate how MTHFR impacts risk of cardiovascular disease; it is important to note that the authors explicitly describe this source as the first reported association between MTHFR C677T and CRP and that this CRP data was log-transformed and adjusted using ANCOVA for a variety of factors including: age, diet, pack-years smoking, BMI, physical activity, total cholesterol, and education (Dedoussis et al., 2005). The results after transformation showed hs-CRP was significantly elevated in TT individuals across both sexes (Male: CC 1.3 ± 1.1, CT 1.8 ± 0.9, TT 2.1 ± 0.4 mg/dL, P = 0.01; females: CC 1.2 ± 1.1, CT 1.6 ± 1.2, TT 1.9 ± 0.9 mg/dL), at P = 0.03 (Dedoussis et al., 2005). While this evidence numerically shows a directional trend, the authors specifically noted a TT-elevated trend instead of TT-stepwise gradient since only comparisons between homozygotes (CC vs TT) reached statistical significance at Bonferroni, α < 0.01 (Dedoussis et al., 2005).

This evidence is corroborated by a smaller cross-sectional analysis (n=489) that spans three Nepali ethnic groups and observed the same TT > CT > CC trend in hs-CRP levels, with CC individuals averaging 1.7 ± 1.2 mg/L, CT individuals averaging 1.9 ± 1.3 mg/L, and TT individuals averaging 2.0 ± 1.4 mg/L, though these results were not considered statistically significant at P = 0.16 (Bhatt et al., 2025). However, this study confirmed a homocysteine-CRP correlation that was established in the previously mentioned study (Dedoussis r = 0.45, Bhatt r = 0.47, both P < 0.001), which is noteworthy given the near identical association value found across these cohorts that span two independent populations on different continents (Dedoussis et al., 2005; Bhatt et al., 2025).

Directional Trend and Confidence Assessment:

The cross-cohort homocysteine-CRP correlation combined with the explicit findings of Dedoussis et al. (2005), which included the explicit finding that this association is independent of MTHFR genotype, supports a secondary effect more than a directional genotypic trend (Dedoussis et al., 2005; Bhatt et al., 2025). Specifically, it indicates that while C677T may direct homocysteine level elevation, homocysteine itself functions as the proximate mechanism driver of this inflammation marker in this specific context. In this framing, CRP elevation functions as a conditional, downstream effect in TT carriers where elevated homocysteine is already present. While evidence-based, these conclusions are supported exclusively by small-sized cross-sectional studies that inherently preclude causal relationships (Dedoussis et al., 2005; Bhatt et al., 2025). Therefore, this portion of the profile should be viewed as a potential mechanistic link between MTHFR genotype, homocysteine, and systemic inflammation, rather than direct evidence, though further investigation is warranted.

1.3.3.3: GGT & Ferritin

Gamma-Glutamyl Transferase (GGT) is a hepatic enzyme involved in glutathione metabolism and oxidative stress pathways (Dillon & Miller, 2016). Because of its sensitivity to hepatic oxidative stress generated by homocysteine accumulation and its association with hyperhomocysteinemia, GGT was a strong candidate for consideration in the biomarker profile construction of the MTHFR C677T variant (Lippi et al., 2008). However, there is a lack of peer-reviewed studies that report GGT stratified by MTHFR C677T genotype in a healthy population using biologically measured, extractable data. This literature gap likely reflects the indirect relationship between GGT and the folate cycle, rather than a true absence of association. However, at this time there is not enough peer-reviewed literature to determine a relationship between GGT and MTHFR C677T.

Ferritin, the storage protein for iron, was also an initial candidate for the profile since elevated levels are also associated with oxidative stress and inflammation (Salem et al., 2024). Like GGT, ferritin exhibits indirect interactions with the folate cycle, with elevated levels functioning as a responsive marker of oxidative stress and inflammatory signaling. Ferritin also appears sparsely in the C677T genotype-stratified literature in a form that meets the inclusion criteria of this analysis.

GGT and ferritin are both plausible contextual markers, but they lack direct genotype-stratified evidence in the currently available literature. They could not reasonably be incorporated into this profile, but will be revisited in the Part 2 analysis on Fatty Liver Disease (FLD) classification and associated biomarkers.

