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Primary topic: B12 transport and recycling pathway

B12 Transport and Recycling Pathway: Why Intake Alone Can Mislead

This pathway matters when B12 intake looks acceptable on paper but functional support still looks weak, uneven, or harder to explain.

What this pathway does

The pathway moves B12 into tissues, recycles it into an active form, and helps determine whether methylation-related work can proceed efficiently inside cells.

Why it matters

It matters because transport and recycling bottlenecks can make symptoms, methylmalonic acid, or homocysteine more useful than intake alone when deciding what deserves follow-up.

What creates pressure on this pathway

  • lower biologically available B12 transport into tissues
  • slower recycling of active B12 inside the methylation network
  • overlap with methylation demand that makes weak transport more visible

Validation markers to consider

  • methylmalonic acid
  • holotranscobalamin
  • homocysteine and broader B12 context

Genes and SNPs connected to this pathway

B12 is one specific B vitamin. It helps nerves, red blood cells, energy, and brain function.

Study rows support the SNP/gene claim. The pathway connection comes from the curated gene-to-pathway map.

What may run higher

Blood B12 may look high. That does not automatically mean there is a problem, but it may need context.

What may work more slowly

B12 may not reach your cells well, even if a basic blood test looks normal.

What to check next

Check serum B12, methylmalonic acid, homocysteine, holotranscobalamin if available, CBC, MCV, diet, and medication context.

4mapped genes
8mapped SNPs
12SNP/gene claims
8report eligible
MMACHC4 SNPs - 4 claimsShow SNPsOpen gene page
rs1219182411 claims - 2 study rows

biomarker tendency - TT or compound-heterozygous MMACHC deficiency context

Intracellular cobalamin processing tendency

Strong

rs121918241 T / MMACHC p.Arg132Ter is associated with lower intracellular cobalamin processing tendency in recessive or compound-heterozygous cblC contexts.

MMACHC rs121918241 T is staged as a recessive cblC loss-of-function allele affecting intracellular B12 processing and methylmalonate/homocysteine biology.

Likely effectLower biomarker tendency
Signal sizeModerate signal
Evidence supportStrong support
Report useIncluded in pathway scoring
Show study evidence
ClinVar Variation 1423clinical curation - directClinVar maps rs121918241 to MMACHC c.394C>T / p.Arg132Ter, classifies it as pathogenic with multiple submitters and no conflicts, and links it to cobalamin C disease and methylmalonic acidemia with homocystinuria.MedlinePlus methylmalonic acidemia with homocystinuriaclinical curation - directThe checked MedlinePlus cblC condition source supports MMACHC involvement in vitamin B12 processing, conversion to adenosylcobalamin and methylcobalamin, and consequent methylmalonic acidemia plus homocystinuria when processing is impaired.
rs1219182421 claims - 2 study rows

biomarker tendency - TT or compound-heterozygous MMACHC deficiency context

Intracellular cobalamin processing tendency

Strong

rs121918242 T / MMACHC c.331C>T / p.Arg111Ter is associated with lower intracellular cobalamin processing tendency in recessive or compound-heterozygous cblC contexts.

MMACHC rs121918242 T is staged as a recessive cblC loss-of-function allele affecting intracellular B12 processing and methylmalonate/homocysteine biology.

Likely effectLower biomarker tendency
Signal sizeModerate signal
Evidence supportStrong support
Report useIncluded in pathway scoring
Show study evidence
ClinVar Variation 1424clinical curation - directClinVar maps rs121918242 to MMACHC c.331C>T / p.Arg111Ter and cobalamin C disease / methylmalonic acidemia with homocystinuria records.MedlinePlus methylmalonic acidemia with homocystinuriaclinical curation - directThe checked MedlinePlus cblC condition source supports MMACHC involvement in vitamin B12 processing, conversion to adenosylcobalamin and methylcobalamin, and consequent methylmalonic acidemia plus homocystinuria when processing is impaired.
rs1219182431 claims - 2 study rows

biomarker tendency - AA or compound-heterozygous MMACHC deficiency context

Intracellular cobalamin processing tendency

Strong

rs121918243 A / MMACHC p.Arg161Gln is associated with lower intracellular cobalamin processing tendency in recessive or compound-heterozygous cblC contexts.