--- title: "1.3 Individual Biomarker Analysis" format: html: toc-depth: 5 engine: knitr --- For this profile, the following biomarkers were investigated: homocysteine, serum folate, folate (RBC), vitamin B6, betaine, vitamin B12, methylmalonic acid (MMA), gamma-glutamyl transferase (GGT), ferritin, C-reactive protein (CRP), and methionine. These biomarkers are grouped into three tiers based on both metabolic role and evidence strength. The first tier focuses on primary folate pathway biomarkers. Biomarkers like homocysteine and folate forms are consistently studied in conjunction with polymorphisms in folate-related genes due to their direct relationship with the MTHFR mechanism (Liew & Gupta, 2015). Due to their strong, closely-linked metabolic pathways, these markers are most often studied, and therefore have some of the most established track records and patterns in current literature (Colson et al., 2017). The second set focuses on cofactors and compensatory pathway biomarkers, which accounts for conditional or interactive effects. This set will include factors like vitamin B6, which is a cofactor for the transsulfuration pathway, or betaine, which is involved in another major compensatory pathway, the BHMT route, which is further discussed in Part 1.6 (Hustad et al., 2007; Abratte et al., 2008). The third set focuses on downstream and diagnostic markers. These are biomarkers like CRP, which indicates systemic inflammation, or Vitamin B12, which is mechanistically relevant, but effects are seen downstream with weak, null, or conflicting results (Bhatt et al., 2025; Shiran et al., 2015). ## **1.3.1 Set 1 — Primary Folate Pathway Biomarkers ** This section examines homocysteine, serum folate, and RBC folate for trends in conjunction with MTHFR C677T polymorphisms. ### 1.3.1.1: Homocysteine Due to its prominent role in the methionine cycle, homocysteine is one of the most investigated and documented biomarkers in relation to MTHFR’s C677T polymorphism (Raghubeer et al., 2021; Liew & Gupta, 2015). As described in section 1.3, homocysteine is a byproduct of methylation that is normally recycled back to methionine via the folate-dependent re-methylation pathway; when MTHFR activity is reduced, like in TT homozygotes, 5-MTHF availability decreases, which impairs this recycling process and allows homocysteine to accumulate in circulation (Raghubeer et al., 2021). Main Evidence: This assertion that TT carriers display elevated homocysteine is supported by multiple studies, including a meta-analysis published in 2017 that included 10 folate intervention studies, which reported that the TT genotype was associated with plasma homocysteine concentrations 2.7 μmol/L higher than CT/CC combined groups, and 2.8 μmol/L higher than the CC group alone (Colson et al., 2017). It is worth noting that 20% of the studies included in the meta-analysis demonstrated TT groups with mean homocysteine concentrations >15 μmol/L, which is the clinical threshold for hyperhomocysteinemia (Colson et al., 2017). This concentration only appears in an estimated 5-7% of the general populace, and therefore suggests that the TT genotype effect is clinically meaningful under certain conditions (Qureshi et al., 2019). A GWAS published in 2018 from a large, healthy Irish population had similar findings, confirming a clear stepwise increase with T allele dosage, with CC (wildtype) individuals averaging 7.8–8.4 μmol/L, CT individuals averaging 8.0–8.9 μmol/L, and TT individuals averaging 9.2–11.0 μmol/L at a confidence of P = 1.26 × 10-19 (Shane et al., 2018). These results are particularly significant when considering the GWAS’s design, which ruled out confounding factors via population stratification, confirming MTHFR locus as a genuine determinant of homocysteine concentration rather than an associated metric (Shane et al., 2018). This consistent stepwise pattern is further supported by a paper published in 2007 that reported median homocysteine of 10.4 μmol/L in CC carriers, 10.9 μmol/L in CT carriers, and 13.3 μmol/L in TT carriers from the Norwegian NORCCAP cohort, which consisted of 10,601 adults (Hustad et al., 2007). Additionally, two smaller studies further corroborated these findings. One reported concentrations of 9.3 ± 2.5 μmol/L in CC carriers and 10.4 ± 3.0 μmol/L in TT carriers (P = 0.043) in 115 healthy adults, which is directionally consistent with other results despite limited sample size (Rooney et al., 2020). The second study reported similar levels, but they incorporated sex-stratification, finding TT men displayed particularly elevated levels, as shown in the following report: men (TT: 19 ± 7 vs CC: 12 ± 8 μmol/L, P < 0.001) and women (TT: 14 ± 7 vs CC: 9 ± 7 μmol/L, P = 0.01) (Dedoussis et al., 2005). It is important to note that these values are derived from a log-transformed analysis, which means they cannot be directly quantitatively compared to other included studies. However, an independent European cohort that used a different assay platform and significant confounder adjustment supported the directional findings, which strengthened overall confidence in this source’s contribution to the homocysteine biomarker synthesis. This is strengthened further by the fact that the proportionally larger TT elevation observed in men is consistent with known sex-based differences in homocysteine metabolism reported across the broader literature (Dedoussis et al., 2005). Directional Trend and Confidence Assessment: Across all five reviewed studies, the directional trend is unanimous: increased T allele dosage is directionally associated with increased homocysteine levels (TT > CT > CC). This trend was replicated across GWAS, meta-analyses, large observational studies and smaller, independent cohort-based designs. Together, they support the assertion that elevated homocysteine in TT carriers is the most robustly established downstream consequence of reduced MTHFR C677T enzyme activity in the current literature, and it serves as the mechanistic anchor for the biomarker profile that is developed in the following sections (Colson et al., 2017; Shane et al., 2018; Hustad et al., 2007; Rooney et al., 2020; Dedoussis et al., 2005). ### 1.3.1.2: Serum Folate Serum folate is one of the primary substrates depleted by MTHFR dysfunction, and like homocysteine, is a frequently examined biomarker in literature on MTHFR C677T (Liew & Gupta, 2015; Raghubeer et al., 2021; Tsang et al., 2015). However, unlike homocysteine, which reflects downstream metabolic consequences of impaired folate systems, serum folate reflects the folate status more directly; this makes it a mechanistically distinct but complementary indicator in the TT biomarker profile (Colson et al., 2017; Raghubeer et al., 2021). Main Evidence: A meta-analysis published in European Journal of Nutrition in 2017 found that TT carriers exhibited an average decrease in serum folate levels at 2.5 nmol/L when compared to CC/CT carriers, and decreased by 3.6 nmol/L when compared to CC carriers alone (Colson et al., 2017). This supports the assertion that there is a directional effect of the C677T genotype on serum folate, with T allele dosage correlating to a decrease in serum folate (Colson et al., 2017). Additionally, while this investigation is not focused on treatment-based improvement, it is worth noting that this publication included an intervention study that provided folic acid supplementation of 1,670 μg DFEs for at least 4 weeks that displayed some relevant results (Colson et al., 2017). After supplementation, all genotypes displayed non-significant levels of homocysteine, but TT individuals showed a persistently low serum folate response, with a difference of 7.2 nmol/L versus CT/CC and 8.7 nmol/L versus CC (Colson et al., 2017). Since TT group serum folate levels remained lower even after intervention, it indicates there may be a weaker biochemical response or persistently low folate status despite folic acid exposure, and supports serum folate as a mechanistically distinct biomarker from homocysteine (Colson et al., 2017). Another peer-reviewed study supported this directionality by identifying genotype-stratified serum folate thresholds of 7.5, 10.2, and 11.1 nmol/L for CC, CT, and TT genotypes, respectively (Girelli et al., 2003). The values represent the lowest folate levels needed to maintain mean homocysteine levels within normal range per genotype, which indicates that increasing T allele dosage requires progressively increasing folate levels to maintain normal metabolic function (Girelli et al., 2003). These findings are supported by meta-analytic evidence indicating that serum folate concentrations are 10-20% lower in TT carriers than CC carriers, with CT carriers showing intermediate levels (Tsang et al., 2015). This is consistent with the co-dominant model in which each T allele present contributes incrementally to reduce serum folate status (Tsang et al., 2015). In corroboration with this, a smaller study that uses LC-MS/MS (n=115) found mean serum folate of 6.7 ± 4.0 nmol/L in TT carriers versus 12.2 ± 8.0 nmol/L in CC carriers at P < 0.001 confidence (Rooney et al., 2020). This ~45% difference exceeds the meta-analytic estimates and likely reflects considerable in-group variability, though it still fully supports the directionality aspect consistently (Rooney et al., 2020). Directional Trend and Confidence Assessment: Reduced folate status is a stable component of the TT biomarker profile across meta-analytic, GWAS, and individual studies (Colson et al., 2017; Shane et al., 2018; Tsang et al., 2015; Rooney et al., 2020). At the GWAS level, the 677T allele has been confirmed to be a significant determinant of lowered serum folate at genome-wide significance (P = 2.82 × 10⁻¹¹), though genotype-stratified values are not available from this source (Shane et al., 2018). The results of these studies help to distinguish folate as a separate indicator biomarker from homocysteine (Colson et al., 2017). Homocysteine differences can disappear after supplementation, but serum folate differences indicate persistence despite supplementation, which suggests that serum folate may be the more durable genotype-sensitive indicator of the two (Colson et al., 2017). This is an important distinction to note for analysis and future study since homocysteine can function as a modifiable marker while serum folate appears to reflect the underlying enzymatic constraint of the TT genotype consistently. ### 1.3.1.3: RBC Folate RBC folate, like serum folate, is a folate metric used frequently to assess disease states, but unlike serum folate, RBC folate reflects longer-term status and is less susceptible to short-term dietary fluctuations (Raghubeer et al., 2021). This stability makes it a more mechanistically informative complement to serum folate while still demonstrating a consistent genotype-stratified pattern across C677T literature. Main Evidence: The strongest and most rigorous evidence comes from a Bayesian meta-analysis that restricted studies based on microbiological assay methodology, which is the gold standard for population folate assessment (Tsang et al., 2015). The six studies that were included showed RBC folate followed a clear CC > CT > TT stepwise pattern, with TT carriers averaging 16% lower than CC (95% CrI: 12%, 20%), CT carriers 8% lower than CC (95% CrI: 4%, 12%), and TT carriers 9% lower than CT (95% CrI: 5%, 13%); in this study’s TT carriers, the RBC folate deficiency exceeded the serum folate deficiency (16% and 13%, respectively) (Tsang et al., 2015). This discrepancy suggests that RBC folate may be a more sensitive genotype-specific marker, which is mechanistically consistent with established pathways given that RBC folate reflects longer-term status instead of circulating concentration (Raghubeer et al., 2021). These findings are corroborated by another large population-based RCT that provided full genotype-stratified baseline metrics: TT at 552.2 nmol/L, CT at 616.3 nmol/L, and CC at 669.0 nmol/L (Crider et al., 2011). This shows TT RBC folate as significantly lower than both other genotypes (CT/CC), both P < 0.001, though CC vs. CT fell just short of significance at P = 0.06 (Crider et al., 2011). These results are consistent with the asymmetric co-dominant effect suggested by the Tsang estimates, especially given