MMACHC rs121918243 A is staged as a recessive cblC allele affecting intracellular B12 processing and methylmalonate/homocysteine biology.

Likely effectLower biomarker tendency
Signal sizeModerate signal
Evidence supportStrong support
Report useIncluded in pathway scoring
Show study evidence
ClinVar RCV000001490clinical curation - directClinVar maps rs121918243 to MMACHC c.482G>A / p.Arg161Gln and classifies the allele in methylmalonic acidemia with homocystinuria, cblC type.PMCID PMC4219318clinical curation - directThe checked cblC clinical review supports MMACHC disease as impaired intracellular synthesis of adenosylcobalamin and methylcobalamin with elevated methylmalonic acid and homocysteine and decreased methionine.
rs3981242921 claims - 2 study rows

biomarker tendency - dupA/dupA or compound-heterozygous MMACHC deficiency context

Intracellular cobalamin processing tendency

Strong

rs398124292 dupA / MMACHC c.271dupA p.Arg91fs is associated with lower intracellular cobalamin processing tendency in recessive or compound-heterozygous cblC contexts.

MMACHC rs398124292 dupA is staged as a recurrent cblC frameshift allele affecting intracellular B12 processing and methylmalonate/homocysteine biology.

Likely effectLower biomarker tendency
Signal sizeModerate signal
Evidence supportStrong support
Report useIncluded in pathway scoring
Show study evidence
ClinVar Variation 1421clinical curation - directClinVar maps rs398124292 to MMACHC c.271dup / p.Arg91fs and classifies the allele as pathogenic for methylmalonic acidemia with homocystinuria, cblC type.PMCID PMC4219318clinical curation - directThe checked cblC clinical review describes c.271dupA as a frequent MMACHC disease-causing frameshift and supports the biochemical pattern of elevated methylmalonic acid and homocysteine with decreased methionine.
MTHFR2 SNPs - 6 claimsShow SNPsOpen gene page
rs18011313 claims - 9 study rows

enzyme activity - GG

MTHFR enzyme activity

Moderate

rs1801131 GG is associated with reduced MTHFR enzyme activity.

MTHFR rs1801131 is scored as a mild lower-activity tendency, with stronger scoring for the two-copy genotype than the one-copy genotype.

Likely effectLower enzyme activity signal
Signal sizeSmall signal
Evidence supportLimited support
Report useIncluded in pathway scoring
Show study evidence
ClinVar recordidentity only - unknownClinVar Variation ID 3521 identifies rs1801131 as MTHFR NM_005957.5:c.1286A>C, protein p.Glu429Ala, also named MTHFR 1298A-C / GLU429ALA. The canonical SPDI shown by ClinVar is NC_000001.11:11794418:T:G, so this file uses the dbSNP/GRCh38 forward-strand genotype letters T and G. ClinVar classifications for listed germline conditions are mostly benign or benign/likely benign, so this record is limited to enzyme-activity tendency.PubMed 9719624functional assay - directWeisberg et al. 1998 characterized MTHFR A1298C as an E-to-A substitution and reported that the polymorphism was associated with decreased enzyme activity; homozygotes had approximately 60% of control activity in lymphocytes. The abstract also reported lower activity in people heterozygous for both C677T and A1298C, but it does not give an equally direct single-heterozygote A1298C estimate, so GT is assigned a small magnitude and lower certainty.PubMed 23288205clinical curation - close proxyThe 2013 ACMG practice guideline says MTHFR polymorphism testing has minimal clinical utility in routine thrombophilia evaluation and discusses disproven or weak disease-risk associations. This supports conservative certainty and prevents using this genotype-effect row as a diagnosis, disease-risk estimate, or treatment rule.

enzyme activity - GT

MTHFR enzyme activity

Not used for pathway scoring

rs1801131 GT is associated with reduced MTHFR enzyme activity.

MTHFR rs1801131 is scored as a mild lower-activity tendency, with stronger scoring for the two-copy genotype than the one-copy genotype.