that the TT < CC trend was maintained across all doses in the trial (Tsang et al., 2015; Crider et al., 2011). However, throughout the trial, even at the highest supplementation doses (4000 μg/d), TT carriers never achieved CC-level RBC folate concentrations, which reinforces the emerging pattern that the genotype effect is not recovered with supplementation (Crider et al., 2011). Supporting this, a smaller LC-MS/MS study (n = 115) found RBC folate approximately 23% lower in TT versus CC carriers at P = 0.045, which is directionally consistent, though it’s important to note this study is limited by absent CT data and displays within-group variability, though that feature consistent with other studies as well (Rooney et al., 2020). Additionally, a GWAS across 2,232 healthy Irish adults was able to confirm MTHFR C677T as the dominant genetic modifier of RBC folate at a genome-wide significance (P = 1.37 × 10⁻17); this was done by mapping all 32 SNPs of significance onto the MTHFR region of chromosome 1, and C677T displayed the strongest signal by a margin of over 4 orders to the next strongest variant (Shane et al., 2018). When the experiment was repeated with homozygotes (CC) alone, none of the MTHFR-region SNPs remained significant, which confirmed that the signal is driven specifically by T allele dosage rather than linkage with other variants (Shane et al., 2018). Additionally, this study was able to determine that the C677T polymorphism accounts for 7% of total variance in RBC folate concentrations (Shane et al., 2018). This is analytically significant since a single SNP explaining 7% of total variance in a continuously distributed biomarker is a large effect in the context of complex genetic traits. Directional Trend and Confidence Assessment: Decreased RBC folate in the stepwise pattern of TT < CT < CC is highly directionally consistent and supported across meta-analytic, RCT, and GWAS studies (Tsang et al., 2015; Crider et al., 2011; Shane et al., 2018). For RBC folate specifically, the CC versus CT distinction is less statistically robust than TT versus either group in the other biomarker syntheses, so caution is warranted surrounding heterozygote-specific assertions. Additionally, since the RBC folate deficit in TT carriers appeared to exceed the folate serum deficit even with supplementation, the assertion that RBC folate is a stable, genotype-sensitive biomarker is strengthened (Tsang et al., 2015; Crider et al., 2011). The only primary limitation across the evidence is the sex-based restriction to female populations in both the Tsang meta-analysis and Crider RCT, though generalizability to mixed-sex adult populations was directionally supported by the Irish GWAS (Tsang et al., 2015; Crider et al., 2011; Shane et al., 2018). ## **1.3.2 Set 2 — Cofactor and Compensatory Pathway Biomarkers ** This section examines vitamin B6/PLP, betaine, vitamin B12, and methionine for trends in conjunction with MTHFR C677T polymorphisms. ### 1.3.2.1: Vitamin B6 Unlike the previously reviewed biomarkers, vitamin B6, which is measured in its actively circulating form, pyridoxal-5'-phosphate (PLP), does not show a simple, genotype-stratified effect in current C677T literature (Midttun et al., 2007; Hustad et al., 2007). Rather, the relationship between C677T and PLP appears to function more akin to a modifier effect: the TT genotype seems to amplify existing metabolic consequences of low B6 status rather than producing lower B6 levels independently, which is not only mechanistically distinct, but also is supported across available research (Midttun et al., 2007; Jarrett et al., 2022). Main Evidence: The strongest evidence comes from the largest available dataset, the NORCCAP cohort, which measured PLP via LC-MS/MS with full MTHFR genotyping of 10,601 individuals. The results showed median concentrations of 47.9 nmol/L for CC carriers, 49.0 nmol/L for CT carriers, and 44.4 nmol/L for TT carriers, which while directionally consistent with the TT disadvantage established in other biomarkers, was not statistically significant, at P = 0.34 (Midttun et al., 2007). However, the same dataset was able to establish a significant genotype interaction with B6 on homocysteine at P = 0.006, when stratifying CC/CT versus TT. CC and CT carriers showed a 0.5 μmol/L homocysteine difference in the highest and lowest PLP, whereas TT carriers showed a 2.1 μmol/L difference (Hustad et al., 2007). This difference indicates that TT carriers are substantially more dependent on sufficient B6 status in order to properly regulate homocysteine (Hustad et al., 2007; Midttun et al., 2007). This conditional status is further supported by a large multi-cohort study (n = 5,612) that examined PLP across MTHFR genotype and riboflavin (B2) status (Jarrett et al., 2022). The results showed that when B2 levels were sufficient, TT carriers showed no significant difference in PLP concentrations compared to non-TT carriers at P = 0.704, which indicates that sufficient B2 can maintain PLP regardless of genotype; however, under B2-deficient conditions, TT carriers showed PLP levels of 52.1 ± 2.87 nmol/L compared to approximately 60 nmol/L in CC/CT deficient individuals, at a confidence of P = 0.016 (Jarrett et al., 2022). This further indicates additional vulnerability to PLP depletion specifically in B2-deficient TT carriers (Jarrett et al., 2022). This is mechanistically coherent, since B2 is a required cofactor for pyridoxine-5'-phosphate oxidase, the enzyme that produces PLP, which would explain why B2-deficient TT carriers would be further exposed to PLP depletion than other cohorts (Jarrett et al., 2022). Additionally, a smaller LC-MS/MS study (N = 115; CC n = 68, TT n = 47) found mean PLP approximately 35% lower in TT versus CC carriers with 47.5 ± 22.2 vs. 72.0 ± 38.3 nmol/L, respectively, at P < 0.001 (Rooney et al., 2020). While this does provide significant and direct genotype comparison, the large standard deviations and lack of heterozygosity consideration limit its contribution to the directionality assertion. Directional Trend and Confidence Assessment: In summary, the directional genotype-stratified trend of PLP is not statistically supported, with the strongest study returning P = 0.34 (Midttun et al., 2007). This is likely reflective of biological realities and not a gap in existing literature. Even with the three-way genotype stratification from the NORCCAP cohort displaying a directionality trend across CC, CT, and TT, it did not reach statistical significance, which limits the strength of the directional claim for PLP as a direct genotype indicator (Midttun et al., 2007). However, the evidence does establish a high confidence that TT carriers demonstrate significantly amplified sensitivity to B6 and B2 deficiencies, which can produce meaningfully lower PLP in those specific conditions (Midttun et al., 2007; Jarrett et al., 2022). This suggests PLP functions as a conditional vulnerability biomarker rather than a stable depletion or inflation signal, which is the mechanistically distinct finding. ### 1.3.2.2: Betaine As mentioned above in section 1.1.6, betaine functions as an alternative methyl donor through the betaine-homocysteine methyltransferase (BHMT) pathway, unlike primary folate pathway metabolites, which allows betaine to operate independently of folate availability (Raghubeer et al., 2021). Current literature supports the importance of the BHMT pathway, particularly for individuals suffering from folate pathway dysfunction, like TT carriers, since methylation dysfunction causes an increased reliance on BHMT for homocysteine recycling (Holm et al., 2007). Main Evidence: The strongest evidence of this pattern comes from the NORCCAP cohort study that also supported the PLP profile investigation, which demonstrated that betaine functioned as a strong determinant of plasma homocysteine in TT individuals with low serum folate concentrations, especially those with low B vitamin status; specifically, for TT carriers with low serum folate and B vitamin deficiencies, the difference in homocysteine across betaine quartiles was 8.8 μmol/L (95% CI: 1.3–16.2), indicating a substantial metabolic dependence on the betaine pathway function (Holm et al., 2007). A recent supplementation study provided further direct evidence of genotype-stratified altered betaine metabolism, with plasma betaine base concentrations occurring at 4.52 ± 1.20 μg/mL for CC carriers and 5.31 ± 1.88 μg/mL for CT/TT carriers, showing numerically elevated betaine concentrations for individuals carrying the T allele (Zawieja et al., 2024). Additionally, after betaine supplementation, T-allele carriers demonstrated significantly greater responses than their CC counterparts to a statistically significant degree of P = 0.027; this difference in response pattern supports the assertion that betaine is used in enhanced capacity for TT carriers (Zawieja et al., 2024). A smaller LC-MS/MS study confirmed no significant genotype-stratified concentration differences with CC individuals displaying concentrations of 53.1 ± 13.7 μmol/L vs TT individuals showing concentrations of 50.5 ± 15.8 μmol/L at P = 0.194, further indicating that betaine's role in the TT biomarker profile is primarily functional rather than concentration-based (Rooney et al., 2020). In addition, a smaller isotopic flux study further confirmed that while baseline betaine concentrations were similar across homozygous genotypes (CC: 56 ± 7.9 μmol/L vs. TT: 51 ± 3.5 μmol/L, at a P = 0.418), TT carriers demonstrated significantly higher betaine pathway usage through multiple flux indicators, including elevated betaine:choline enrichment ratios (P = 0.041) and increased urinary sarcosine enrichment (P = 0.041) (Yan et al., 2011). This reflects compensatory pathway upregulation since elevated betaine:choline enrichment ratios measure the conversion rate of choline to betaine, meaning these values demonstrate greater demand for betaine synthesis in TT carriers (Yan et al., 2011). Additionally, urinary sarcosine enrichment (USE) is a metric for betaine oxidation; since sarcosine is a downstream metabolite of the BHMT pathway, increased USE levels in this context further support the idea that TT carriers show increased active BHMT pathway utilization as a compensatory method against impaired folate-dependent methylation (Yan et al., 2011). Directional Trend and Confidence Assessment: Overall, the literature indicates that betaine demonstrates a pattern of enhanced metabolic importance and utilization for TT carriers specifically, rather than standard genotype-stratified concentration differences. The directional signaling present reflects compensatory upregulation, with TT carriers showing similar baseline concentrations with greater metabolic responsiveness to betaine availability (Zawieja et al., 2024; Holm et al., 2007). These findings are particularly relevant for liver biology, since betaine metabolism is involved in hepatic lipid regulation through the choline-betaine-phosphatidylcholine axis, which could provide a potential mechanistic bridge between MTHFR C677T genotype and fatty liver pathophysiology (Kitagawa et al., 2017; Christensen et al., 2025). This relationship will be explored more in depth in the integrated discussion portion of the project. ### 1.3.2.3: Vitamin B12 While betaine’s role in one-carbon metabolism is primarily within the alternative BHMT pathway, vitamin B12 is an essential cofactor for the primary folate re-methylation cycle (Leclerc et al., 2013). Population-level B12 concentrations show no significant association with C677T genotypes under normal conditions, though emerging evidence suggests TT carriers exhibit increased vulnerability to B12 deficiency, and experience conditionally amplified metabolic consequences when deficient (Shane et al., 2018; Shiran et al., 2015). Main Evidence: Large population studies in healthy adults consistently show a lack of significant differences in MTHFR genotype-stratified B12 concentrations; a comprehensive analysis of 2,232 Irish adults reported mean B12 concentrations of approximately 319 and 350 pmol/L for women and men, respectively (Shane et al., 2018). This study measured B12 as a covariant and found no significant association with the C677T genotype, which establishes that in healthy individuals with