Likely effectLower enzyme activity signal
Signal sizeMinimal signal
Evidence supportLimited support
Report useEvidence only, not scored
Show study evidence
ClinVar recordidentity only - unknownClinVar Variation ID 3521 identifies rs1801131 as MTHFR NM_005957.5:c.1286A>C, protein p.Glu429Ala, also named MTHFR 1298A-C / GLU429ALA. The canonical SPDI shown by ClinVar is NC_000001.11:11794418:T:G, so this file uses the dbSNP/GRCh38 forward-strand genotype letters T and G. ClinVar classifications for listed germline conditions are mostly benign or benign/likely benign, so this record is limited to enzyme-activity tendency.PubMed 9719624functional assay - directWeisberg et al. 1998 characterized MTHFR A1298C as an E-to-A substitution and reported that the polymorphism was associated with decreased enzyme activity; homozygotes had approximately 60% of control activity in lymphocytes. The abstract also reported lower activity in people heterozygous for both C677T and A1298C, but it does not give an equally direct single-heterozygote A1298C estimate, so GT is assigned a small magnitude and lower certainty.PubMed 23288205clinical curation - close proxyThe 2013 ACMG practice guideline says MTHFR polymorphism testing has minimal clinical utility in routine thrombophilia evaluation and discusses disproven or weak disease-risk associations. This supports conservative certainty and prevents using this genotype-effect row as a diagnosis, disease-risk estimate, or treatment rule.

enzyme activity - TT

MTHFR enzyme activity

Not used for pathway scoring

rs1801131 TT has no scored directional claim for MTHFR enzyme activity.

MTHFR rs1801131 is scored as a mild lower-activity tendency, with stronger scoring for the two-copy genotype than the one-copy genotype.

Likely effectNo clear enzyme signal
Signal sizeMinimal signal
Evidence supportVery limited support
Report useEvidence only, not scored
Show study evidence
ClinVar recordidentity only - unknownClinVar Variation ID 3521 identifies rs1801131 as MTHFR NM_005957.5:c.1286A>C, protein p.Glu429Ala, also named MTHFR 1298A-C / GLU429ALA. The canonical SPDI shown by ClinVar is NC_000001.11:11794418:T:G, so this file uses the dbSNP/GRCh38 forward-strand genotype letters T and G. ClinVar classifications for listed germline conditions are mostly benign or benign/likely benign, so this record is limited to enzyme-activity tendency.PubMed 9719624functional assay - directWeisberg et al. 1998 characterized MTHFR A1298C as an E-to-A substitution and reported that the polymorphism was associated with decreased enzyme activity; homozygotes had approximately 60% of control activity in lymphocytes. The abstract also reported lower activity in people heterozygous for both C677T and A1298C, but it does not give an equally direct single-heterozygote A1298C estimate, so GT is assigned a small magnitude and lower certainty.PubMed 23288205clinical curation - close proxyThe 2013 ACMG practice guideline says MTHFR polymorphism testing has minimal clinical utility in routine thrombophilia evaluation and discusses disproven or weak disease-risk associations. This supports conservative certainty and prevents using this genotype-effect row as a diagnosis, disease-risk estimate, or treatment rule.
rs18011333 claims - 9 study rows

enzyme activity - AA

MTHFR enzyme activity

Moderate

rs1801133 AA is associated with reduced MTHFR enzyme activity.

MTHFR rs1801133 is scored as a lower-activity tendency, with stronger scoring for the two-copy genotype than the one-copy genotype.

Likely effectLower enzyme activity signal
Signal sizeModerate signal
Evidence supportModerate support
Report useIncluded in pathway scoring
Show study evidence
ClinVar recordidentity only - unknownClinVar Variation ID 3520 identifies rs1801133 as MTHFR NM_005957.5:c.665C>T, protein p.Ala222Val, also named MTHFR 677C-T / ALA222VAL. The canonical SPDI shown by ClinVar is NC_000001.11:11796320:G:A, so this file uses the dbSNP/GRCh38 forward-strand genotype letters G and A. ClinVar condition classifications are mixed and include benign, uncertain, conflicting, and drug-response contexts, so this record is limited to enzyme-activity tendency rather than disease interpretation.PubMed 7647779functional assay - directFrosst et al. 1995 reported a common MTHFR mutation that changes a conserved amino acid. The PubMed abstract states that heterozygous or homozygous mutation status correlated with reduced enzyme activity and increased thermolability in lymphocyte extracts, and that homozygous individuals had significantly elevated plasma homocysteine. This supports a negative enzyme-activity direction for A-containing genotypes, with a larger magnitude for AA than AG.PubMed 23288205clinical curation - close proxyThe 2013 ACMG practice guideline says MTHFR polymorphism testing has minimal clinical utility in routine thrombophilia evaluation and discusses disproven or weak disease-risk associations. This supports conservative certainty and prevents using this genotype-effect row as a diagnosis, disease-risk estimate, or treatment rule.