adequate nutrition, MTHFR genotype does not impact circulating B12 levels (Shane et al., 2018). However, when B12 status drops to marginal or deficient levels, a conditional vulnerability emerges. One case-control study of 360 adults showed that TT carriers had significantly increased rates of B12 deficiency compared to CC/CT individuals, with 29.8% and 9.2% deficient rates, respectively at P < 0.0001 (Zittan et al., 2007). This meant that at a 95% confidence interval, the authors concluded TT individuals had a 4.2 times increased risk of B12 deficiency compared to CC/CT individuals (95% CI: 2.1–8.3), which is a significant difference that indicates substantial genotype-stratified susceptibility (Zittan et al., 2007). It is important to note that TT carriers who were also B12-deficient exhibited markedly elevated homocysteine concentrations (20.6 ± 18.8 μmol/L) compared to non-TT B12-deficient carriers (9.4 ± 3.2 μmol/L, at P < 0.0001), which is a notable demonstration of the conditional, compounding, and downstream metabolic effects of nutritional status on specific genotypes, like C677T polymorphisms (Zittan et al., 2007). These trends were independently confirmed by another case-control study that showed similar statistically significant findings of B12 deficiency rates (TT: 28% vs CC/CT: 15%, at P = 0.005) and elevated homocysteine levels under B12-deficient conditions (TT: 21.2 ± 16 vs. CC/CT: 12.3 ± 5.6 μmol/L, at P = 0.008), confirming the trend that TT genotype is at higher risk of B12 deficiency and exacerbates the metabolic impact of this deficiency downstream (Shiran et al., 2015). Directional Trend and Confidence Assessment: Large-scale population studies, cross-sectional examinations, and targeted genotype analyses all support the conclusion that a conditional B12 vulnerability pattern exists for TT carriers, and it is characterized by normal baseline concentrations with an increased susceptibility to deficiency that can amplify downstream metabolic consequences when B12 status is compromised (Shane et al., 2018; Zittan et al., 2007; Shiran et al., 2015). This conditional gene-nutrient interaction indicates that while MTHFR C677T does not influence baseline B12 levels, it can alter susceptibility to metabolic dysfunction. This aligns with established mechanistic literature; since B12 is a required cofactor of methionine synthase, reduced B12 availability would logically amplify any downstream accumulation of homocysteine, which for individuals with impaired folate pathways, like TT carriers, could substantially worsen an already existing homocysteine:methionine imbalance (Leclerc et al., 2013; Shane et al., 2018). ### 1.3.2.4: Methionine Methionine is the direct product of folate-dependent remethylation of homocysteine and functions as a precursor to S-adenosylmethionine (SAM), which acts as the universal methyl donor in one-carbon metabolism (Leclerc et al., 2013). Since impaired MTHFR activity in T allele carriers reduces remethylation efficiency, lower circulating methionine levels could be plausibly anticipated in this genotype profile (Leclerc et al., 2013). Main Evidence: However, the limited available literature does not consistently support this expectation. The strongest data comes from the NORCCAP cohort study (n=10,601), which reported mean plasma methionine of 23.5, 23.6, and 23.4 μmol/L for CC, CT, and TT carriers respectively, at P = 0.186, showing no significant difference in concentrations across genotypes (Midttun et al., 2007). This study also included a partial Spearman correlation analysis between homocysteine and methionine, finding r = -0.07 (Midttun et al., 2007). This very weak, negative association further suggests that methionine does not track in conjunction with genotype-driven elevated homocysteine pattern despite close pathway associations and interactions. Two small, independent studies corroborated these null findings. One of these was a smaller LC-MS/MS study that reported plasma methionine concentrations of 29.5 ± 7.2 μmol/L for CC and 30.3 ± 6.7 μmol/L for TT individuals at P = 0.450, which, while statistically insignificant, still counters the established pattern expectation given that the CC cohort displayed numerically lower methionine concentrations than the TT individuals (Rooney et al., 2020). Another study further supported these null findings through urinary methionine data reports that showed a similar absence of genotypic effect, though it does mirror the unexpected pattern previously observed of TT carriers displaying higher average concentrations than their CC counterparts (TT: 9.5 ± 1.5 vs CC: 7.4 ± 1.2 μmol/L, at P = 0.356) (Yan et al., 2011). This consistent directional pattern spans three independent studies that use three distinct measurement approaches, supporting the plausibility of a genuine mechanistic phenomenon rather than measurement artifact (Midttun et al., 2007; Rooney et al., 2020; Yan et al., 2011). One plausible contributor to the maintained circulating methionine levels despite impaired folate methylation pathways is compensatory upregulation of the BHMT pathway, which remethylates homocysteine independently of the folate pathway (Holm et al., 2007). That would be mechanistically consistent with the established upregulation of BHMT observed in TT carriers, which is discussed more in depth in section 1.3.2.2 (Holm et al., 2007; Yan et al., 2011). However, there are likely other methylation pathways and cellular mechanisms contributing to this specific phenomenon, warranting further and expanded investigation (Martinez et al., 2017). Importantly, the LC-MS/MS study data revealed that while circulating methionine is preserved regardless of genotype, downstream methylation capacity is not, which could provide a mechanistic explanation for the observed directional pattern. The study showed that TT carriers displayed significantly lower plasma SAM concentrations (TT: 74.7 ± 21.0 vs CC: 85.2 ± 22.6 nmol/L, at P = 0.013) compared to CC individuals (Rooney et al., 2020). As the universal methyl donor, SAM concentration reflects active methylation capacity; therefore, a decrease in SAM in this context suggests that despite normal methionine supply, TT carriers experience a compromised conversion rate between methionine and usable methyl groups (Martinez et al., 2017). This is further supported by the SAM:SAH ratio results which demonstrate the same directional trend (TT: 1.66 ± 0.55 vs CC: 1.85 ± 0.51, at P = 0.043) (Rooney et al., 2020). S-adenosylhomocysteine (SAH) is the direct byproduct of SAM after methyl group donation, and the SAM:SAH ratio is used to directly reflect active methylation potential (Martinez et al., 2017; Rooney et al., 2020). Together, a decrease in both SAM and SAM:SAH levels indicates that TT carriers exhibit methylation dysfunction driven not only by methyl group supply deficits, but also active inhibition of methylation potential, even when methionine concentrations appear normal (Rooney et al., 2020). Directional Trend and Confidence Assessment: Overall, the literature indicates that circulating methionine levels are not significantly impacted by the C677T genotype, but T allele dosage is still associated with a consistent pattern of downstream methylation dysfunction regardless; confidence in this null finding is high given the statistical power of the NORCCAP cohort studies and the cross-methodological consistency in trends across all three independent studies (Midttun et al., 2007; Rooney et al., 2020; Yan et al., 2011). The SAM and SAM:SAH evidence further indicates that preserved methionine should not be interpreted as preserved methylation capacity, which is a distinction with meaningful implications that will be discussed in the integrated profile (Rooney et al., 2020). ## **1.3.3 Set 3 — Downstream and Diagnostic Biomarkers ** This section examines methylmalonic acid (MMA), C-reactive protein (CRP), ferritin, and gamma-glutamyl transferase (GGT) for trends in conjunction with MTHFR C677T polymorphisms. ### 1.3.3.1: MMA Methylmalonic acid (MMA) is a byproduct created when a vitamin B12-dependent enzyme, methylmalonyl-CoA mutase (MCM), converts methylmalonyl-CoA (MM-CoA) to succinyl-CoA in fat and protein metabolism processes (Molloy et al., 2016). This means circulating MMA levels are dependent on, and therefore indicative of, vitamin B12 enzymatic activity, which is indirectly linked to folate-dependent remethylation pathway function through their shared B12 pathway reliance (Leclerc et al., 2013; Molloy et al., 2016). Elevated homocysteine is associated with both folate and B12 deficiencies, which makes pathway-specific (folate vs. B12) attributions difficult to determine (Liew & Gupta, 2015). However, since MMA is not directly involved in the folate cycle and only increases when B12-specific pathways are impaired, MMA biomarker synthesis can help isolate that determination, given we would expect MMA levels to increase with homocysteine if B12-deficiency was the predominant driving pathway of the profile (Molloy et al., 2016; Fredriksen et al., 2007). That indirect link is why MMA’s inclusion in the TT MTHFR profile is valuable: it functions as a diagnostic metric of mechanism to determine if B12 deficiency is a confounding or legitimate contributor of elevated homocysteine. Main Evidence: All reviewed literature indicates that elevated MMA levels are not associated with genotype-specific responses for MTHFR C677T (Molloy et al., 2016; Fredriksen et al., 2007; Barbosa et al., 2008). The strongest evidence comes from a GWAS (n=2,210) that was specifically designed to identify genetic determinants of elevated MMA, screening 758,443 candidate SNPs for association and measuring plasma MMA at a cohort-wide median of 0.17 μmol/L (IQR: 0.14–0.21) (Molloy et al., 2016). It determined the top dominant genetic determinants of MMA were HIBCH (rs291466) and ACSF3 (~12% of MMA), which are related to the valine catabolism and MM-CoA synthesis, respectively, and are both entirely unrelated to both folate and B12 remethylation cycles; it also directly stated that MTHFR C677T produced no genome-wide significant signal for MMA in both the discovery cohort and in an independent replication cohort of 1,481 older Irish adults, further confirming the null findings (Molloy et al., 2016). It is important to note the power scale of this study: it is designed and executed to detect effects at genome-wide level, and its determination of a notable absence of MTHFR related signaling avoids the inconclusiveness often associated with null results given their frequent partial attribution to the power-limitation of their studies. A small Brazilian study (n=102) that used the validated GC-MS assay method for MMA quantification and included C677T genotyping provided additional null corroboration; it determined that serum cobalamin and creatinine were the sole examined determinants of MMA concentration and incidentally reported MMA experienced no genotypic effect during their investigation into cobalamin deficiency pathways (Barbosa et al., 2008). The null findings are further backed by another study that used the same NORCCAP cohort (n=10,601) included in some of the earlier biomarker profile syntheses, which measured MMA and 12 additional one-carbon metabolites across genotyped individuals, looking specifically at 13 one-carbon-related polymorphisms, including MTHFR C677T (Fredriksen et al., 2007). It confirmed MTHFR as the highest predictor of folate and homocysteine out of all measured polymorphisms, which aligns with this analysis’s findings but returned no significant association with MMA in the same individuals; however, MMA was significantly associated with TCN2 C776G at P < 0.001, which is a polymorphism of the transcobalamin-II gene and is involved in B12 transport pathways, unrelated to folate pathways (Fredriksen et al., 2007). This internal contrast of strong homocysteine and folate detection with no MMA detection suggests the two pathways are metabolically and genetically distinct, at both genomic and population levels. Directional Trend and Confidence Assessment: All three reviewed studies, which included a GWAS, a large-scale population-based metabolic phenotyping, and an independent observational cohort, unanimously found that MTHFR C677T genotype does not predict MMA