enzyme activity - AG

MTHFR enzyme activity

Not used for pathway scoring

rs1801133 AG is associated with reduced MTHFR enzyme activity.

MTHFR rs1801133 is scored as a lower-activity tendency, with stronger scoring for the two-copy genotype than the one-copy genotype.

Likely effectLower enzyme activity signal
Signal sizeSmall signal
Evidence supportLimited support
Report useEvidence only, not scored
Show study evidence
ClinVar recordidentity only - unknownClinVar Variation ID 3520 identifies rs1801133 as MTHFR NM_005957.5:c.665C>T, protein p.Ala222Val, also named MTHFR 677C-T / ALA222VAL. The canonical SPDI shown by ClinVar is NC_000001.11:11796320:G:A, so this file uses the dbSNP/GRCh38 forward-strand genotype letters G and A. ClinVar condition classifications are mixed and include benign, uncertain, conflicting, and drug-response contexts, so this record is limited to enzyme-activity tendency rather than disease interpretation.PubMed 7647779functional assay - directFrosst et al. 1995 reported a common MTHFR mutation that changes a conserved amino acid. The PubMed abstract states that heterozygous or homozygous mutation status correlated with reduced enzyme activity and increased thermolability in lymphocyte extracts, and that homozygous individuals had significantly elevated plasma homocysteine. This supports a negative enzyme-activity direction for A-containing genotypes, with a larger magnitude for AA than AG.PubMed 23288205clinical curation - close proxyThe 2013 ACMG practice guideline says MTHFR polymorphism testing has minimal clinical utility in routine thrombophilia evaluation and discusses disproven or weak disease-risk associations. This supports conservative certainty and prevents using this genotype-effect row as a diagnosis, disease-risk estimate, or treatment rule.

enzyme activity - GG

MTHFR enzyme activity

Not used for pathway scoring

rs1801133 GG has no scored directional claim for MTHFR enzyme activity.

MTHFR rs1801133 is scored as a lower-activity tendency, with stronger scoring for the two-copy genotype than the one-copy genotype.

Likely effectNo clear enzyme signal
Signal sizeMinimal signal
Evidence supportVery limited support
Report useEvidence only, not scored
Show study evidence
ClinVar recordidentity only - unknownClinVar Variation ID 3520 identifies rs1801133 as MTHFR NM_005957.5:c.665C>T, protein p.Ala222Val, also named MTHFR 677C-T / ALA222VAL. The canonical SPDI shown by ClinVar is NC_000001.11:11796320:G:A, so this file uses the dbSNP/GRCh38 forward-strand genotype letters G and A. ClinVar condition classifications are mixed and include benign, uncertain, conflicting, and drug-response contexts, so this record is limited to enzyme-activity tendency rather than disease interpretation.PubMed 7647779functional assay - directFrosst et al. 1995 reported a common MTHFR mutation that changes a conserved amino acid. The PubMed abstract states that heterozygous or homozygous mutation status correlated with reduced enzyme activity and increased thermolability in lymphocyte extracts, and that homozygous individuals had significantly elevated plasma homocysteine. This supports a negative enzyme-activity direction for A-containing genotypes, with a larger magnitude for AA than AG.PubMed 23288205clinical curation - close proxyThe 2013 ACMG practice guideline says MTHFR polymorphism testing has minimal clinical utility in routine thrombophilia evaluation and discusses disproven or weak disease-risk associations. This supports conservative certainty and prevents using this genotype-effect row as a diagnosis, disease-risk estimate, or treatment rule.
FUT21 SNPs - 1 claimsShow SNPsOpen gene page
rs4926021 claims - 3 study rows

biomarker tendency - A

plasma vitamin B12 tendency

Moderate

rs492602 A at FUT2 is associated with lower circulating vitamin B12 biomarker tendency.