concentration, and no TT > CT > CC gradient appears in any dataset (Molloy et al., 2016; Fredriksen et al., 2007; Barbosa et al., 2008). The methodological rigor and population coverage across these studies indicates a high confidence in these consistently null results, which are mechanistically significant for the TT profile. Because B12 deficiency would increase both homocysteine and MMA levels, the absence of elevated MMA indicates that, under baseline conditions, elevated homocysteine associated with the TT profile is primarily folate-driven, which is mechanistically coherent with earlier biomarker syntheses and strengthens the profile (Leclerc et al., 2013; Molloy et al., 2016). ### 1.3.3.2: CRP C-reactive protein (CRP) is a hepatic protein that is used commonly as a low-grade, chronic inflammation marker for various disease states, which is measured frequently in studies as high-sensitivity CRP (hs-CRP) (Yeniova et al., 2014). While CRP is not a one-carbon metabolite, it is implicated in the MTHFR C677T profile since the polymorphism is strongly associated with elevated homocysteine levels, which is, in turn, a known trigger for systemic inflammation (Matté et al., 2009). While literature investigating the relationship between MTHFR C677T and CRP is sparse, existing sources largely indicate a conditional, modest, directional relationship (Dedoussis et al., 2005; Bhatt et al., 2025). Main Evidence: The strongest evidence for this relationship comes from the ATTICA study, which is a cross-sectional population analysis of Greek adults (n=574) that stratified their data by age, sex, and genotype to investigate how MTHFR impacts risk of cardiovascular disease; it is important to note that the authors explicitly describe this source as the first reported association between MTHFR C677T and CRP and that this CRP data was log-transformed and adjusted using ANCOVA for a variety of factors including: age, diet, pack-years smoking, BMI, physical activity, total cholesterol, and education (Dedoussis et al., 2005). The results after transformation showed hs-CRP was significantly elevated in TT individuals across both sexes (Male: CC 1.3 ± 1.1, CT 1.8 ± 0.9, TT 2.1 ± 0.4 mg/dL, P = 0.01; females: CC 1.2 ± 1.1, CT 1.6 ± 1.2, TT 1.9 ± 0.9 mg/dL), at P = 0.03 (Dedoussis et al., 2005). While this evidence numerically shows a directional trend, the authors specifically noted a TT-elevated trend instead of TT-stepwise gradient since only comparisons between homozygotes (CC vs TT) reached statistical significance at Bonferroni, α < 0.01 (Dedoussis et al., 2005). This evidence is corroborated by a smaller cross-sectional analysis (n=489) that spans three Nepali ethnic groups and observed the same TT > CT > CC trend in hs-CRP levels, with CC individuals averaging 1.7 ± 1.2 mg/L, CT individuals averaging 1.9 ± 1.3 mg/L, and TT individuals averaging 2.0 ± 1.4 mg/L, though these results were not considered statistically significant at P = 0.16 (Bhatt et al., 2025). However, this study confirmed a homocysteine-CRP correlation that was established in the previously mentioned study (Dedoussis r = 0.45, Bhatt r = 0.47, both P < 0.001), which is noteworthy given the near identical association value found across these cohorts that span two independent populations on different continents (Dedoussis et al., 2005; Bhatt et al., 2025). Directional Trend and Confidence Assessment: The cross-cohort homocysteine-CRP correlation combined with the explicit findings of Dedoussis et al. (2005), which included the explicit finding that this association is independent of MTHFR genotype, supports a secondary effect more than a directional genotypic trend (Dedoussis et al., 2005; Bhatt et al., 2025). Specifically, it indicates that while C677T may direct homocysteine level elevation, homocysteine itself functions as the proximate mechanism driver of this inflammation marker in this specific context. In this framing, CRP elevation functions as a conditional, downstream effect in TT carriers where elevated homocysteine is already present. While evidence-based, these conclusions are supported exclusively by small-sized cross-sectional studies that inherently preclude causal relationships (Dedoussis et al., 2005; Bhatt et al., 2025). Therefore, this portion of the profile should be viewed as a potential mechanistic link between MTHFR genotype, homocysteine, and systemic inflammation, rather than direct evidence, though further investigation is warranted. ### 1.3.3.3: GGT & Ferritin Gamma-Glutamyl Transferase (GGT) is a hepatic enzyme involved in glutathione metabolism and oxidative stress pathways (Dillon & Miller, 2016). Because of its sensitivity to hepatic oxidative stress generated by homocysteine accumulation and its association with hyperhomocysteinemia, GGT was a strong candidate for consideration in the biomarker profile construction of the MTHFR C677T variant (Lippi et al., 2008). However, there is a lack of peer-reviewed studies that report GGT stratified by MTHFR C677T genotype in a healthy population using biologically measured, extractable data. This literature gap likely reflects the indirect relationship between GGT and the folate cycle, rather than a true absence of association. However, at this time there is not enough peer-reviewed literature to determine a relationship between GGT and MTHFR C677T. Ferritin, the storage protein for iron, was also an initial candidate for the profile since elevated levels are also associated with oxidative stress and inflammation (Salem et al., 2024). Like GGT, ferritin exhibits indirect interactions with the folate cycle, with elevated levels functioning as a responsive marker of oxidative stress and inflammatory signaling. Ferritin also appears sparsely in the C677T genotype-stratified literature in a form that meets the inclusion criteria of this analysis. GGT and ferritin are both plausible contextual markers, but they lack direct genotype-stratified evidence in the currently available literature. They could not reasonably be incorporated into this profile, but will be revisited in the Part 2 analysis on Fatty Liver Disease (FLD) classification and associated biomarkers.