FUT2 rs492602 A is staged as a lower total plasma vitamin B12 tendency allele with supporting holo-haptocorrin/secretor biology.

Likely effectLower biomarker tendency
Signal sizeSmall signal
Evidence supportStrong support
Report useIncluded in pathway scoring
Show study evidence
PMID 18776911gwas - directThe checked GWAS record supports rs492602 at FUT2 as a genome-wide plasma vitamin B12 locus, with the A allele reported in follow-up summaries as the lower-B12 allele.PMCID PMC3243448biomarker association - directThe checked plasma B12 study reports FUT2 204G>A / rs492602 among B12-related SNPs strongly associated with lower plasma B12 concentrations and includes holo-haptocorrin measurements.PMCID PMC5886113biomarker association - close proxyThe checked FUT2 secretor-variant source supports a mechanism in which FUT2 genotype influences haptocorrin-bound B12 and haptocorrin glycosylation, providing close transport/status support for the rs492602 plasma B12 signal.
MTR1 SNPs - 1 claimsShow SNPsOpen gene page
rs11314501 claims - 3 study rows

biomarker tendency - AA

MTR-mediated homocysteine remethylation

Moderate

rs1131450 AA is associated with lower MTR-mediated homocysteine remethylation capacity.

MTR rs1131450 AA is scored as a lower MTR/remethylation tendency based on functional reporter data and homocysteine-related biomarkers.

Likely effectLower biomarker tendency
Signal sizeSmall signal
Evidence supportModerate support
Report useIncluded in pathway scoring
Show study evidence
NCBI RefSNP recordidentity only - unknownNCBI RefSNP places rs1131450 on chromosome 1 and lists G and A alleles on the current preferred genomic placement. This supports using A-containing genotype labels for the MTR G>A source claim, but does not itself assign effect direction.Scientific Reports 2016 MTR functional variant studyfunctional assay - directQu et al. reported rs1131450 G>A in the MTR 3 prime UTR. In luciferase assays, the A-allele construct showed lower reporter activity than the G construct in LNCaP and PC3 cells. The same paper notes that prostate tissue qPCR did not show a direct mRNA-expression association for rs1131450, so this evidence supports a conservative lower MTR pathway-function tendency rather than a high-certainty expression claim.Scientific Reports 2016 plasma homocysteine analysisbiomarker association - close proxyThe same study measured plasma homocysteine in 201 prostate cancer patients and 210 matched controls. For rs1131450 G>A, AA carriers had the highest plasma homocysteine concentration, approximately 1.5-fold higher than GG carriers in the case cohort, control cohort, and combined analysis. Because MTR remethylates homocysteine to methionine, this is close metabolic biomarker support for lower MTR-mediated remethylation capacity in AA.

Where DNA analysis helps

DNA helps decide whether B12 transport and recycling deserve attention before assuming the problem is simply low intake or an isolated methylation variant.

Example interpretation

Your B12 pathway may deserve follow-up because transport and recycling signals overlap strongly enough to justify direct marker validation.

Suggested validation: methylmalonic acid plus holotranscobalamin.

What to do next

  • check methylmalonic acid and holotranscobalamin before assuming intake is the limiting factor
  • compare TCN2 and MTRR with broader methylation genes to see whether B12 handling changes the interpretation
  • use functional markers before escalating supplement strategy

Related pages

TCN2

TCN2 B12 transport gene

MTRR

MTRR B12 recycling gene

Low energy

low energy metabolism causes

Methylmalonic acid

methylmalonic acid and B12

Holotranscobalamin

holotranscobalamin and B12 transport

Is DNA metabolism analysis worth it?

is DNA metabolism analysis worth it

DNA nutrition analysis report

DNA nutrition analysis report

Get the full report

Use your DNA file to see which pathways may be worth validating first.